Unverified Commit 2fee9583 authored by H. Nikolaus Schaller's avatar H. Nikolaus Schaller Committed by Mark Brown
Browse files

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



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: default avatarH. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/3bed61807fff6268789e7d411412fbc5cd6ffe2a.1607507863.git.hns@goldelico.com


Signed-off-by: default avatarMark Brown <broonie@kernel.org>
parent 49ab19a4

Documentation/devicetree/bindings/spi/spi-controller.yaml

+27 −0
Original line number Diff line number Diff line
@@ -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:

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