spi: Enable controllers to extend the SPI protocol with MOSI idle configuration

The behavior of an SPI controller data output line (SDO or MOSI or COPI
(Controller Output Peripheral Input) for disambiguation) is usually not
specified when the controller is not clocking out data on SCLK edges.
However, there do exist SPI peripherals that require specific MOSI line
state when data is not being clocked out of the controller.

Conventional SPI controllers may set the MOSI line on SCLK edges then bring
it low when no data is going out or leave the line the state of the last
transfer bit. More elaborated controllers are capable to set the MOSI idle
state according to different configurable levels and thus are more suitable
for interfacing with demanding peripherals.

Add SPI mode bits to allow peripherals to request explicit MOSI idle state
when needed.

When supporting a particular MOSI idle configuration, the data output line
state is expected to remain at the configured level when the controller is
not clocking out data. When a device that needs a specific MOSI idle state
is identified, its driver should request the MOSI idle configuration by
setting the proper SPI mode bit.

Acked-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Tested-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/9802160b5e5baed7f83ee43ac819cb757a19be55.1720810545.git.marcelo.schmitt@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>

1 files changed, 83 insertions, 0 deletions

diff --git a/Documentation/spi/spi-summary.rst b/Documentation/spi/spi-summary.rst
index 7f8accfae6f9..6e21e6f86912 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,