spi: sh-msiof: Document hardware limitations related to chip selects

Guide users to maintain the proper balance between native and GPIO chip
selects.

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

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/spi/sh-msiof.txt b/Documentation/devicetree/bindings/spi/sh-msiof.txt
index bc8c16a6cfc8..80710f0f0448 100644
--- a/Documentation/devicetree/bindings/spi/sh-msiof.txt
+++ b/Documentation/devicetree/bindings/spi/sh-msiof.txt
@@ -41,6 +41,16 @@ Optional properties:
                            0: MSIOF_SYNC
                            1: MSIOF_SS1
                            2: MSIOF_SS2
+                         Hardware limitations related to chip selects:
+                           - Native chip selects are always deasserted in
+                             between transfers that are part of the same
+                             message.  Use cs-gpios to work around this.
+                           - All slaves using native chip selects must use the
+                             same spi-cs-high configuration.  Use cs-gpios to
+                             work around this.
+                           - When using GPIO chip selects, at least one native
+                             chip select must be left unused, as it will be
+                             driven anyway.
 - dmas                 : Must contain a list of two references to DMA
                          specifiers, one for transmission, and one for
                          reception.