linux-kernel - Re: [PATCH v1 1/1] Documentation: firmware-guide: gpio-properties: Add note to SPI CS case

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Date:   Wed, 28 Apr 2021 19:12:41 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH v1 1/1] Documentation: firmware-guide: gpio-properties:
 Add note to SPI CS case

On Sat, Apr 24, 2021 at 4:39 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> Historically ACPI has no means of the GPIO polarity and thus
> the SPISerialBus() resource defines it on the per-chip basis.
> In order to avoid an ambiguity, the GPIO polarity is considered
> being always Active High.
>
> Add note about this to the respective documentation file.
>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  Documentation/firmware-guide/acpi/gpio-properties.rst | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/Documentation/firmware-guide/acpi/gpio-properties.rst b/Documentation/firmware-guide/acpi/gpio-properties.rst
> index 4e264c16ddff..df4b711053ee 100644
> --- a/Documentation/firmware-guide/acpi/gpio-properties.rst
> +++ b/Documentation/firmware-guide/acpi/gpio-properties.rst
> @@ -99,6 +99,12 @@ native::
>        }
>    }
>
> +Note, that historically ACPI has no means of the GPIO polarity and thus
> +the SPISerialBus() resource defines it on the per-chip basis. In order
> +to avoid a chain of negations, the GPIO polarity is considered being
> +Active High. Even for the cases when _DSD() is involved (see the example
> +above) the GPIO CS polarity must be defined Active High to avoid ambiguity.
> +
>  Other supported properties
>  ==========================
>
> --

Applied as 5.13-rc material, thanks!