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gpio: dt-bindings: document the official use of "ngpios"

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There are a bunch of drivers that utilize the "ngpios" DT property
without any vendor prefix. Try to start cleaning up the mess by
defining what we mean by this property.

Cc: devicetree@vger.kernel.org
Cc: Pramod Kumar <pramodku@broadcom.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
This commit is contained in:
Linus Walleij 2015-10-30 11:58:28 +01:00
parent aad7a21199
commit aacaffd1d9
1 changed files with 24 additions and 0 deletions
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24
Documentation/devicetree/bindings/gpio/gpio.txt
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@ -129,6 +129,30 @@ Every GPIO controller node must contain both an empty "gpio-controller"
property, and a #gpio-cells integer property, which indicates the number of
cells in a gpio-specifier.
Optionally, a GPIO controller may have a "ngpios" property. This property
indicates the number of in-use slots of available slots for GPIOs. The
typical example is something like this: the hardware register is 32 bits
wide, but only 18 of the bits have a physical counterpart. The driver is
generally written so that all 32 bits can be used, but the IP block is reused
in a lot of designs, some using all 32 bits, some using 18 and some using
12. In this case, setting "ngpios = <18>;" informs the driver that only the
first 18 GPIOs, at local offset 0 .. 17, are in use.
If these GPIOs do not happen to be the first N GPIOs at offset 0...N-1, an
additional bitmask is needed to specify which GPIOs are actually in use,
and which are dummies. The bindings for this case has not yet been
specified, but should be specified if/when such hardware appears.
Example:
gpio-controller@00000000 {
compatible = "foo";
reg = <0x00000000 0x1000>;
gpio-controller;
#gpio-cells = <2>;
ngpios = <18>;
}
The GPIO chip may contain GPIO hog definitions. GPIO hogging is a mechanism
providing automatic GPIO request and configuration as part of the
gpio-controller's driver probe function.