[PATCH v4 0/2] spi: support inter-word delays

[PATCH v4 0/2] spi: support inter-word delays

[Jonas Bonn]

Changed in v4:
* Rename word_delay to word_delay_us and slot it in _beside_ the
  existing word_delay parameter in spi_transfer (see commit message for
  more info).
* Add code to __spi_validate to make sure transfer and device align with
  respect to the word_delay_us parameter

Changed in v3:
* Drop setting of inter-word delay via device tree

Changed in v2:
* Fix atmel-spi driver to not unconditionally set minimal delay if no
  delay is required (erroneous clamping)

This short series adds support for SPI inter-word delays and configures
the spi-atmel driver to honour the setting.

Some SPI slaves are so slow that they are unable to keep up even at the
SPI controller's lowest available clock frequency.  I have such a
configuration where an AVR-based SPI slave is unable to feed the SPI bus
fast enough even the SPI master runs at the lowest possible clock speed.

Jonas Bonn (2):
  spi: support inter-word delay requirement for devices
  spi-atmel: support inter-word delay

 drivers/spi/spi-atmel.c | 18 +++++++++++++-----
 drivers/spi/spi.c       |  5 +++++
 include/linux/spi/spi.h |  6 ++++++
 3 files changed, 24 insertions(+), 5 deletions(-)

-- 
2.19.1

[PATCH v4 1/2] spi: support inter-word delay requirement for devices

[Jonas Bonn]

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(+)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 9a7def7c3237..64f13204bcb7 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2961,6 +2961,8 @@ 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) {
@@ -3031,6 +3033,9 @@ 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 314d922ca607..e04622a01836 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -118,6 +118,8 @@ void spi_statistics_add_transfer_stats(struct spi_statistics *stats,
  *	for driver coldplugging, and in uevents used for hotplugging
  * @cs_gpio: gpio number of the chipselect line (optional, -ENOENT 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
  *
@@ -164,6 +166,7 @@ struct spi_device {
 	char			modalias[SPI_NAME_SIZE];
 	const char		*driver_override;
 	int			cs_gpio;	/* chip select gpio */
+	uint8_t			word_delay_us;	/* inter-word delay */
 
 	/* the statistics */
 	struct spi_statistics	statistics;
@@ -706,6 +709,8 @@ 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
@@ -788,6 +793,7 @@ 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;
-- 
2.19.1

Re: [PATCH v4 1/2] spi: support inter-word delay requirement for devices

[Mark Brown]

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On Tue, Jan 29, 2019 at 09:38:43AM +0100, Jonas Bonn 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.

This doesn't apply against current code, please rebase against for-next
and resend.

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[PATCH v4 2/2] spi-atmel: support inter-word delay

[Jonas Bonn]

If the SPI slave requires an inter-word delay, configure the DLYBCT
register accordingly.

Tested on a SAMA5D2 board (derived from SAMA5D2-Xplained reference
board).

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
CC: Nicolas Ferre <nicolas.ferre@microchip.com>
CC: Mark Brown <broonie@kernel.org>
CC: Alexandre Belloni <alexandre.belloni@bootlin.com>
CC: Ludovic Desroches <ludovic.desroches@microchip.com>
CC: linux-spi@vger.kernel.org
CC: linux-arm-kernel@lists.infradead.org
---
 drivers/spi/spi-atmel.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/spi/spi-atmel.c b/drivers/spi/spi-atmel.c
index 74fddcd3282b..6389a228d2f5 100644
--- a/drivers/spi/spi-atmel.c
+++ b/drivers/spi/spi-atmel.c
@@ -1209,13 +1209,21 @@ static int atmel_spi_setup(struct spi_device *spi)
 		csr |= SPI_BIT(CSAAT);
 
 	/* DLYBS is mostly irrelevant since we manage chipselect using GPIOs.
-	 *
-	 * DLYBCT would add delays between words, slowing down transfers.
-	 * It could potentially be useful to cope with DMA bottlenecks, but
-	 * in those cases it's probably best to just use a lower bitrate.
 	 */
 	csr |= SPI_BF(DLYBS, 0);
-	csr |= SPI_BF(DLYBCT, 0);
+
+	/* DLYBCT adds delays between words.  This is useful for slow devices
+	 * that need a bit of time to setup the next transfer.
+	 */
+	if (spi->word_delay_us) {
+		csr |= SPI_BF(DLYBCT,
+			clamp_t(u8,
+				(as->spi_clk/1000000*spi->word_delay_us)>>5,
+				1, 255));
+	} else {
+		csr |= SPI_BF(DLYBCT, 0);
+	}
+
 
 	/* chipselect must have been muxed as GPIO (e.g. in board setup) */
 	npcs_pin = (unsigned long)spi->controller_data;
-- 
2.19.1

Re: [PATCH v4 2/2] spi-atmel: support inter-word delay

[Nicolas.Ferre]

On 29/01/2019 at 09:38, Jonas Bonn wrote:
> If the SPI slave requires an inter-word delay, configure the DLYBCT
> register accordingly.
> 
> Tested on a SAMA5D2 board (derived from SAMA5D2-Xplained reference
> board).
> 
> Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
> CC: Nicolas Ferre <nicolas.ferre@microchip.com>
> CC: Mark Brown <broonie@kernel.org>
> CC: Alexandre Belloni <alexandre.belloni@bootlin.com>
> CC: Ludovic Desroches <ludovic.desroches@microchip.com>
> CC: linux-spi@vger.kernel.org
> CC: linux-arm-kernel@lists.infradead.org
> ---
>   drivers/spi/spi-atmel.c | 18 +++++++++++++-----
>   1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/spi/spi-atmel.c b/drivers/spi/spi-atmel.c
> index 74fddcd3282b..6389a228d2f5 100644
> --- a/drivers/spi/spi-atmel.c
> +++ b/drivers/spi/spi-atmel.c
> @@ -1209,13 +1209,21 @@ static int atmel_spi_setup(struct spi_device *spi)
>   		csr |= SPI_BIT(CSAAT);
>   
>   	/* DLYBS is mostly irrelevant since we manage chipselect using GPIOs.
> -	 *
> -	 * DLYBCT would add delays between words, slowing down transfers.
> -	 * It could potentially be useful to cope with DMA bottlenecks, but
> -	 * in those cases it's probably best to just use a lower bitrate.
>   	 */
>   	csr |= SPI_BF(DLYBS, 0);
> -	csr |= SPI_BF(DLYBCT, 0);
> +
> +	/* DLYBCT adds delays between words.  This is useful for slow devices
> +	 * that need a bit of time to setup the next transfer.
> +	 */
> +	if (spi->word_delay_us) {

Well...

> +		csr |= SPI_BF(DLYBCT,
> +			clamp_t(u8,
> +				(as->spi_clk/1000000*spi->word_delay_us)>>5,
> +				1, 255));

... why not simplifying to:
+				0, 255));
and remove the test altogether, after all?

> +	} else {
> +		csr |= SPI_BF(DLYBCT, 0);
> +	}
> +
>   
>   	/* chipselect must have been muxed as GPIO (e.g. in board setup) */
>   	npcs_pin = (unsigned long)spi->controller_data;
> 


-- 
Nicolas Ferre

Re: [PATCH v4 2/2] spi-atmel: support inter-word delay

[Jonas Bonn]

Hi,

On 29/01/2019 15:27, Nicolas.Ferre@microchip.com wrote:
> On 29/01/2019 at 09:38, Jonas Bonn wrote:
>> 
>> +	/* DLYBCT adds delays between words.  This is useful for slow devices
>> +	 * that need a bit of time to setup the next transfer.
>> +	 */
>> +	if (spi->word_delay_us) {
> 
> Well...
> 
>> +		csr |= SPI_BF(DLYBCT,
>> +			clamp_t(u8,
>> +				(as->spi_clk/1000000*spi->word_delay_us)>>5,
>> +				1, 255));
> 
> ... why not simplifying to:
> +				0, 255));
> and remove the test altogether, after all?

Hmm... that seemed too easy!  This started out as something else and 
looking at it now I think even the clamp_t() is unnecessary.  The value 
is already 0-255 and the way SPI_BF works any overflow is already 
truncated...  I'll rework this and resubmit once I get some feedback on 
the word_delay_us bits.

Thanks,
/Jonas

Re: [PATCH v4 2/2] spi-atmel: support inter-word delay

[Alexandre Belloni]

Hi,

On 29/01/2019 15:56:31+0100, Jonas Bonn wrote:
> On 29/01/2019 15:27, Nicolas.Ferre@microchip.com wrote:
> > On 29/01/2019 at 09:38, Jonas Bonn wrote:
> > > 
> > > +	/* DLYBCT adds delays between words.  This is useful for slow devices
> > > +	 * that need a bit of time to setup the next transfer.
> > > +	 */
> > > +	if (spi->word_delay_us) {
> > 
> > Well...
> > 
> > > +		csr |= SPI_BF(DLYBCT,
> > > +			clamp_t(u8,
> > > +				(as->spi_clk/1000000*spi->word_delay_us)>>5,
> > > +				1, 255));
> > 
> > ... why not simplifying to:
> > +				0, 255));
> > and remove the test altogether, after all?
> 
> Hmm... that seemed too easy!  This started out as something else and looking
> at it now I think even the clamp_t() is unnecessary.  The value is already
> 0-255 and the way SPI_BF works any overflow is already truncated...  I'll
> rework this and resubmit once I get some feedback on the word_delay_us bits.
> 

While at it, note that you need to add spaces around the operators.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com