[PATCH v3 1/2] spi: support inter-word delay requirement for devices

[Jonas Bonn <jonas@norrbonn.se>]

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
---
 include/linux/spi/spi.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 314d922ca607..8e410de02af4 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: 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 */
+	uint16_t		word_delay;	/* inter-word delay (us) */
 
 	/* the statistics */
 	struct spi_statistics	statistics;
-- 
2.19.1