diff --git a/Documentation/spi/spi-summary.rst b/Documentation/spi/spi-summary.rst
index 7f8accfae6f98f1f8f84e2e370afb7933b37e4aa..6e21e6f86912ba9f2407b4b60858d97281a869f3 100644
--- a/Documentation/spi/spi-summary.rst
+++ b/Documentation/spi/spi-summary.rst
@@ -614,6 +614,89 @@ queue, and then start some asynchronous transfer engine (unless it's
already running).
+Extensions to the SPI protocol
+------------------------------
+The fact that SPI doesn't have a formal specification or standard permits chip
+manufacturers to implement the SPI protocol in slightly different ways. In most
+cases, SPI protocol implementations from different vendors are compatible among
+each other. For example, in SPI mode 0 (CPOL=0, CPHA=0) the bus lines may behave
+like the following:
+
+::
+
+ nCSx ___ ___
+ \_________________________________________________________________/
+ • •
+ • •
+ SCLK ___ ___ ___ ___ ___ ___ ___ ___
+ _______/ \___/ \___/ \___/ \___/ \___/ \___/ \___/ \_____
+ • : ; : ; : ; : ; : ; : ; : ; : ; •
+ • : ; : ; : ; : ; : ; : ; : ; : ; •
+ MOSI XXX__________ _______ _______ ________XXX
+ 0xA5 XXX__/ 1 \_0_____/ 1 \_0_______0_____/ 1 \_0_____/ 1 \_XXX
+ • ; ; ; ; ; ; ; ; •
+ • ; ; ; ; ; ; ; ; •
+ MISO XXX__________ _______________________ _______ XXX
+ 0xBA XXX__/ 1 \_____0_/ 1 1 1 \_____0__/ 1 \____0__XXX
+
+Legend::
+
+ • marks the start/end of transmission;
+ : marks when data is clocked into the peripheral;
+ ; marks when data is clocked into the controller;
+ X marks when line states are not specified.
+
+In some few cases, chips extend the SPI protocol by specifying line behaviors
+that other SPI protocols don't (e.g. data line state for when CS is not
+asserted). Those distinct SPI protocols, modes, and configurations are supported
+by different SPI mode flags.
+
+MOSI idle state configuration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Common SPI protocol implementations don't specify any state or behavior for the
+MOSI line when the controller is not clocking out data. However, there do exist
+peripherals that require specific MOSI line state when data is not being clocked
+out. For example, if the peripheral expects the MOSI line to be high when the
+controller is not clocking out data (``SPI_MOSI_IDLE_HIGH``), then a transfer in
+SPI mode 0 would look like the following:
+
+::
+
+ nCSx ___ ___
+ \_________________________________________________________________/
+ • •
+ • •
+ SCLK ___ ___ ___ ___ ___ ___ ___ ___
+ _______/ \___/ \___/ \___/ \___/ \___/ \___/ \___/ \_____
+ • : ; : ; : ; : ; : ; : ; : ; : ; •
+ • : ; : ; : ; : ; : ; : ; : ; : ; •
+ MOSI _____ _______ _______ _______________ ___
+ 0x56 \_0_____/ 1 \_0_____/ 1 \_0_____/ 1 1 \_0_____/
+ • ; ; ; ; ; ; ; ; •
+ • ; ; ; ; ; ; ; ; •
+ MISO XXX__________ _______________________ _______ XXX
+ 0xBA XXX__/ 1 \_____0_/ 1 1 1 \_____0__/ 1 \____0__XXX
+
+Legend::
+
+ • marks the start/end of transmission;
+ : marks when data is clocked into the peripheral;
+ ; marks when data is clocked into the controller;
+ X marks when line states are not specified.
+
+In this extension to the usual SPI protocol, the MOSI line state is specified to
+be kept high when CS is asserted but the controller is not clocking out data to
+the peripheral and also when CS is not asserted.
+
+Peripherals that require this extension must request it by setting the
+``SPI_MOSI_IDLE_HIGH`` bit into the mode attribute of their ``struct
+spi_device`` and call spi_setup(). Controllers that support this extension
+should indicate it by setting ``SPI_MOSI_IDLE_HIGH`` in the mode_bits attribute
+of their ``struct spi_controller``. The configuration to idle MOSI low is
+analogous but uses the ``SPI_MOSI_IDLE_LOW`` mode bit.
+
+
THANKS TO
---------
Contributors to Linux-SPI discussions include (in alphabetical order,
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 6ebe5dd9bbb18530afb1f6a807dd6ce5f40e6b5f..60a2b5a0c85dcb0ed70438b3599db35582c63bfa 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -3921,6 +3921,12 @@ int spi_setup(struct spi_device *spi)
(SPI_TX_DUAL | SPI_TX_QUAD | SPI_TX_OCTAL |
SPI_RX_DUAL | SPI_RX_QUAD | SPI_RX_OCTAL)))
return -EINVAL;
+ /* Check against conflicting MOSI idle configuration */
+ if ((spi->mode & SPI_MOSI_IDLE_LOW) && (spi->mode & SPI_MOSI_IDLE_HIGH)) {
+ dev_err(&spi->dev,
+ "setup: MOSI configured to idle low and high at the same time.\n");
+ return -EINVAL;
+ }
/*
* Help drivers fail *cleanly* when they need options
* that aren't supported with their current controller.
diff --git a/include/uapi/linux/spi/spi.h b/include/uapi/linux/spi/spi.h
index ca56e477d161906257d023de03bf3df01f998d18..ee4ac812b8f8bcb1735d50349fe4bd0c028cd4ed 100644
--- a/include/uapi/linux/spi/spi.h
+++ b/include/uapi/linux/spi/spi.h
@@ -28,7 +28,8 @@
#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 */
+#define SPI_MOSI_IDLE_LOW _BITUL(17) /* leave MOSI line low when idle */
+#define SPI_MOSI_IDLE_HIGH _BITUL(18) /* leave MOSI line high when idle */
/*
* All the bits defined above should be covered by SPI_MODE_USER_MASK.
@@ -38,6 +39,6 @@
* 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(18) - 1)
+#define SPI_MODE_USER_MASK (_BITUL(19) - 1)
#endif /* _UAPI_SPI_H */