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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@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Andy Shevchenko 2021-04-24 17:39:35 +03:00 committed by Rafael J. Wysocki
parent d8f9176b4e
commit ec3576eac1
1 changed files with 6 additions and 0 deletions
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6
Documentation/firmware-guide/acpi/gpio-properties.rst
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@ -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
==========================