[PATCH v4 1/3] spi: add SPI_MOSI_IDLE_LOW mode bit

[Boerge Struempfel]

Some spi controller switch the mosi line to high, whenever they are
idle. This may not be desired in all use cases. For example neopixel
leds can get confused and flicker due to misinterpreting the idle state.
Therefore, we introduce a new spi-mode bit, with which the idle behaviour
can be overwritten on a per device basis.

Signed-off-by: Boerge Struempfel <boerge.struempfel@xxxxxxxxx>


Link for versions:
  v1 and v2: https://lore.kernel.org/linux-spi/20230511135632.78344-1-bstruempfel@xxxxxxxxxxxxxx/
  v3: https://lore.kernel.org/linux-spi/20230517103007.26287-1-boerge.struempfel@xxxxxxxxx/T/#t

Changes from V3:
  - Added missing paranthesis which caused builderrors

Changes from V2:
  - Removed the device-tree binding since this should not be managed by
    the DT but by the device itself.
  - Replaced all occurences of spi->chip_select with the corresponding 
    macro spi_get_chipselect(spi,0)

Changes from V1:
  - Added patch, introducing the new devicetree binding flag
  - Split the generic spi part of the patch from the imx-spi specific
    part
  - Replaced SPI_CPOL and SPI_CPHA by the combined SPI_MODE_X_MASK bit
    in the imx-spi.c modebits.
  - Added the SPI_MOSI_IDLE_LOW bit to spidev

---
 include/uapi/linux/spi/spi.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/spi/spi.h b/include/uapi/linux/spi/spi.h
index 9d5f58059703..ca56e477d161 100644
--- a/include/uapi/linux/spi/spi.h
+++ b/include/uapi/linux/spi/spi.h
@@ -28,6 +28,7 @@
 #define	SPI_RX_OCTAL		_BITUL(14)	/* receive with 8 wires */
 #define	SPI_3WIRE_HIZ		_BITUL(15)	/* high impedance turnaround */
 #define	SPI_RX_CPHA_FLIP	_BITUL(16)	/* flip CPHA on Rx only xfer */
+#define SPI_MOSI_IDLE_LOW	_BITUL(17)	/* leave mosi line low when idle */
 
 /*
  * All the bits defined above should be covered by SPI_MODE_USER_MASK.
@@ -37,6 +38,6 @@
  * These bits must not overlap. A static assert check should make sure of that.
  * If adding extra bits, make sure to increase the bit index below as well.
  */
-#define SPI_MODE_USER_MASK	(_BITUL(17) - 1)
+#define SPI_MODE_USER_MASK	(_BITUL(18) - 1)
 
 #endif /* _UAPI_SPI_H */
-- 
2.25.1