[PATCH] spi: use specific last_cs instead of last_cs_enable

[Yun Zhou <yun.zhou@windriver.com>]

Commit d40f0b6f2e21 instroduced last_cs_enable to avoid setting
chipselect if it's not necessary, but it also introduces a bug. The
chipselect may not be set correctly on multi-device SPI busses. The
reason is that we can't judge the chipselect by bool last_cs_enable,
since chipselect may be modified after other devices were accessed.

So we should record the specific state of chipselect in case of
confusion.

Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
 drivers/spi/spi.c       | 4 ++--
 include/linux/spi/spi.h | 5 +++--
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 4599b121d744..3114b8e5c954 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -936,13 +936,13 @@ static void spi_set_cs(struct spi_device *spi, bool enable, bool force)
 	 * Avoid calling into the driver (or doing delays) if the chip select
 	 * isn't actually changing from the last time this was called.
 	 */
-	if (!force && (spi->controller->last_cs_enable == enable) &&
+	if (!force && (spi->controller->last_cs == spi->chip_select) &&
 	    (spi->controller->last_cs_mode_high == (spi->mode & SPI_CS_HIGH)))
 		return;
 
 	trace_spi_set_cs(spi, activate);
 
-	spi->controller->last_cs_enable = enable;
+	spi->controller->last_cs = enable ? spi->chip_select : -1;
 	spi->controller->last_cs_mode_high = spi->mode & SPI_CS_HIGH;
 
 	if ((spi->cs_gpiod || gpio_is_valid(spi->cs_gpio) ||
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 7ab3fed7b804..5a54ea354053 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -373,7 +373,8 @@ extern struct spi_device *spi_new_ancillary_device(struct spi_device *spi, u8 ch
  * @cur_msg_prepared: spi_prepare_message was called for the currently
  *                    in-flight message
  * @cur_msg_mapped: message has been mapped for DMA
- * @last_cs_enable: was enable true on the last call to set_cs.
+ * @last_cs: the last chip_select that is recorded by set_cs, -1 on non chip
+ *           selected
  * @last_cs_mode_high: was (mode & SPI_CS_HIGH) true on the last call to set_cs.
  * @xfer_completion: used by core transfer_one_message()
  * @busy: message pump is busy
@@ -611,7 +612,7 @@ struct spi_controller {
 	bool				auto_runtime_pm;
 	bool                            cur_msg_prepared;
 	bool				cur_msg_mapped;
-	bool				last_cs_enable;
+	char				last_cs;
 	bool				last_cs_mode_high;
 	bool                            fallback;
 	struct completion               xfer_completion;
-- 
2.26.1