[PATCH 1/4][V2] drivers: spi: core: Add optional delay between cs_change transfers

[Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>]

Some devices like the ADIS16460 IMU require a stall 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.

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 | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 5e75944ad5d1..02fd00bcaace 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_usecs ?
+				       xfer->cs_change_delay_usecs : 10);
 				spi_set_cs(msg->spi, true);
 			}
 		}
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 053abd22ad31..c884b3b94841 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -734,6 +734,8 @@ 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_usecs: 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 +825,7 @@ struct spi_transfer {
 #define	SPI_NBITS_QUAD		0x04 /* 4bits transfer */
 	u8		bits_per_word;
 	u8		word_delay_usecs;
+	u8		cs_change_delay_usecs;
 	u16		delay_usecs;
 	u32		speed_hz;
 	u16		word_delay;
-- 
2.20.1