[PATCH 1/4][V3] spi: Add optional stall delay between cs_change transfers

[Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>]

Some devices like the ADIS16460 IMU require a longer period between
transfers, i.e. between when the CS is de-asserted and re-asserted. The
default value of 10us is not enough. This change makes the delay
configurable for when the next CS change goes active, allowing the default
to remain 10us is case it is unspecified.

Signed-off-by: Michael Hennerich <michael.hennerich@xxxxxxxxxx>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
---
 drivers/spi/spi.c       | 3 ++-
 include/linux/spi/spi.h | 2 ++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 5e75944ad5d1..fe7fa7fb25c5 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1163,7 +1163,8 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 				keep_cs = true;
 			} else {
 				spi_set_cs(msg->spi, false);
-				udelay(10);
+				udelay(xfer->cs_change_delay ?
+				       xfer->cs_change_delay : 10);
 				spi_set_cs(msg->spi, true);
 			}
 		}
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 053abd22ad31..6de287f8e465 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -734,6 +734,7 @@ extern void spi_res_release(struct spi_controller *ctlr,
  *      transfer. If 0 the default (from @spi_device) is used.
  * @bits_per_word: select a bits_per_word other than the device default
  *      for this transfer. If 0 the default (from @spi_device) is used.
+ * @cs_change_delay: microseconds to delay between cs_change transfers.
  * @cs_change: affects chipselect after this transfer completes
  * @delay_usecs: microseconds to delay after this transfer before
  *	(optionally) changing the chipselect status, then starting
@@ -823,6 +824,7 @@ struct spi_transfer {
 #define	SPI_NBITS_QUAD		0x04 /* 4bits transfer */
 	u8		bits_per_word;
 	u8		word_delay_usecs;
+	u8		cs_change_delay;
 	u16		delay_usecs;
 	u32		speed_hz;
 	u16		word_delay;
-- 
2.20.1