spi: Document SPI slave controller support

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

1 files changed, 20 insertions, 7 deletions

diff --git a/Documentation/spi/spi-summary b/Documentation/spi/spi-summary
index d1824b399b2d..1721c1b570c3 100644
--- a/Documentation/spi/spi-summary
+++ b/Documentation/spi/spi-summary
@@ -62,8 +62,8 @@ chips described as using "three wire" signaling: SCK, data, nCSx.
 (That data line is sometimes called MOMI or SISO.)
 
 Microcontrollers often support both master and slave sides of the SPI
-protocol.  This document (and Linux) currently only supports the master
-side of SPI interactions.
+protocol.  This document (and Linux) supports both the master and slave
+sides of SPI interactions.
 
 
 Who uses it?  On what kinds of systems?
@@ -154,9 +154,8 @@ control audio interfaces, present touchscreen sensors as input interfaces,
 or monitor temperature and voltage levels during industrial processing.
 And those might all be sharing the same controller driver.
 
-A "struct spi_device" encapsulates the master-side interface between
-those two types of driver.  At this writing, Linux has no slave side
-programming interface.
+A "struct spi_device" encapsulates the controller-side interface between
+those two types of drivers.
 
 There is a minimal core of SPI programming interfaces, focussing on
 using the driver model to connect controller and protocol drivers using
@@ -177,10 +176,24 @@ shows up in sysfs in several locations:
    /sys/bus/spi/drivers/D ... driver for one or more spi*.* devices
 
    /sys/class/spi_master/spiB ... symlink (or actual device node) to
-        a logical node which could hold class related state for the
-        controller managing bus "B".  All spiB.* devices share one
+        a logical node which could hold class related state for the SPI
+        master controller managing bus "B".  All spiB.* devices share one
         physical SPI bus segment, with SCLK, MOSI, and MISO.
 
+   /sys/devices/.../CTLR/slave ... virtual file for (un)registering the
+        slave device for an SPI slave controller.
+        Writing the driver name of an SPI slave handler to this file
+        registers the slave device; writing "(null)" unregisters the slave
+        device.
+        Reading from this file shows the name of the slave device ("(null)"
+        if not registered).
+
+   /sys/class/spi_slave/spiB ... symlink (or actual device node) to
+        a logical node which could hold class related state for the SPI
+        slave controller on bus "B".  When registered, a single spiB.*
+        device is present here, possible sharing the physical SPI bus
+        segment with other SPI slave devices.
+
 Note that the actual location of the controller's class state depends
 on whether you enabled CONFIG_SYSFS_DEPRECATED or not.  At this time,
 the only class-specific state is the bus number ("B" in "spiB"), so