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@xxxxxxxxx>
---
Changes v1 -> v2:
- Newly inserted patch.
.../devicetree/bindings/common-properties.txt | 26 ++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/Documentation/devicetree/bindings/common-properties.txt b/Documentation/devicetree/bindings/common-properties.txt
index 697714f8d75c..a3448bfa1c82 100644
--- a/Documentation/devicetree/bindings/common-properties.txt
+++ b/Documentation/devicetree/bindings/common-properties.txt
@@ -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>;
+};
--
2.11.0
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