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

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Message-Id: <20210424143935.69487-1-andriy.shevchenko@linux.intel.com>
Date:   Sat, 24 Apr 2021 17:39:35 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-gpio@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>
Subject: [PATCH v1 1/1] Documentation: firmware-guide: gpio-properties: Add note to SPI CS case

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
 ==========================
 
-- 
2.30.2