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authorJonas Bonn <jonas@norrbonn.se>2019-01-30 09:40:04 +0100
committerMark Brown <broonie@kernel.org>2019-01-30 23:02:10 +0000
commitb7bb367afa4bf9de60830683305c63030c3e581d (patch)
tree9da3b46b4f696b5cb021a71831448e38cf7a8727 /include/linux/spi/spi.h
parentf3fdea3af405f3269099a695d670675cddcd0bfc (diff)
spi: support inter-word delay requirement for devices
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 Signed-off-by: Mark Brown <broonie@kernel.org>
Diffstat (limited to 'include/linux/spi/spi.h')
-rw-r--r--include/linux/spi/spi.h6
1 files changed, 6 insertions, 0 deletions
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 916bba47d156..662b336aa2e4 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_usecs: 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_usecs; /* 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_usecs: 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_usecs;
u16 delay_usecs;
u32 speed_hz;
u16 word_delay;