[V4,1/2] clk: bcm2835: mark enabled clocks with CLK_IS_CRITICAL

Commit Message

Martin Sperl April 29, 2016, 5:42 p.m. UTC

From: Martin Sperl <[e-mail]>

The bcm2835 firmware enables several clocks and plls before
booting the linux kernel.

These plls should never get disabled as it may result in a
stopped system clock and more.

So during probing we check if the clock is enabled
and if it is then mark that clock with CLK_IS_CRITICAL.

As a consequence this will also enable the corresponding
parent plls and pll-divs.

This is intended as a stop-gap until CLK_ENABLE_HAND_OFF
becomes available, at which point it should be used instead.

Signed-off-by: Martin Sperl <[e-mail]>
---
 drivers/clk/bcm/clk-bcm2835.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Eric Anholt May 2, 2016, 3:36 p.m. UTC | #1

[e-mail] writes:

> From: Martin Sperl <[e-mail]>
>
> The bcm2835 firmware enables several clocks and plls before
> booting the linux kernel.
>
> These plls should never get disabled as it may result in a
> stopped system clock and more.
>
> So during probing we check if the clock is enabled
> and if it is then mark that clock with CLK_IS_CRITICAL.
>
> As a consequence this will also enable the corresponding
> parent plls and pll-divs.
>
> This is intended as a stop-gap until CLK_ENABLE_HAND_OFF
> becomes available, at which point it should be used instead.
>
> Signed-off-by: Martin Sperl <[e-mail]>

I still think that we don't want this patch.  We should be able to
disable clocks that the firmware turned on, unless they really need to
stay on.  If you have troubles on the upstream DT, let's talk about
individual clocks.

Martin Sperl May 2, 2016, 4:16 p.m. UTC | #2

> On 02.05.2016, at 17:36, Eric Anholt <[e-mail]> wrote:
> 
> [e-mail] writes:
> 
>> From: Martin Sperl <[e-mail]>
>> 
>> The bcm2835 firmware enables several clocks and plls before
>> booting the linux kernel.
>> 
>> These plls should never get disabled as it may result in a
>> stopped system clock and more.
>> 
>> So during probing we check if the clock is enabled
>> and if it is then mark that clock with CLK_IS_CRITICAL.
>> 
>> As a consequence this will also enable the corresponding
>> parent plls and pll-divs.
>> 
>> This is intended as a stop-gap until CLK_ENABLE_HAND_OFF
>> becomes available, at which point it should be used instead.
>> 
>> Signed-off-by: Martin Sperl <[e-mail]>
> 
> I still think that we don't want this patch.  We should be able to
> disable clocks that the firmware turned on, unless they really need to
> stay on.  If you have troubles on the upstream DT, let's talk about
> individual clocks.

May I ask you what is your main concern about using
CLK_IS_CRITICAL as a stop-gap/in general?

This is just a stop-gap until we get CLK_ENABLE_HAND_OFF merged
and I really want to use HAND_OFF as soon as possible, that is why
I have split it into 2 patches.

We essentially just replace the CLK_IS_CRITICAL with
CLK_ENABLE_HAND_OFF (plus comments and debug messages).

Any custom temporary code for specific clocks (which you propose)
is much more complicated than this simple patch and does not handle
things better in any way.

Also the current situation of the machine crashing when releasing the
PCM clock when the parent is PLLC or PLLD is worse than having some
clocks/pll running unnecessarily.

Also this is an incentive to merge HAND_OFF - there is now an
immediate user that would make immediate use of it.


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Eric Anholt May 3, 2016, 1:13 a.m. UTC | #3

Martin Sperl <[e-mail]> writes:

>> On 02.05.2016, at 17:36, Eric Anholt <[e-mail]> wrote:
>> 
>> [e-mail] writes:
>> 
>>> From: Martin Sperl <[e-mail]>
>>> 
>>> The bcm2835 firmware enables several clocks and plls before
>>> booting the linux kernel.
>>> 
>>> These plls should never get disabled as it may result in a
>>> stopped system clock and more.
>>> 
>>> So during probing we check if the clock is enabled
>>> and if it is then mark that clock with CLK_IS_CRITICAL.
>>> 
>>> As a consequence this will also enable the corresponding
>>> parent plls and pll-divs.
>>> 
>>> This is intended as a stop-gap until CLK_ENABLE_HAND_OFF
>>> becomes available, at which point it should be used instead.
>>> 
>>> Signed-off-by: Martin Sperl <[e-mail]>
>> 
>> I still think that we don't want this patch.  We should be able to
>> disable clocks that the firmware turned on, unless they really need to
>> stay on.  If you have troubles on the upstream DT, let's talk about
>> individual clocks.
>
> May I ask you what is your main concern about using
> CLK_IS_CRITICAL as a stop-gap/in general?

Burning power when you shouldn't, which is a bug.

> Also the current situation of the machine crashing when releasing the
> PCM clock when the parent is PLLC or PLLD is worse than having some
> clocks/pll running unnecessarily.

Are you saying this happens on the upstream kernel?  This sounds like a
bug you'd see in the downstream kernel because they haven't hooked up
the clocks in their DT.

Martin Sperl May 3, 2016, 5:07 a.m. UTC | #4

> On 03.05.2016, at 03:13, Eric Anholt <[e-mail]> wrote:
> 
> Martin Sperl <[e-mail]> writes:
> 
>>> On 02.05.2016, at 17:36, Eric Anholt <[e-mail]> wrote:
>>> 
>>> [e-mail] writes:
>>> 
>>>> From: Martin Sperl <[e-mail]>
>>>> 
>>>> The bcm2835 firmware enables several clocks and plls before
>>>> booting the linux kernel.
>>>> 
>>>> These plls should never get disabled as it may result in a
>>>> stopped system clock and more.
>>>> 
>>>> So during probing we check if the clock is enabled
>>>> and if it is then mark that clock with CLK_IS_CRITICAL.
>>>> 
>>>> As a consequence this will also enable the corresponding
>>>> parent plls and pll-divs.
>>>> 
>>>> This is intended as a stop-gap until CLK_ENABLE_HAND_OFF
>>>> becomes available, at which point it should be used instead.
>>>> 
>>>> Signed-off-by: Martin Sperl <[e-mail]>
>>> 
>>> I still think that we don't want this patch.  We should be able to
>>> disable clocks that the firmware turned on, unless they really need to
>>> stay on.  If you have troubles on the upstream DT, let's talk about
>>> individual clocks.
>> 
>> May I ask you what is your main concern about using
>> CLK_IS_CRITICAL as a stop-gap/in general?
> 
> Burning power when you shouldn't, which is a bug.
It is a bug, but it is better than a crash that happens currently
because of a stopped clock.
Also there is a proposed way forward when hand-off
becomes available.

> 
>> Also the current situation of the machine crashing when releasing the
>> PCM clock when the parent is PLLC or PLLD is worse than having some
>> clocks/pll running unnecessarily.
> 
> Are you saying this happens on the upstream kernel?  This sounds like a
> bug you'd see in the downstream kernel because they haven't hooked up
> the clocks in their DT.
This happens for both upstream and downstream when using
cprman...

You remember the main-uart crashing discussion (when 
loaded as a module or not used as the kernel console?) it is
exactly what an unclaimed clock is triggering when the clock
Is prepared/enabled and then released.

I have also seen the same issue happen with PCM without
the patch - as soon as we request a frequency which will use
pllc/d-per as the parent, and we later release the clock the
system crashes -  but there only if we run the dt with minimal
clocks using cprman, as it is hidden as soon as you increase enable
count of the plls because of an enabled clock by a different
device that also consumes the same pll.

So this bug is mostly hidden behind the complete dts that
just consumes lots of parent clocks, but it still may occur 
when we select a frequency that will select the firmware 
enabled but not claimed plla_per - no idea what stopping
that pll would trigger in the system.

Hence we need this patch for the running clocks , but we
also need it for the pll_divs, so actually my patch is
incomplete!
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Patch

diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
index 35f8de7..03b7f01 100644
--- a/drivers/clk/bcm/clk-bcm2835.c
+++ b/drivers/clk/bcm/clk-bcm2835.c
@@ -1251,6 +1251,14 @@  static struct clk *bcm2835_register_clock(struct bcm2835_cprman *cprman,
 		init.flags |= CLK_SET_RATE_GATE | CLK_SET_PARENT_GATE;
 	}
 
+	/* if the clock is running, then enable CRITICAL */
+	if (cprman_read(cprman, data->ctl_reg) & CM_ENABLE) {
+		dev_dbg(cprman->dev,
+			"found firmware enabled clock %s - enabling critical\n",
+			data->name);
+		init.flags |= CLK_IS_CRITICAL;
+	}
+
 	clock = devm_kzalloc(cprman->dev, sizeof(*clock), GFP_KERNEL);
 	if (!clock)
 		return NULL;