[v5,1/2] spi: support inter-word delay requirement for devices
diff mbox series

Message ID 20190129205502.7741-2-jonas@norrbonn.se
State New, archived
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  • [v5,1/2] spi: support inter-word delay requirement for devices
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Commit Message

Jonas Bonn Jan. 29, 2019, 8:55 p.m. UTC
Some devices are slow and cannot keep up with the SPI bus and therefore
require a short delay between words of the SPI transfer.

The example of this that I'm looking at is a SAMA5D2 with a minimum SPI
clock of 400kHz talking to an AVR-based SPI slave.  The AVR cannot put
bytes on the bus fast enough to keep up with the SoC's SPI controller
even at the lowest bus speed.

This patch introduces the ability to specify a required inter-word
delay for SPI devices.  It is up to the controller driver to configure
itself accordingly in order to introduce the requested delay.

Note that, for spi_transfer, there is already a field word_delay that
provides similar functionality.  This field, however, is specified in
clock cycles (and worse, SPI controller cycles, not SCK cycles); that
makes this value dependent on the master clock instead of the device
clock for which the delay is intended to provide some relief.  This
patch leaves this old word_delay in place and provides a time-based
word_delay_us alongside it; the new field fits in the struct padding
so struct size is constant.  There is only one in-kernel user of the
word_delay field and presumably that driver could be reworked to use
the time-based value instead.

The time-based delay is limited to 8 bits as these delays are intended
to be short.  The SAMA5D2 that I've tested this on limits delays to a
maximum of ~100us, which is already many word-transfer periods even at
the minimum transfer speed supported by the controller.

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
CC: Mark Brown <broonie@kernel.org>
CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: linux-spi@vger.kernel.org
CC: devicetree@vger.kernel.org
---
 drivers/spi/spi.c       | 5 +++++
 include/linux/spi/spi.h | 6 ++++++
 2 files changed, 11 insertions(+)

Comments

Geert Uytterhoeven Jan. 30, 2019, 7:35 a.m. UTC | #1
Hi Jonas,

On Tue, Jan 29, 2019 at 9:55 PM Jonas Bonn <jonas@norrbonn.se> wrote:
> Some devices are slow and cannot keep up with the SPI bus and therefore
> require a short delay between words of the SPI transfer.
>
> The example of this that I'm looking at is a SAMA5D2 with a minimum SPI
> clock of 400kHz talking to an AVR-based SPI slave.  The AVR cannot put
> bytes on the bus fast enough to keep up with the SoC's SPI controller
> even at the lowest bus speed.
>
> This patch introduces the ability to specify a required inter-word
> delay for SPI devices.  It is up to the controller driver to configure
> itself accordingly in order to introduce the requested delay.
>
> Note that, for spi_transfer, there is already a field word_delay that
> provides similar functionality.  This field, however, is specified in
> clock cycles (and worse, SPI controller cycles, not SCK cycles); that
> makes this value dependent on the master clock instead of the device
> clock for which the delay is intended to provide some relief.  This
> patch leaves this old word_delay in place and provides a time-based
> word_delay_us alongside it; the new field fits in the struct padding
> so struct size is constant.  There is only one in-kernel user of the
> word_delay field and presumably that driver could be reworked to use
> the time-based value instead.

Thanks for your patch!

> The time-based delay is limited to 8 bits as these delays are intended
> to be short.  The SAMA5D2 that I've tested this on limits delays to a
> maximum of ~100us, which is already many word-transfer periods even at
> the minimum transfer speed supported by the controller.

Still, the similar delay_usecs uses a u16.

> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h

> @@ -803,6 +808,7 @@ struct spi_transfer {
>  #define        SPI_NBITS_DUAL          0x02 /* 2bits transfer */
>  #define        SPI_NBITS_QUAD          0x04 /* 4bits transfer */
>         u8              bits_per_word;
> +       u8              word_delay_us;

us for µs

>         u16             delay_usecs;

usecs for µs

Can we please try to be consistent?

Gr{oetje,eeting}s,

                        Geert
Jonas Bonn Jan. 30, 2019, 8:10 a.m. UTC | #2
Hi Geert,

On 30/01/2019 08:35, Geert Uytterhoeven wrote:
> Hi Jonas,
> 
> On Tue, Jan 29, 2019 at 9:55 PM Jonas Bonn <jonas@norrbonn.se> wrote:
>> Some devices are slow and cannot keep up with the SPI bus and therefore
>> require a short delay between words of the SPI transfer.
>>
>> The example of this that I'm looking at is a SAMA5D2 with a minimum SPI
>> clock of 400kHz talking to an AVR-based SPI slave.  The AVR cannot put
>> bytes on the bus fast enough to keep up with the SoC's SPI controller
>> even at the lowest bus speed.
>>
>> This patch introduces the ability to specify a required inter-word
>> delay for SPI devices.  It is up to the controller driver to configure
>> itself accordingly in order to introduce the requested delay.
>>
>> Note that, for spi_transfer, there is already a field word_delay that
>> provides similar functionality.  This field, however, is specified in
>> clock cycles (and worse, SPI controller cycles, not SCK cycles); that
>> makes this value dependent on the master clock instead of the device
>> clock for which the delay is intended to provide some relief.  This
>> patch leaves this old word_delay in place and provides a time-based
>> word_delay_us alongside it; the new field fits in the struct padding
>> so struct size is constant.  There is only one in-kernel user of the
>> word_delay field and presumably that driver could be reworked to use
>> the time-based value instead.
> 
> Thanks for your patch!
> 
>> The time-based delay is limited to 8 bits as these delays are intended
>> to be short.  The SAMA5D2 that I've tested this on limits delays to a
>> maximum of ~100us, which is already many word-transfer periods even at
>> the minimum transfer speed supported by the controller.
> 
> Still, the similar delay_usecs uses a u16.

The delays are not comparable.  delay_usecs is the "end of transfer" 
delay and represents the processing time to handle a command or a bundle 
of data.  The inter-word delay is just a stutter to give the slave time 
to get the next word set up.

On the Microchip (Atmel... these name changes take a year or two to sink 
in!) board that I'm using (SAMA5D2), the inter-word delay is limited to 
8192 SPI-controller-clock cycles (not SPI bus clock, input clock).  At 
83Mhz, that's about 100us, and it only gets shorter as you clock up. 
The Spreadtrum board has an upper limit of 130 SPI-controller-clock 
cycles which is just a few microseconds.

Aside from that limitation, consider what's reasonable in terms of 
delay.  More than 5x the word-transfer time and things are pretty 
questionable.  A 250us delay with a word transfer time of 50us gives an 
SPI-bus clock 160kHz.  That's already pretty slow in SPI terms and well 
below what the SAMA5D2 is capable of driving with it's 83MHz input clock 
(max divider is 255 which gives roughtly 400kHz minimum SPI bus clock).

So with that, setting an inter-word delay larger than 255us is barely 
reasonable.  If somebody finds a concrete use for such a setting, then 
the size of the field can be expanded at that time.

Just for info, the inter-word delay that I need to communicate with an 
Atmega MCU running at 500kHz is ~20us which is on the order of the word 
transfer time itself.  This chip is a poor example of how to design an 
SPI slave so I suspect it can only get better from here! :)

And with that said, if you insist having a u16 for this, I'll change it. 
  Just let me know.


> 
>> --- a/include/linux/spi/spi.h
>> +++ b/include/linux/spi/spi.h
> 
>> @@ -803,6 +808,7 @@ struct spi_transfer {
>>   #define        SPI_NBITS_DUAL          0x02 /* 2bits transfer */
>>   #define        SPI_NBITS_QUAD          0x04 /* 4bits transfer */
>>          u8              bits_per_word;
>> +       u8              word_delay_us;
> 
> us for µs
> 
>>          u16             delay_usecs;
> 
> usecs for µs
> 
> Can we please try to be consistent?



I can change it to usecs if you want.  Is this a serious request to do 
so?  _usecs and _us are used pretty interchangeably across the kernel 
with a slight advantage to _us.

/Jonas

> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>
Geert Uytterhoeven Jan. 30, 2019, 8:15 a.m. UTC | #3
Hi Jonas,

On Wed, Jan 30, 2019 at 9:10 AM Jonas Bonn <jonas@norrbonn.se> wrote:
> On 30/01/2019 08:35, Geert Uytterhoeven wrote:
> > On Tue, Jan 29, 2019 at 9:55 PM Jonas Bonn <jonas@norrbonn.se> wrote:
> >> Some devices are slow and cannot keep up with the SPI bus and therefore
> >> require a short delay between words of the SPI transfer.
> >>
> >> The example of this that I'm looking at is a SAMA5D2 with a minimum SPI
> >> clock of 400kHz talking to an AVR-based SPI slave.  The AVR cannot put
> >> bytes on the bus fast enough to keep up with the SoC's SPI controller
> >> even at the lowest bus speed.
> >>
> >> This patch introduces the ability to specify a required inter-word
> >> delay for SPI devices.  It is up to the controller driver to configure
> >> itself accordingly in order to introduce the requested delay.
> >>
> >> Note that, for spi_transfer, there is already a field word_delay that
> >> provides similar functionality.  This field, however, is specified in
> >> clock cycles (and worse, SPI controller cycles, not SCK cycles); that
> >> makes this value dependent on the master clock instead of the device
> >> clock for which the delay is intended to provide some relief.  This
> >> patch leaves this old word_delay in place and provides a time-based
> >> word_delay_us alongside it; the new field fits in the struct padding
> >> so struct size is constant.  There is only one in-kernel user of the
> >> word_delay field and presumably that driver could be reworked to use
> >> the time-based value instead.
> >
> > Thanks for your patch!
> >
> >> The time-based delay is limited to 8 bits as these delays are intended
> >> to be short.  The SAMA5D2 that I've tested this on limits delays to a
> >> maximum of ~100us, which is already many word-transfer periods even at
> >> the minimum transfer speed supported by the controller.
> >
> > Still, the similar delay_usecs uses a u16.
>
> The delays are not comparable.  delay_usecs is the "end of transfer"
> delay and represents the processing time to handle a command or a bundle
> of data.  The inter-word delay is just a stutter to give the slave time
> to get the next word set up.
>
> On the Microchip (Atmel... these name changes take a year or two to sink
> in!) board that I'm using (SAMA5D2), the inter-word delay is limited to
> 8192 SPI-controller-clock cycles (not SPI bus clock, input clock).  At
> 83Mhz, that's about 100us, and it only gets shorter as you clock up.
> The Spreadtrum board has an upper limit of 130 SPI-controller-clock
> cycles which is just a few microseconds.
>
> Aside from that limitation, consider what's reasonable in terms of
> delay.  More than 5x the word-transfer time and things are pretty
> questionable.  A 250us delay with a word transfer time of 50us gives an
> SPI-bus clock 160kHz.  That's already pretty slow in SPI terms and well
> below what the SAMA5D2 is capable of driving with it's 83MHz input clock
> (max divider is 255 which gives roughtly 400kHz minimum SPI bus clock).
>
> So with that, setting an inter-word delay larger than 255us is barely
> reasonable.  If somebody finds a concrete use for such a setting, then
> the size of the field can be expanded at that time.
>
> Just for info, the inter-word delay that I need to communicate with an
> Atmega MCU running at 500kHz is ~20us which is on the order of the word
> transfer time itself.  This chip is a poor example of how to design an
> SPI slave so I suspect it can only get better from here! :)
>
> And with that said, if you insist having a u16 for this, I'll change it.
>   Just let me know.

I'll defer that to the maintainer (Mark).

> >> --- a/include/linux/spi/spi.h
> >> +++ b/include/linux/spi/spi.h
> >
> >> @@ -803,6 +808,7 @@ struct spi_transfer {
> >>   #define        SPI_NBITS_DUAL          0x02 /* 2bits transfer */
> >>   #define        SPI_NBITS_QUAD          0x04 /* 4bits transfer */
> >>          u8              bits_per_word;
> >> +       u8              word_delay_us;
> >
> > us for µs
> >
> >>          u16             delay_usecs;
> >
> > usecs for µs
> >
> > Can we please try to be consistent?
>
> I can change it to usecs if you want.  Is this a serious request to do
> so?  _usecs and _us are used pretty interchangeably across the kernel
> with a slight advantage to _us.

Personally, I prefer "us", too. Unfortunately not everyone has been
converted to metric, SI, and EURO yet ;-)

However, it looks really odd if the next line uses "usecs".

Gr{oetje,eeting}s,

                        Geert
Mark Brown Jan. 31, 2019, 12:46 p.m. UTC | #4
On Wed, Jan 30, 2019 at 09:15:09AM +0100, Geert Uytterhoeven wrote:
> On Wed, Jan 30, 2019 at 9:10 AM Jonas Bonn <jonas@norrbonn.se> wrote:

> > And with that said, if you insist having a u16 for this, I'll change it.
> >   Just let me know.

> I'll defer that to the maintainer (Mark).

It's an internal kernel thing, if we end up needing more space we can
just change it.

Patch
diff mbox series

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 0e0f2c62973c..448443adee3a 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -3050,6 +3050,8 @@  static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 	 * it is not set for this transfer.
 	 * Set transfer tx_nbits and rx_nbits as single transfer default
 	 * (SPI_NBITS_SINGLE) if it is not set for this transfer.
+	 * Ensure transfer word_delay is at least as long as that required by
+	 * device itself.
 	 */
 	message->frame_length = 0;
 	list_for_each_entry(xfer, &message->transfers, transfer_list) {
@@ -3120,6 +3122,9 @@  static int __spi_validate(struct spi_device *spi, struct spi_message *message)
 				!(spi->mode & SPI_RX_QUAD))
 				return -EINVAL;
 		}
+
+		if (xfer->word_delay_us < spi->word_delay_us)
+			xfer->word_delay_us = spi->word_delay_us;
 	}
 
 	message->status = -EINPROGRESS;
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 916bba47d156..555ba125bb89 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -122,6 +122,8 @@  void spi_statistics_add_transfer_stats(struct spi_statistics *stats,
  *	the spi_master.
  * @cs_gpiod: gpio descriptor of the chipselect line (optional, NULL when
  *	not using a GPIO line)
+ * @word_delay_us: microsecond delay to be inserted between consecutive
+ *	words of a transfer
  *
  * @statistics: statistics for the spi_device
  *
@@ -169,6 +171,7 @@  struct spi_device {
 	const char		*driver_override;
 	int			cs_gpio;	/* LEGACY: chip select gpio */
 	struct gpio_desc	*cs_gpiod;	/* chip select gpio desc */
+	uint8_t			word_delay_us;	/* inter-word delay */
 
 	/* the statistics */
 	struct spi_statistics	statistics;
@@ -721,6 +724,8 @@  extern void spi_res_release(struct spi_controller *ctlr,
  * @delay_usecs: microseconds to delay after this transfer before
  *	(optionally) changing the chipselect status, then starting
  *	the next transfer or completing this @spi_message.
+ * @word_delay_us: microseconds to inter word delay after each word size
+ *	(set by bits_per_word) transmission.
  * @word_delay: clock cycles to inter word delay after each word size
  *	(set by bits_per_word) transmission.
  * @transfer_list: transfers are sequenced through @spi_message.transfers
@@ -803,6 +808,7 @@  struct spi_transfer {
 #define	SPI_NBITS_DUAL		0x02 /* 2bits transfer */
 #define	SPI_NBITS_QUAD		0x04 /* 4bits transfer */
 	u8		bits_per_word;
+	u8		word_delay_us;
 	u16		delay_usecs;
 	u32		speed_hz;
 	u16		word_delay;