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>

diff --git a/Documentation/devicetree/bindings/spi/sh-msiof.txt b/Documentation/devicetree/bindings/spi/sh-msiof.txt
index bc8c16a6cfc826859461f81c1927d1158b5e285f..80710f0f044891748a506374c68dfef635cf2853 100644 (file)
--- 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.