Behavior of CS signal in combination of spi-cs-high and gpio descriptors
is not clearly defined and documented. So clarify the documentation
Cc: linus.walleij@xxxxxxxxxx
Cc: linux-gpio@xxxxxxxxxxxxxxx
Signed-off-by: H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>
---
.../bindings/spi/spi-controller.yaml | 27 +++++++++++++++++++
1 file changed, 27 insertions(+)
diff --git a/Documentation/devicetree/bindings/spi/spi-controller.yaml b/Documentation/devicetree/bindings/spi/spi-controller.yaml
index 1b56d5e40f1fc..5f505810104dd 100644
--- a/Documentation/devicetree/bindings/spi/spi-controller.yaml
+++ b/Documentation/devicetree/bindings/spi/spi-controller.yaml
@@ -42,6 +42,33 @@ properties:
cs2 : &gpio1 1 0
cs3 : &gpio1 2 0
+ The second flag of a gpio descriptor can be GPIO_ACTIVE_HIGH (0)
+ or GPIO_ACTIVE_LOW(1). Legacy device trees often use 0.
+
+ There is a special rule set for combining the second flag of an
+ cs-gpio with the optional spi-cs-high flag for SPI slaves.
+
+ Each table entry defines how the CS pin is to be physically
+ driven (not considering potential gpio inversions by pinmux):
+
+ device node | cs-gpio | CS pin state active | Note
+ ================+===============+=====================+=====
+ spi-cs-high | - | H |
+ - | - | L |
+ spi-cs-high | ACTIVE_HIGH | H |
+ - | ACTIVE_HIGH | L | 1
+ spi-cs-high | ACTIVE_LOW | H | 2
+ - | ACTIVE_LOW | L |
+
+ Notes:
+ 1) Should print a warning about polarity inversion.
+ Here it would be wise to avoid and define the gpio as
+ ACTIVE_LOW.
+ 2) Should print a warning about polarity inversion
+ because ACTIVE_LOW is overridden by spi-cs-high.
+ Should be generally avoided and be replaced by
+ spi-cs-high + ACTIVE_HIGH.
+
num-cs:
$ref: /schemas/types.yaml#/definitions/uint32
description:
--
2.26.2
