[V3,1/4] ACPI / CPPC: Optimize PCC Read Write operations

Commit Message

Prakash, Prashanth Feb. 10, 2016, 8:05 p.m. UTC

From: Ashwin Chaugule <[e-mail]>

Previously the send_pcc_cmd() code checked if the
PCC operation had completed before returning from
the function. This check was performed regardless
of the PCC op type (i.e. Read/Write). Knowing
the type of cmd can be used to optimize the check
and avoid needless waiting. e.g. with Write ops,
the actual Writing is done before calling send_pcc_cmd().
And the subsequent Writes will check if the channel is
free at the entry of send_pcc_cmd() anyway.

However, for Read cmds, we need to wait for the cmd
completion bit to be flipped, since the actual Read
ops follow after returning from the send_pcc_cmd(). So,
only do the looping check at the end for Read ops.

Also, instead of using udelay() calls, use ktime as a
means to check for deadlines. The current deadline
in which the Remote should flip the cmd completion bit
is defined as N * Nominal latency. Where N is arbitrary
and large enough to work on slow emulators and Nominal
latency comes from the ACPI table (PCCT). This helps
in working around the CONFIG_HZ effects on udelay()
and also avoids needing different ACPI tables for Silicon
and Emulation platforms.

Signed-off-by: Ashwin Chaugule <[e-mail]>
Signed-off-by: Prashanth Prakash <[e-mail]>
---
 drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 71 insertions(+), 28 deletions(-)

Comments

Timur Tabi Feb. 10, 2016, 8:42 p.m. UTC | #1

Prashanth Prakash wrote:

> +static int check_pcc_chan(void)
> +{
> +	int ret = -EIO;
> +	struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
> +	ktime_t next_deadline = ktime_add(ktime_get(), deadline);
> +
> +	/* Retry in case the remote processor was too slow to catch up. */
> +	while (!ktime_after(ktime_get(), next_deadline)) {
> +		if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
> +			ret = 0;
> +			break;
> +		}
> +		/*
> +		 * Reducing the bus traffic in case this loop takes longer than
> +		 * a few retries.
> +		 */
> +		udelay(3);
> +	}

Like I said last time, you really should use 
readq_relaxed_poll_timeout(), which does exactly the same thing as this 
loop, but more elegantly.  I think this will work:

u32 status;
ret = readq_relaxed_poll_timeout(&generic_comm_base->status, status, 
status & PCC_CMD_COMPLETE, 3, deadline);
return ret ? -EIO : 0;
...
deadline = NUM_RETRIES * cppc_ss->latency;

This lets you completely eliminate all usage of ktime.  You can 
eliminate the global variable 'deadline' also, if you can figure out how 
to pass the cppc_ss object to check_pcc_chan().

> -	/* Wait for a nominal time to let platform process command. */
> -	udelay(cmd_latency);
> -
> -	/* Retry in case the remote processor was too slow to catch up. */
> -	for (retries = NUM_RETRIES; retries > 0; retries--) {
> -		if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
> -			result = 0;
> -			break;
> -		}
> -	}
> +	/*
> +	 * For READs we need to ensure the cmd completed to ensure
> +	 * the ensuing read()s can proceed. For WRITEs we dont care

"don't"

Ashwin Chaugule Feb. 10, 2016, 9:15 p.m. UTC | #2

On 10 February 2016 at 15:42, Timur Tabi <[e-mail]> wrote:
> Prashanth Prakash wrote:
>
>> +static int check_pcc_chan(void)
>> +{
>> +       int ret = -EIO;
>> +       struct acpi_pcct_shared_memory __iomem *generic_comm_base =
>> pcc_comm_addr;
>> +       ktime_t next_deadline = ktime_add(ktime_get(), deadline);
>> +
>> +       /* Retry in case the remote processor was too slow to catch up. */
>> +       while (!ktime_after(ktime_get(), next_deadline)) {
>> +               if (readw_relaxed(&generic_comm_base->status) &
>> PCC_CMD_COMPLETE) {
>> +                       ret = 0;
>> +                       break;
>> +               }
>> +               /*
>> +                * Reducing the bus traffic in case this loop takes longer
>> than
>> +                * a few retries.
>> +                */
>> +               udelay(3);
>> +       }
>
>
> Like I said last time, you really should use readq_relaxed_poll_timeout(),
> which does exactly the same thing as this loop, but more elegantly.

Nope.

>I think
> this will work:
>
> u32 status;
> ret = readq_relaxed_poll_timeout(&generic_comm_base->status, status, status
> & PCC_CMD_COMPLETE, 3, deadline);
> return ret ? -EIO : 0;
> ...
> deadline = NUM_RETRIES * cppc_ss->latency;
>
> This lets you completely eliminate all usage of ktime.

..Only to be re substituted by the macro all over again. So, there
really is no value in replacing this with a macro.  Also,
readx_relaxed_poll_timeout() has a usleep(), which will kill all the
optimization here. Its also horribly wrong in this context.

The alternative readx_poll_timeout_atomic() has a udelay() but it also
adds way more conditionals than this code. So, there is no need to
change things for the sake of cosmetic reasons.

> You can eliminate
> the global variable 'deadline' also, if you can figure out how to pass the
> cppc_ss object to check_pcc_chan().
>
>> -       /* Wait for a nominal time to let platform process command. */
>> -       udelay(cmd_latency);
>> -
>> -       /* Retry in case the remote processor was too slow to catch up. */
>> -       for (retries = NUM_RETRIES; retries > 0; retries--) {
>> -               if (readw_relaxed(&generic_comm_base->status) &
>> PCC_CMD_COMPLETE) {
>> -                       result = 0;
>> -                       break;
>> -               }
>> -       }
>> +       /*
>> +        * For READs we need to ensure the cmd completed to ensure
>> +        * the ensuing read()s can proceed. For WRITEs we dont care
>
>
> "don't"
>

Ashwin
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Timur Tabi Feb. 10, 2016, 9:57 p.m. UTC | #3

Ashwin Chaugule wrote:
> ..Only to be re substituted by the macro all over again. So, there
> really is no value in replacing this with a macro.  Also,
> readx_relaxed_poll_timeout() has a usleep(), which will kill all the
> optimization here. Its also horribly wrong in this context.
>
> The alternative readx_poll_timeout_atomic() has a udelay() but it also
> adds way more conditionals than this code. So, there is no need to
> change things for the sake of cosmetic reasons.

This is not a cosmetic change.  Calling a built-in macro instead of 
re-inventing the wheel is not cosmetic.  That's like saying that people 
shouldn't bother using min() or max() because they can just calculate 
the minimum and maximum themselves.  The macros exist for a reason, and 
people should be using them when applicable.

Also, the number of conditionals is not really much of an issue, I think.

The while loop has two conditionals:

1) The while-expression !ktime_after(ktime_get(), next_deadline)
2) The if-statement readw_relaxed(&generic_comm_base->status) & 
PCC_CMD_COMPLETE

readl_relaxed_poll_timeout_atomic() has four:

1) if (cond) break;
2) if (timeout_us && ktime_compare(ktime_get(), timeout) > 0)
3) if (delay_us) udelay(delay_us);	
4) (cond) ? 0 : -ETIMEDOUT; \

The third one is compiled-out because delay_us is a constant.  The 
fourth test is to handle the race condition between timeout and success. 
  If we time out, but the register value changes at the last 
microsecond, then we report success anyway.

And since this is a macro instead of a function, the code is generally 
optimized better.  The compiler typically can optimize macros better 
than functions, so there's still a chance that the macro version will be 
more optimized than the function anyway.

Finally, there's the convention of using a built-in macro/function over 
hand-coding something, even if it's not quite as optimized.  I don't 
think check_pcc_chan() is that time-critical that a single additional 
conditional is really a problem.  This while-loop really does re-invent 
the wheel.

Anyway, I don't want to belabor this point.  The decision is Rafael's now.

Ashwin Chaugule Feb. 10, 2016, 10:17 p.m. UTC | #4

On 10 February 2016 at 16:57, Timur Tabi <[e-mail]> wrote:

> Anyway, I don't want to belabor this point.  The decision is Rafael's now.

Thank you.
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Alexey Klimov Feb. 15, 2016, 4:37 p.m. UTC | #5

Hi Prashanth and Ashwin,

On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
> From: Ashwin Chaugule <[e-mail]>
> 
> Previously the send_pcc_cmd() code checked if the
> PCC operation had completed before returning from
> the function. This check was performed regardless
> of the PCC op type (i.e. Read/Write). Knowing
> the type of cmd can be used to optimize the check
> and avoid needless waiting. e.g. with Write ops,
> the actual Writing is done before calling send_pcc_cmd().
> And the subsequent Writes will check if the channel is
> free at the entry of send_pcc_cmd() anyway.
> 
> However, for Read cmds, we need to wait for the cmd
> completion bit to be flipped, since the actual Read
> ops follow after returning from the send_pcc_cmd(). So,
> only do the looping check at the end for Read ops.
> 
> Also, instead of using udelay() calls, use ktime as a
> means to check for deadlines. The current deadline
> in which the Remote should flip the cmd completion bit
> is defined as N * Nominal latency. Where N is arbitrary
> and large enough to work on slow emulators and Nominal
> latency comes from the ACPI table (PCCT). This helps
> in working around the CONFIG_HZ effects on udelay()
> and also avoids needing different ACPI tables for Silicon
> and Emulation platforms.
> 
> Signed-off-by: Ashwin Chaugule <[e-mail]>
> Signed-off-by: Prashanth Prakash <[e-mail]>
> ---
>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 71 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 6730f96..6ae327e 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -39,6 +39,7 @@
>  
>  #include <linux/cpufreq.h>
>  #include <linux/delay.h>
> +#include <linux/ktime.h>
>  
>  #include <acpi/cppc_acpi.h>
>  /*
> @@ -63,25 +64,53 @@ static struct mbox_chan *pcc_channel;
>  static void __iomem *pcc_comm_addr;
>  static u64 comm_base_addr;
>  static int pcc_subspace_idx = -1;
> -static u16 pcc_cmd_delay;
>  static bool pcc_channel_acquired;
> +static ktime_t deadline;
>  
>  /*
>   * Arbitrary Retries in case the remote processor is slow to respond
> - * to PCC commands.
> + * to PCC commands. Keeping it high enough to cover emulators where
> + * the processors run painfully slow.
>   */
>  #define NUM_RETRIES 500
>  
> +static int check_pcc_chan(void)
> +{
> +	int ret = -EIO;
> +	struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
> +	ktime_t next_deadline = ktime_add(ktime_get(), deadline);
> +
> +	/* Retry in case the remote processor was too slow to catch up. */
> +	while (!ktime_after(ktime_get(), next_deadline)) {
> +		if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {

What do you think about polling for error too?
Firmware might set error bit and kernel will spin here for full cycle (maybe even
more than one time).


> +			ret = 0;
> +			break;
> +		}
> +		/*
> +		 * Reducing the bus traffic in case this loop takes longer than
> +		 * a few retries.
> +		 */
> +		udelay(3);
> +	}
> +
> +	return ret;
> +}
> +
>  static int send_pcc_cmd(u16 cmd)
>  {
> -	int retries, result = -EIO;
> -	struct acpi_pcct_hw_reduced *pcct_ss = pcc_channel->con_priv;
> +	int ret = -EIO;
>  	struct acpi_pcct_shared_memory *generic_comm_base =
>  		(struct acpi_pcct_shared_memory *) pcc_comm_addr;
> -	u32 cmd_latency = pcct_ss->latency;
>  
> -	/* Min time OS should wait before sending next command. */
> -	udelay(pcc_cmd_delay);

Ouch.
This delay is Minimum Request Turnaround Time and specification is strict about this:
The minimum amount of time that OSPM must wait after the
completion of a command before issuing the next command, in microseconds.

And you're going to completely remove delay that described as "must wait" regardless if it's
write or read command. In my view completion of a command is when CMD complete bit is set to 1.
This field in table is not optional.

Even worse, what if platform report latency = 0 (or very close to zero) and turnaround time as very large number?

Well, my other comments here are minor things but this one looks like main issue of this patch.



> +	/*
> +	 * For CMD_WRITE we know for a fact the caller should have checked
> +	 * the channel before writing to PCC space
> +	 */
> +	if (cmd == CMD_READ) {
> +		ret = check_pcc_chan();
> +		if (ret)
> +			return ret;
> +	}
>  
>  	/* Write to the shared comm region. */
>  	writew(cmd, &generic_comm_base->command);
> @@ -90,26 +119,25 @@ static int send_pcc_cmd(u16 cmd)
>  	writew(0, &generic_comm_base->status);
>  
>  	/* Ring doorbell */
> -	result = mbox_send_message(pcc_channel, &cmd);
> -	if (result < 0) {
> +	ret = mbox_send_message(pcc_channel, &cmd);
> +	if (ret < 0) {
>  		pr_err("Err sending PCC mbox message. cmd:%d, ret:%d\n",
> -				cmd, result);
> -		return result;
> +				cmd, ret);
> +		return ret;
>  	}
>  
> -	/* Wait for a nominal time to let platform process command. */
> -	udelay(cmd_latency);
> -
> -	/* Retry in case the remote processor was too slow to catch up. */
> -	for (retries = NUM_RETRIES; retries > 0; retries--) {
> -		if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
> -			result = 0;
> -			break;
> -		}
> -	}
> +	/*
> +	 * For READs we need to ensure the cmd completed to ensure
> +	 * the ensuing read()s can proceed. For WRITEs we dont care
> +	 * because the actual write()s are done before coming here
> +	 * and the next READ or WRITE will check if the channel
> +	 * is busy/free at the entry of this call.
> +	 */
> +	if (cmd == CMD_READ)
> +		ret = check_pcc_chan();
>  
> -	mbox_client_txdone(pcc_channel, result);
> -	return result;
> +	mbox_client_txdone(pcc_channel, ret);
> +	return ret;
>  }
>  
>  static void cppc_chan_tx_done(struct mbox_client *cl, void *msg, int ret)
> @@ -306,6 +334,7 @@ static int register_pcc_channel(int pcc_subspace_idx)
>  {
>  	struct acpi_pcct_hw_reduced *cppc_ss;
>  	unsigned int len;
> +	u64 usecs_lat;
>  
>  	if (pcc_subspace_idx >= 0) {
>  		pcc_channel = pcc_mbox_request_channel(&cppc_mbox_cl,
> @@ -335,7 +364,14 @@ static int register_pcc_channel(int pcc_subspace_idx)
>  		 */
>  		comm_base_addr = cppc_ss->base_address;
>  		len = cppc_ss->length;
> -		pcc_cmd_delay = cppc_ss->min_turnaround_time;
> +
> +		/*
> +		 * cppc_ss->latency is just a Nominal value. In reality
> +		 * the remote processor could be much slower to reply.
> +		 * So add an arbitrary amount of wait on top of Nominal.

Specification requires platform to report accurate values.
If remote party requires arbitrary amount on top that means someone screwed up
values in firmware.
That's not only slowness of remote processor but broken/inaccurate values given by firmware.
I think if you want this comment you need to blame firmware too.


> +		 */
> +		usecs_lat = NUM_RETRIES * cppc_ss->latency;
> +		deadline = ns_to_ktime(usecs_lat * NSEC_PER_USEC);
>  
>  		pcc_comm_addr = acpi_os_ioremap(comm_base_addr, len);
>  		if (!pcc_comm_addr) {
> @@ -604,7 +640,7 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
>  			(ref_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
>  			(nom_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
>  		/* Ring doorbell once to update PCC subspace */
> -		if (send_pcc_cmd(CMD_READ)) {
> +		if (send_pcc_cmd(CMD_READ) < 0) {

Somewhere in previous email I asked you to point me to specs with words about init values before
issuing first PCC command. Thanks for that.
However I still think that comment should be added about init value of status register.
Please add it unless you have something to come up with. I think such comment will be useful.


>  			ret = -EIO;
>  			goto out_err;
>  		}
> @@ -662,7 +698,7 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
>  	if ((delivered_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
>  			(reference_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
>  		/* Ring doorbell once to update PCC subspace */
> -		if (send_pcc_cmd(CMD_READ)) {
> +		if (send_pcc_cmd(CMD_READ) < 0) {
>  			ret = -EIO;
>  			goto out_err;
>  		}
> @@ -713,6 +749,13 @@ int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
>  
>  	spin_lock(&pcc_lock);
>  
> +	/* If this is PCC reg, check if channel is free before writing */
> +	if (desired_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
> +		ret = check_pcc_chan();
> +		if (ret)
> +			goto busy_channel;
> +	}
> +
>  	/*
>  	 * Skip writing MIN/MAX until Linux knows how to come up with
>  	 * useful values.
> @@ -722,10 +765,10 @@ int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
>  	/* Is this a PCC reg ?*/
>  	if (desired_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
>  		/* Ring doorbell so Remote can get our perf request. */
> -		if (send_pcc_cmd(CMD_WRITE))
> +		if (send_pcc_cmd(CMD_WRITE) < 0)
>  			ret = -EIO;
>  	}
> -
> +busy_channel:
>  	spin_unlock(&pcc_lock);
>  
>  	return ret;

Best regards,
Alexey.
 
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Ashwin Chaugule Feb. 16, 2016, 6:47 p.m. UTC | #6

Hi Alexey,

On 15 February 2016 at 11:37, Alexey Klimov <[e-mail]> wrote:
> Hi Prashanth and Ashwin,
>
> On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
>> From: Ashwin Chaugule <[e-mail]>
>>
>> Previously the send_pcc_cmd() code checked if the
>> PCC operation had completed before returning from
>> the function. This check was performed regardless
>> of the PCC op type (i.e. Read/Write). Knowing
>> the type of cmd can be used to optimize the check
>> and avoid needless waiting. e.g. with Write ops,
>> the actual Writing is done before calling send_pcc_cmd().
>> And the subsequent Writes will check if the channel is
>> free at the entry of send_pcc_cmd() anyway.
>>
>> However, for Read cmds, we need to wait for the cmd
>> completion bit to be flipped, since the actual Read
>> ops follow after returning from the send_pcc_cmd(). So,
>> only do the looping check at the end for Read ops.
>>
>> Also, instead of using udelay() calls, use ktime as a
>> means to check for deadlines. The current deadline
>> in which the Remote should flip the cmd completion bit
>> is defined as N * Nominal latency. Where N is arbitrary
>> and large enough to work on slow emulators and Nominal
>> latency comes from the ACPI table (PCCT). This helps
>> in working around the CONFIG_HZ effects on udelay()
>> and also avoids needing different ACPI tables for Silicon
>> and Emulation platforms.
>>
>> Signed-off-by: Ashwin Chaugule <[e-mail]>
>> Signed-off-by: Prashanth Prakash <[e-mail]>
>> ---
>>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
>>  1 file changed, 71 insertions(+), 28 deletions(-)

[..]

>>
>>  /*
>>   * Arbitrary Retries in case the remote processor is slow to respond
>> - * to PCC commands.
>> + * to PCC commands. Keeping it high enough to cover emulators where
>> + * the processors run painfully slow.
>>   */
>>  #define NUM_RETRIES 500
>>
>> +static int check_pcc_chan(void)
>> +{
>> +     int ret = -EIO;
>> +     struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
>> +     ktime_t next_deadline = ktime_add(ktime_get(), deadline);
>> +
>> +     /* Retry in case the remote processor was too slow to catch up. */
>> +     while (!ktime_after(ktime_get(), next_deadline)) {
>> +             if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
>
> What do you think about polling for error too?
> Firmware might set error bit and kernel will spin here for full cycle (maybe even
> more than one time).
>

Hm. I dont think thats useful since we dont do anything special if the
firmware errors out. Perhaps at some later point we can stop sending
new requests if there are firmware errors, but then how would we know
when to resume?

>
>> +                     ret = 0;
>> +                     break;
>> +             }
>> +             /*
>> +              * Reducing the bus traffic in case this loop takes longer than
>> +              * a few retries.
>> +              */
>> +             udelay(3);
>> +     }
>> +
>> +     return ret;
>> +}
>> +
>>  static int send_pcc_cmd(u16 cmd)
>>  {
>> -     int retries, result = -EIO;
>> -     struct acpi_pcct_hw_reduced *pcct_ss = pcc_channel->con_priv;
>> +     int ret = -EIO;
>>       struct acpi_pcct_shared_memory *generic_comm_base =
>>               (struct acpi_pcct_shared_memory *) pcc_comm_addr;
>> -     u32 cmd_latency = pcct_ss->latency;
>>
>> -     /* Min time OS should wait before sending next command. */
>> -     udelay(pcc_cmd_delay);
>
> Ouch.
> This delay is Minimum Request Turnaround Time and specification is strict about this:
> The minimum amount of time that OSPM must wait after the
> completion of a command before issuing the next command, in microseconds.
>
> And you're going to completely remove delay that described as "must wait" regardless if it's
> write or read command. In my view completion of a command is when CMD complete bit is set to 1.
> This field in table is not optional.

The spec has a whole bunch of delays which are highly questionable.
e.g. the Min Request Turnaround time and the Max periodic access rate.
Both seem quite useless since their original intention was to avoid
the OS from flooding the BMC. But one can argue that the firmware can
throttle the OS requests via the Command Completion bit anyway. I also
didnt like the usage of udelays and prefer the ktime math, but that
seems overkill given the questionable value of these fields.

>
> Even worse, what if platform report latency = 0 (or very close to zero) and turnaround time as very large number?
>

Such a firmware would need serious rework. Besides, if for whatever
reason they choose to have it, the command complete bit can be used to
throttle the OS requests.

>> +             /*
>> +              * cppc_ss->latency is just a Nominal value. In reality
>> +              * the remote processor could be much slower to reply.
>> +              * So add an arbitrary amount of wait on top of Nominal.
>
> Specification requires platform to report accurate values.
> If remote party requires arbitrary amount on top that means someone screwed up
> values in firmware.
> That's not only slowness of remote processor but broken/inaccurate values given by firmware.
> I think if you want this comment you need to blame firmware too.
>

Not necessarily. The firmware could have other threads going on, which
sporadically cause additional ( > Nominal) latencies and the OS must
honor that. Its only a "Nominal" value after all.

>
>> +              */
>> +             usecs_lat = NUM_RETRIES * cppc_ss->latency;
>> +             deadline = ns_to_ktime(usecs_lat * NSEC_PER_USEC);
>>
>>               pcc_comm_addr = acpi_os_ioremap(comm_base_addr, len);
>>               if (!pcc_comm_addr) {
>> @@ -604,7 +640,7 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
>>                       (ref_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
>>                       (nom_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
>>               /* Ring doorbell once to update PCC subspace */
>> -             if (send_pcc_cmd(CMD_READ)) {
>> +             if (send_pcc_cmd(CMD_READ) < 0) {
>
> Somewhere in previous email I asked you to point me to specs with words about init values before
> issuing first PCC command. Thanks for that.
> However I still think that comment should be added about init value of status register.
> Please add it unless you have something to come up with. I think such comment will be useful.
>
>

Ok. We'll add it if we need to revise this patchset.


Thanks,
Ashwin.
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Rafael J. Wysocki Feb. 16, 2016, 7:10 p.m. UTC | #7

On Tue, Feb 16, 2016 at 7:47 PM, Ashwin Chaugule
<[e-mail]> wrote:
> Hi Alexey,
>
> On 15 February 2016 at 11:37, Alexey Klimov <[e-mail]> wrote:
>> Hi Prashanth and Ashwin,
>>
>> On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
>>> From: Ashwin Chaugule <[e-mail]>
>>>
>>> Previously the send_pcc_cmd() code checked if the
>>> PCC operation had completed before returning from
>>> the function. This check was performed regardless
>>> of the PCC op type (i.e. Read/Write). Knowing
>>> the type of cmd can be used to optimize the check
>>> and avoid needless waiting. e.g. with Write ops,
>>> the actual Writing is done before calling send_pcc_cmd().
>>> And the subsequent Writes will check if the channel is
>>> free at the entry of send_pcc_cmd() anyway.
>>>
>>> However, for Read cmds, we need to wait for the cmd
>>> completion bit to be flipped, since the actual Read
>>> ops follow after returning from the send_pcc_cmd(). So,
>>> only do the looping check at the end for Read ops.
>>>
>>> Also, instead of using udelay() calls, use ktime as a
>>> means to check for deadlines. The current deadline
>>> in which the Remote should flip the cmd completion bit
>>> is defined as N * Nominal latency. Where N is arbitrary
>>> and large enough to work on slow emulators and Nominal
>>> latency comes from the ACPI table (PCCT). This helps
>>> in working around the CONFIG_HZ effects on udelay()
>>> and also avoids needing different ACPI tables for Silicon
>>> and Emulation platforms.
>>>
>>> Signed-off-by: Ashwin Chaugule <[e-mail]>
>>> Signed-off-by: Prashanth Prakash <[e-mail]>
>>> ---
>>>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
>>>  1 file changed, 71 insertions(+), 28 deletions(-)
>
> [..]
>
>>>
>>>  /*
>>>   * Arbitrary Retries in case the remote processor is slow to respond
>>> - * to PCC commands.
>>> + * to PCC commands. Keeping it high enough to cover emulators where
>>> + * the processors run painfully slow.
>>>   */
>>>  #define NUM_RETRIES 500
>>>
>>> +static int check_pcc_chan(void)
>>> +{
>>> +     int ret = -EIO;
>>> +     struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
>>> +     ktime_t next_deadline = ktime_add(ktime_get(), deadline);
>>> +
>>> +     /* Retry in case the remote processor was too slow to catch up. */
>>> +     while (!ktime_after(ktime_get(), next_deadline)) {
>>> +             if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
>>
>> What do you think about polling for error too?
>> Firmware might set error bit and kernel will spin here for full cycle (maybe even
>> more than one time).
>>
>
> Hm. I dont think thats useful since we dont do anything special if the
> firmware errors out. Perhaps at some later point we can stop sending
> new requests if there are firmware errors, but then how would we know
> when to resume?
>
>>
>>> +                     ret = 0;
>>> +                     break;
>>> +             }
>>> +             /*
>>> +              * Reducing the bus traffic in case this loop takes longer than
>>> +              * a few retries.
>>> +              */
>>> +             udelay(3);
>>> +     }
>>> +
>>> +     return ret;
>>> +}
>>> +
>>>  static int send_pcc_cmd(u16 cmd)
>>>  {
>>> -     int retries, result = -EIO;
>>> -     struct acpi_pcct_hw_reduced *pcct_ss = pcc_channel->con_priv;
>>> +     int ret = -EIO;
>>>       struct acpi_pcct_shared_memory *generic_comm_base =
>>>               (struct acpi_pcct_shared_memory *) pcc_comm_addr;
>>> -     u32 cmd_latency = pcct_ss->latency;
>>>
>>> -     /* Min time OS should wait before sending next command. */
>>> -     udelay(pcc_cmd_delay);
>>
>> Ouch.
>> This delay is Minimum Request Turnaround Time and specification is strict about this:
>> The minimum amount of time that OSPM must wait after the
>> completion of a command before issuing the next command, in microseconds.
>>
>> And you're going to completely remove delay that described as "must wait" regardless if it's
>> write or read command. In my view completion of a command is when CMD complete bit is set to 1.
>> This field in table is not optional.
>
> The spec has a whole bunch of delays which are highly questionable.

Why does your opinion matter here if I may ask?

If everybody follows this line of reasoning, there's no point in
having any specifications in the first place.

If you don't like the spec, please submit requests to modify it.  Or
follow it otherwise.

> e.g. the Min Request Turnaround time and the Max periodic access rate.
> Both seem quite useless since their original intention was to avoid
> the OS from flooding the BMC. But one can argue that the firmware can
> throttle the OS requests via the Command Completion bit anyway.

Yes, they can.  What does that have to do with the other fields?

> I also didnt like the usage of udelays and prefer the ktime math, but that
> seems overkill given the questionable value of these fields.
>
>>
>> Even worse, what if platform report latency = 0 (or very close to zero) and turnaround time as very large number?
>>
>
> Such a firmware would need serious rework. Besides, if for whatever
> reason they choose to have it, the command complete bit can be used to
> throttle the OS requests.

Yes, it can.  Is that a good enough argument for not respecting the
spec on the OS side?  I don't think so.

Thanks,
Rafael
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Ashwin Chaugule Feb. 16, 2016, 7:33 p.m. UTC | #8

Hi Rafael,

On 16 February 2016 at 14:10, Rafael J. Wysocki <[e-mail]> wrote:
> On Tue, Feb 16, 2016 at 7:47 PM, Ashwin Chaugule
> <[e-mail]> wrote:
>> Hi Alexey,
>>
>> On 15 February 2016 at 11:37, Alexey Klimov <[e-mail]> wrote:
>>> Hi Prashanth and Ashwin,
>>>
>>> On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
>>>> From: Ashwin Chaugule <[e-mail]>
>>>>
>>>> Previously the send_pcc_cmd() code checked if the
>>>> PCC operation had completed before returning from
>>>> the function. This check was performed regardless
>>>> of the PCC op type (i.e. Read/Write). Knowing
>>>> the type of cmd can be used to optimize the check
>>>> and avoid needless waiting. e.g. with Write ops,
>>>> the actual Writing is done before calling send_pcc_cmd().
>>>> And the subsequent Writes will check if the channel is
>>>> free at the entry of send_pcc_cmd() anyway.
>>>>
>>>> However, for Read cmds, we need to wait for the cmd
>>>> completion bit to be flipped, since the actual Read
>>>> ops follow after returning from the send_pcc_cmd(). So,
>>>> only do the looping check at the end for Read ops.
>>>>
>>>> Also, instead of using udelay() calls, use ktime as a
>>>> means to check for deadlines. The current deadline
>>>> in which the Remote should flip the cmd completion bit
>>>> is defined as N * Nominal latency. Where N is arbitrary
>>>> and large enough to work on slow emulators and Nominal
>>>> latency comes from the ACPI table (PCCT). This helps
>>>> in working around the CONFIG_HZ effects on udelay()
>>>> and also avoids needing different ACPI tables for Silicon
>>>> and Emulation platforms.
>>>>
>>>> Signed-off-by: Ashwin Chaugule <[e-mail]>
>>>> Signed-off-by: Prashanth Prakash <[e-mail]>
>>>> ---
>>>>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
>>>>  1 file changed, 71 insertions(+), 28 deletions(-)
>>
>> [..]
>>
>>>> -     u32 cmd_latency = pcct_ss->latency;
>>>>
>>>> -     /* Min time OS should wait before sending next command. */
>>>> -     udelay(pcc_cmd_delay);
>>>
>>> Ouch.
>>> This delay is Minimum Request Turnaround Time and specification is strict about this:
>>> The minimum amount of time that OSPM must wait after the
>>> completion of a command before issuing the next command, in microseconds.
>>>
>>> And you're going to completely remove delay that described as "must wait" regardless if it's
>>> write or read command. In my view completion of a command is when CMD complete bit is set to 1.
>>> This field in table is not optional.
>>
>> The spec has a whole bunch of delays which are highly questionable.
>
> Why does your opinion matter here if I may ask?
>

We asked the folks who added those delays to the spec before making
this change. So this is not unfounded.

> If everybody follows this line of reasoning, there's no point in
> having any specifications in the first place.
>
> If you don't like the spec, please submit requests to modify it.  Or
> follow it otherwise.

But I agree that for the sake of spec correctness we should include
this delay and let firmware choose what value to put in it and deal
with the latencies for each request.

>
>> e.g. the Min Request Turnaround time and the Max periodic access rate.
>> Both seem quite useless since their original intention was to avoid
>> the OS from flooding the BMC. But one can argue that the firmware can
>> throttle the OS requests via the Command Completion bit anyway.
>
> Yes, they can.  What does that have to do with the other fields?

If the intention was only to control the OS request rate, then why
isnt Command Completion sufficient for that purpose? The firmware can
flip the command complete bit only when it is ready to receive the
next request.

>
>> I also didnt like the usage of udelays and prefer the ktime math, but that
>> seems overkill given the questionable value of these fields.
>>
>>>
>>> Even worse, what if platform report latency = 0 (or very close to zero) and turnaround time as very large number?
>>>
>>
>> Such a firmware would need serious rework. Besides, if for whatever
>> reason they choose to have it, the command complete bit can be used to
>> throttle the OS requests.
>
> Yes, it can.  Is that a good enough argument for not respecting the
> spec on the OS side?  I don't think so.

In principle I agree, but with CPPC, these fields seemed redundant,
hence the patch. But I'm not strongly opposed either way, so we'll add
the latency field back and leave it up to the platform and vendors.

Thanks,
Ashwin.
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Rafael J. Wysocki Feb. 16, 2016, 7:39 p.m. UTC | #9

On Tuesday, February 16, 2016 02:33:00 PM Ashwin Chaugule wrote:
> Hi Rafael,
> 
> On 16 February 2016 at 14:10, Rafael J. Wysocki <[e-mail]> wrote:
> > On Tue, Feb 16, 2016 at 7:47 PM, Ashwin Chaugule
> > <[e-mail]> wrote:
> >> Hi Alexey,
> >>
> >> On 15 February 2016 at 11:37, Alexey Klimov <[e-mail]> wrote:
> >>> Hi Prashanth and Ashwin,
> >>>
> >>> On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
> >>>> From: Ashwin Chaugule <[e-mail]>
> >>>>
> >>>> Previously the send_pcc_cmd() code checked if the
> >>>> PCC operation had completed before returning from
> >>>> the function. This check was performed regardless
> >>>> of the PCC op type (i.e. Read/Write). Knowing
> >>>> the type of cmd can be used to optimize the check
> >>>> and avoid needless waiting. e.g. with Write ops,
> >>>> the actual Writing is done before calling send_pcc_cmd().
> >>>> And the subsequent Writes will check if the channel is
> >>>> free at the entry of send_pcc_cmd() anyway.
> >>>>
> >>>> However, for Read cmds, we need to wait for the cmd
> >>>> completion bit to be flipped, since the actual Read
> >>>> ops follow after returning from the send_pcc_cmd(). So,
> >>>> only do the looping check at the end for Read ops.
> >>>>
> >>>> Also, instead of using udelay() calls, use ktime as a
> >>>> means to check for deadlines. The current deadline
> >>>> in which the Remote should flip the cmd completion bit
> >>>> is defined as N * Nominal latency. Where N is arbitrary
> >>>> and large enough to work on slow emulators and Nominal
> >>>> latency comes from the ACPI table (PCCT). This helps
> >>>> in working around the CONFIG_HZ effects on udelay()
> >>>> and also avoids needing different ACPI tables for Silicon
> >>>> and Emulation platforms.
> >>>>
> >>>> Signed-off-by: Ashwin Chaugule <[e-mail]>
> >>>> Signed-off-by: Prashanth Prakash <[e-mail]>
> >>>> ---
> >>>>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
> >>>>  1 file changed, 71 insertions(+), 28 deletions(-)
> >>
> >> [..]
> >>
> >>>> -     u32 cmd_latency = pcct_ss->latency;
> >>>>
> >>>> -     /* Min time OS should wait before sending next command. */
> >>>> -     udelay(pcc_cmd_delay);
> >>>
> >>> Ouch.
> >>> This delay is Minimum Request Turnaround Time and specification is strict about this:
> >>> The minimum amount of time that OSPM must wait after the
> >>> completion of a command before issuing the next command, in microseconds.
> >>>
> >>> And you're going to completely remove delay that described as "must wait" regardless if it's
> >>> write or read command. In my view completion of a command is when CMD complete bit is set to 1.
> >>> This field in table is not optional.
> >>
> >> The spec has a whole bunch of delays which are highly questionable.
> >
> > Why does your opinion matter here if I may ask?
> >
> 
> We asked the folks who added those delays to the spec before making
> this change. So this is not unfounded.

That's fine, but then the authors of the firmware your code has to deal
with may not have that conversation and may expect certain type of behavior
from it (as indicated by the spec).

> > If everybody follows this line of reasoning, there's no point in
> > having any specifications in the first place.
> >
> > If you don't like the spec, please submit requests to modify it.  Or
> > follow it otherwise.
> 
> But I agree that for the sake of spec correctness we should include
> this delay and let firmware choose what value to put in it and deal
> with the latencies for each request.
> 
> >
> >> e.g. the Min Request Turnaround time and the Max periodic access rate.
> >> Both seem quite useless since their original intention was to avoid
> >> the OS from flooding the BMC. But one can argue that the firmware can
> >> throttle the OS requests via the Command Completion bit anyway.
> >
> > Yes, they can.  What does that have to do with the other fields?
> 
> If the intention was only to control the OS request rate, then why
> isnt Command Completion sufficient for that purpose? The firmware can
> flip the command complete bit only when it is ready to receive the
> next request.
> 
> >
> >> I also didnt like the usage of udelays and prefer the ktime math, but that
> >> seems overkill given the questionable value of these fields.
> >>
> >>>
> >>> Even worse, what if platform report latency = 0 (or very close to zero) and turnaround time as very large number?
> >>>
> >>
> >> Such a firmware would need serious rework. Besides, if for whatever
> >> reason they choose to have it, the command complete bit can be used to
> >> throttle the OS requests.
> >
> > Yes, it can.  Is that a good enough argument for not respecting the
> > spec on the OS side?  I don't think so.
> 
> In principle I agree, but with CPPC, these fields seemed redundant,
> hence the patch. But I'm not strongly opposed either way, so we'll add
> the latency field back and leave it up to the platform and vendors.

Sounds good.

Thanks,
Rafael

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Alexey Klimov Feb. 29, 2016, 5:39 p.m. UTC | #10

On Tue, Feb 16, 2016 at 01:47:15PM -0500, Ashwin Chaugule wrote:
> Hi Alexey,
> 
> On 15 February 2016 at 11:37, Alexey Klimov <[e-mail]> wrote:
> > Hi Prashanth and Ashwin,
> >
> > On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
> >> From: Ashwin Chaugule <[e-mail]>
> >>
> >> Previously the send_pcc_cmd() code checked if the
> >> PCC operation had completed before returning from
> >> the function. This check was performed regardless
> >> of the PCC op type (i.e. Read/Write). Knowing
> >> the type of cmd can be used to optimize the check
> >> and avoid needless waiting. e.g. with Write ops,
> >> the actual Writing is done before calling send_pcc_cmd().
> >> And the subsequent Writes will check if the channel is
> >> free at the entry of send_pcc_cmd() anyway.
> >>
> >> However, for Read cmds, we need to wait for the cmd
> >> completion bit to be flipped, since the actual Read
> >> ops follow after returning from the send_pcc_cmd(). So,
> >> only do the looping check at the end for Read ops.
> >>
> >> Also, instead of using udelay() calls, use ktime as a
> >> means to check for deadlines. The current deadline
> >> in which the Remote should flip the cmd completion bit
> >> is defined as N * Nominal latency. Where N is arbitrary
> >> and large enough to work on slow emulators and Nominal
> >> latency comes from the ACPI table (PCCT). This helps
> >> in working around the CONFIG_HZ effects on udelay()
> >> and also avoids needing different ACPI tables for Silicon
> >> and Emulation platforms.
> >>
> >> Signed-off-by: Ashwin Chaugule <[e-mail]>
> >> Signed-off-by: Prashanth Prakash <[e-mail]>
> >> ---
> >>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
> >>  1 file changed, 71 insertions(+), 28 deletions(-)
> 
> [..]
> 
> >>
> >>  /*
> >>   * Arbitrary Retries in case the remote processor is slow to respond
> >> - * to PCC commands.
> >> + * to PCC commands. Keeping it high enough to cover emulators where
> >> + * the processors run painfully slow.
> >>   */
> >>  #define NUM_RETRIES 500
> >>
> >> +static int check_pcc_chan(void)
> >> +{
> >> +     int ret = -EIO;
> >> +     struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
> >> +     ktime_t next_deadline = ktime_add(ktime_get(), deadline);
> >> +
> >> +     /* Retry in case the remote processor was too slow to catch up. */
> >> +     while (!ktime_after(ktime_get(), next_deadline)) {
> >> +             if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
> >
> > What do you think about polling for error too?
> > Firmware might set error bit and kernel will spin here for full cycle (maybe even
> > more than one time).
> >
> 
> Hm. I dont think thats useful since we dont do anything special if the
> firmware errors out. Perhaps at some later point we can stop sending
> new requests if there are firmware errors, but then how would we know
> when to resume?

The suggestion is to jump out of polling loop quickly instead of spinning for full cycle.
With longer delays given by firmware the spinning issue on error will be worse.
This is not going to change behaviour on error paths (or in other words I don't suggest
to change error path behaviour right now).

Does specification describe any resume after error path? I don't see anything.

<snip>

Best regards,
Alexey Klimov.

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Prakash, Prashanth Feb. 29, 2016, 7:20 p.m. UTC | #11

Hi Alexey,

On 2/29/2016 10:39 AM, Alexey Klimov wrote:
> On Tue, Feb 16, 2016 at 01:47:15PM -0500, Ashwin Chaugule wrote:
>> Hi Alexey,
>>
>> On 15 February 2016 at 11:37, Alexey Klimov <[e-mail]> wrote:
>>> Hi Prashanth and Ashwin,
>>>
>>> On Wed, Feb 10, 2016 at 01:05:59PM -0700, Prashanth Prakash wrote:
>>>> From: Ashwin Chaugule <[e-mail]>
>>>>
>>>> Previously the send_pcc_cmd() code checked if the
>>>> PCC operation had completed before returning from
>>>> the function. This check was performed regardless
>>>> of the PCC op type (i.e. Read/Write). Knowing
>>>> the type of cmd can be used to optimize the check
>>>> and avoid needless waiting. e.g. with Write ops,
>>>> the actual Writing is done before calling send_pcc_cmd().
>>>> And the subsequent Writes will check if the channel is
>>>> free at the entry of send_pcc_cmd() anyway.
>>>>
>>>> However, for Read cmds, we need to wait for the cmd
>>>> completion bit to be flipped, since the actual Read
>>>> ops follow after returning from the send_pcc_cmd(). So,
>>>> only do the looping check at the end for Read ops.
>>>>
>>>> Also, instead of using udelay() calls, use ktime as a
>>>> means to check for deadlines. The current deadline
>>>> in which the Remote should flip the cmd completion bit
>>>> is defined as N * Nominal latency. Where N is arbitrary
>>>> and large enough to work on slow emulators and Nominal
>>>> latency comes from the ACPI table (PCCT). This helps
>>>> in working around the CONFIG_HZ effects on udelay()
>>>> and also avoids needing different ACPI tables for Silicon
>>>> and Emulation platforms.
>>>>
>>>> Signed-off-by: Ashwin Chaugule <[e-mail]>
>>>> Signed-off-by: Prashanth Prakash <[e-mail]>
>>>> ---
>>>>  drivers/acpi/cppc_acpi.c | 99 ++++++++++++++++++++++++++++++++++--------------
>>>>  1 file changed, 71 insertions(+), 28 deletions(-)
>> [..]
>>
>>>>  /*
>>>>   * Arbitrary Retries in case the remote processor is slow to respond
>>>> - * to PCC commands.
>>>> + * to PCC commands. Keeping it high enough to cover emulators where
>>>> + * the processors run painfully slow.
>>>>   */
>>>>  #define NUM_RETRIES 500
>>>>
>>>> +static int check_pcc_chan(void)
>>>> +{
>>>> +     int ret = -EIO;
>>>> +     struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
>>>> +     ktime_t next_deadline = ktime_add(ktime_get(), deadline);
>>>> +
>>>> +     /* Retry in case the remote processor was too slow to catch up. */
>>>> +     while (!ktime_after(ktime_get(), next_deadline)) {
>>>> +             if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
>>> What do you think about polling for error too?
>>> Firmware might set error bit and kernel will spin here for full cycle (maybe even
>>> more than one time).
>>>
>> Hm. I dont think thats useful since we dont do anything special if the
>> firmware errors out. Perhaps at some later point we can stop sending
>> new requests if there are firmware errors, but then how would we know
>> when to resume?
> The suggestion is to jump out of polling loop quickly instead of spinning for full cycle.
> With longer delays given by firmware the spinning issue on error will be worse.
> This is not going to change behaviour on error paths (or in other words I don't suggest
> to change error path behaviour right now).
>
> Does specification describe any resume after error path? I don't see anything.
The specification doesn't really say how to handle the error. While it doesn't say resume after
error, it  also doesn't say abort all commands after an error.
The only thing I see with respect to error bit is "If set, an error occurred executing the last
command."

So, all we know is there was a an error with the execution of the last command and I am not
sure it is correct to assume all the future usage of the channel should be aborted because the
failure of the last command.

Given the above ambiguity, I think it is better to keep the current implementation as is. If
some platforms shows a behavior where after an error the channel becomes completely
useless, then we can maintain some sort of internal state that decides not to use the channel
after say X number of consecutive errors.
> <snip>
>
> Best regards,
> Alexey Klimov.
>
>
>


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Patch

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 6730f96..6ae327e 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -39,6 +39,7 @@ 
 
 #include <linux/cpufreq.h>
 #include <linux/delay.h>
+#include <linux/ktime.h>
 
 #include <acpi/cppc_acpi.h>
 /*
@@ -63,25 +64,53 @@  static struct mbox_chan *pcc_channel;
 static void __iomem *pcc_comm_addr;
 static u64 comm_base_addr;
 static int pcc_subspace_idx = -1;
-static u16 pcc_cmd_delay;
 static bool pcc_channel_acquired;
+static ktime_t deadline;
 
 /*
  * Arbitrary Retries in case the remote processor is slow to respond
- * to PCC commands.
+ * to PCC commands. Keeping it high enough to cover emulators where
+ * the processors run painfully slow.
  */
 #define NUM_RETRIES 500
 
+static int check_pcc_chan(void)
+{
+	int ret = -EIO;
+	struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
+	ktime_t next_deadline = ktime_add(ktime_get(), deadline);
+
+	/* Retry in case the remote processor was too slow to catch up. */
+	while (!ktime_after(ktime_get(), next_deadline)) {
+		if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
+			ret = 0;
+			break;
+		}
+		/*
+		 * Reducing the bus traffic in case this loop takes longer than
+		 * a few retries.
+		 */
+		udelay(3);
+	}
+
+	return ret;
+}
+
 static int send_pcc_cmd(u16 cmd)
 {
-	int retries, result = -EIO;
-	struct acpi_pcct_hw_reduced *pcct_ss = pcc_channel->con_priv;
+	int ret = -EIO;
 	struct acpi_pcct_shared_memory *generic_comm_base =
 		(struct acpi_pcct_shared_memory *) pcc_comm_addr;
-	u32 cmd_latency = pcct_ss->latency;
 
-	/* Min time OS should wait before sending next command. */
-	udelay(pcc_cmd_delay);
+	/*
+	 * For CMD_WRITE we know for a fact the caller should have checked
+	 * the channel before writing to PCC space
+	 */
+	if (cmd == CMD_READ) {
+		ret = check_pcc_chan();
+		if (ret)
+			return ret;
+	}
 
 	/* Write to the shared comm region. */
 	writew(cmd, &generic_comm_base->command);
@@ -90,26 +119,25 @@  static int send_pcc_cmd(u16 cmd)
 	writew(0, &generic_comm_base->status);
 
 	/* Ring doorbell */
-	result = mbox_send_message(pcc_channel, &cmd);
-	if (result < 0) {
+	ret = mbox_send_message(pcc_channel, &cmd);
+	if (ret < 0) {
 		pr_err("Err sending PCC mbox message. cmd:%d, ret:%d\n",
-				cmd, result);
-		return result;
+				cmd, ret);
+		return ret;
 	}
 
-	/* Wait for a nominal time to let platform process command. */
-	udelay(cmd_latency);
-
-	/* Retry in case the remote processor was too slow to catch up. */
-	for (retries = NUM_RETRIES; retries > 0; retries--) {
-		if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
-			result = 0;
-			break;
-		}
-	}
+	/*
+	 * For READs we need to ensure the cmd completed to ensure
+	 * the ensuing read()s can proceed. For WRITEs we dont care
+	 * because the actual write()s are done before coming here
+	 * and the next READ or WRITE will check if the channel
+	 * is busy/free at the entry of this call.
+	 */
+	if (cmd == CMD_READ)
+		ret = check_pcc_chan();
 
-	mbox_client_txdone(pcc_channel, result);
-	return result;
+	mbox_client_txdone(pcc_channel, ret);
+	return ret;
 }
 
 static void cppc_chan_tx_done(struct mbox_client *cl, void *msg, int ret)
@@ -306,6 +334,7 @@  static int register_pcc_channel(int pcc_subspace_idx)
 {
 	struct acpi_pcct_hw_reduced *cppc_ss;
 	unsigned int len;
+	u64 usecs_lat;
 
 	if (pcc_subspace_idx >= 0) {
 		pcc_channel = pcc_mbox_request_channel(&cppc_mbox_cl,
@@ -335,7 +364,14 @@  static int register_pcc_channel(int pcc_subspace_idx)
 		 */
 		comm_base_addr = cppc_ss->base_address;
 		len = cppc_ss->length;
-		pcc_cmd_delay = cppc_ss->min_turnaround_time;
+
+		/*
+		 * cppc_ss->latency is just a Nominal value. In reality
+		 * the remote processor could be much slower to reply.
+		 * So add an arbitrary amount of wait on top of Nominal.
+		 */
+		usecs_lat = NUM_RETRIES * cppc_ss->latency;
+		deadline = ns_to_ktime(usecs_lat * NSEC_PER_USEC);
 
 		pcc_comm_addr = acpi_os_ioremap(comm_base_addr, len);
 		if (!pcc_comm_addr) {
@@ -604,7 +640,7 @@  int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
 			(ref_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
 			(nom_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
 		/* Ring doorbell once to update PCC subspace */
-		if (send_pcc_cmd(CMD_READ)) {
+		if (send_pcc_cmd(CMD_READ) < 0) {
 			ret = -EIO;
 			goto out_err;
 		}
@@ -662,7 +698,7 @@  int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
 	if ((delivered_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
 			(reference_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
 		/* Ring doorbell once to update PCC subspace */
-		if (send_pcc_cmd(CMD_READ)) {
+		if (send_pcc_cmd(CMD_READ) < 0) {
 			ret = -EIO;
 			goto out_err;
 		}
@@ -713,6 +749,13 @@  int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
 
 	spin_lock(&pcc_lock);
 
+	/* If this is PCC reg, check if channel is free before writing */
+	if (desired_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
+		ret = check_pcc_chan();
+		if (ret)
+			goto busy_channel;
+	}
+
 	/*
 	 * Skip writing MIN/MAX until Linux knows how to come up with
 	 * useful values.
@@ -722,10 +765,10 @@  int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
 	/* Is this a PCC reg ?*/
 	if (desired_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
 		/* Ring doorbell so Remote can get our perf request. */
-		if (send_pcc_cmd(CMD_WRITE))
+		if (send_pcc_cmd(CMD_WRITE) < 0)
 			ret = -EIO;
 	}
-
+busy_channel:
 	spin_unlock(&pcc_lock);
 
 	return ret;