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spi: dt-bindings: clarify CS behavior for spi-cs-high and gpio descriptors

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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@linaro.org
Cc: linux-gpio@vger.kernel.org
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/3bed61807fff6268789e7d411412fbc5cd6ffe2a.1607507863.git.hns@goldelico.com
Signed-off-by: Mark Brown <broonie@kernel.org>
This commit is contained in:
H. Nikolaus Schaller 2020-12-09 10:57:44 +01:00 committed by Mark Brown
parent 49ab19a4a5
commit 2fee958319
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27
Documentation/devicetree/bindings/spi/spi-controller.yaml
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@ -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: