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dt-bindings: Document common property for daisy-chained devices

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Many serially-attached GPIO and IIO devices are daisy-chainable.

    Examples for GPIO devices are Maxim MAX3191x and TI SN65HVS88x:
    https://datasheets.maximintegrated.com/en/ds/MAX31913.pdf
    http://www.ti.com/lit/ds/symlink/sn65hvs880.pdf

    Examples for IIO devices are TI DAC128S085 and TI DAC161S055:
    http://www.ti.com/lit/ds/symlink/dac128s085.pdf
    http://www.ti.com/lit/ds/symlink/dac161s055.pdf

We already have drivers for daisy-chainable devices in the tree but
their devicetree bindings are somewhat inconsistent and ill-named:

    The gpio-74x164.c driver uses "registers-number" to convey the
    number of devices in the daisy-chain.  (Sans vendor prefix,
    multiple vendors sell compatible versions of this chip.)

    The gpio-pisosr.c driver takes a different approach and calculates
    the number of devices in the daisy-chain by dividing the common
    "ngpios" property (Documentation/devicetree/bindings/gpio/gpio.txt)
    by 8 (which assumes that each chip has 8 inputs).

Let's standardize on a common "#daisy-chained-devices" property.
That name was chosen because it's the term most frequently used in
datasheets.  (A less frequently used synonym is "cascaded devices".)

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
This commit is contained in:
Lukas Wunner 2017-10-12 12:40:10 +02:00 committed by Linus Walleij
parent 5048f0aefb
commit 1f63fab955
1 changed files with 26 additions and 0 deletions
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26
Documentation/devicetree/bindings/common-properties.txt
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@ -1,4 +1,8 @@
Common properties
=================
Endianness
----------
The Devicetree Specification does not define any properties related to hardware
byteswapping, but endianness issues show up frequently in porting Linux to
@ -58,3 +62,25 @@ dev: dev@40031000 {
...
little-endian;
};
Daisy-chained devices
---------------------
Many serially-attached GPIO and IIO devices are daisy-chainable. To the
host controller, a daisy-chain appears as a single device, but the number
of inputs and outputs it provides is the sum of inputs and outputs provided
by all of its devices. The driver needs to know how many devices the
daisy-chain comprises to determine the amount of data exchanged, how many
inputs and outputs to register and so on.
Optional properties:
- #daisy-chained-devices: Number of devices in the daisy-chain (default is 1).
Example:
gpio@0 {
compatible = "name";
reg = <0>;
gpio-controller;
#gpio-cells = <2>;
#daisy-chained-devices = <3>;
};