[PATCH v2 0/2] spi: support inter-word delays

[PATCH v2 0/2] spi: support inter-word delays

[Jonas Bonn]

Resubmission to fix thinko in spi-atmel patch.

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

 .../devicetree/bindings/spi/spi-bus.txt        |  1 +
 drivers/spi/spi-atmel.c                        | 18 +++++++++++++-----
 drivers/spi/spi.c                              |  4 ++++
 include/linux/spi/spi.h                        |  1 +
 4 files changed, 19 insertions(+), 5 deletions(-)

-- 
2.19.1

[PATCH v2 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.

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
---
 Documentation/devicetree/bindings/spi/spi-bus.txt | 1 +
 drivers/spi/spi.c                                 | 4 ++++
 include/linux/spi/spi.h                           | 1 +
 3 files changed, 6 insertions(+)

diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
index 1f6e86f787ef..a5f20060676d 100644
--- a/Documentation/devicetree/bindings/spi/spi-bus.txt
+++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
@@ -77,6 +77,7 @@ All slave nodes can contain the following optional properties:
 		    Defaults to 1 if not present.
 - spi-rx-delay-us - Microsecond delay after a read transfer.
 - spi-tx-delay-us - Microsecond delay after a write transfer.
+- spi-word-delay-us - Microsecond delay between individual words of a transfer
 
 Some SPI controllers and devices support Dual and Quad SPI transfer mode.
 It allows data in the SPI system to be transferred using 2 wires (DUAL) or 4
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 9a7def7c3237..cd4d4065eca2 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1692,6 +1692,10 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
 	}
 	spi->max_speed_hz = value;
 
+	if (!of_property_read_u32(nc, "spi-word-delay-us", &value)) {
+		spi->word_delay = value;
+	}
+
 	return 0;
 }
 
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 314d922ca607..e5200dd9d750 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -164,6 +164,7 @@ struct spi_device {
 	char			modalias[SPI_NAME_SIZE];
 	const char		*driver_override;
 	int			cs_gpio;	/* chip select gpio */
+	uint16_t		word_delay;	/* inter-word delay (us) */
 
 	/* the statistics */
 	struct spi_statistics	statistics;
-- 
2.19.1

[PATCH v2 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..24445bfbd74e 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) {
+		csr |= SPI_BF(DLYBCT,
+			clamp_t(u8,
+				(as->spi_clk/1000000*spi->word_delay)>>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

[PATCH v2 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..24445bfbd74e 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) {
+		csr |= SPI_BF(DLYBCT,
+			clamp_t(u8,
+				(as->spi_clk/1000000*spi->word_delay)>>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


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