[RFC 2/2] ARM:Tegra: Device Tree Support: Initialize audio card gpio's from the device tree.

[Grant Likely]

On Mon, May 30, 2011 at 12:54 PM, Segher Boessenkool
<segher at kernel.crashing.org> wrote:
>>> I'm currently dealing with an SoC that has over a hundred GPIOs.
>>> Whatever we choose, I think it should be able to handle an insane
>>> number of GPIOs without getting any more cumbersome that is
>>> necessary.
>>
>> This is *consumer* side GPIOs, not bindings for the device providing the
>> GPIOs.  If a single device needs to use hundreds of GPIOs I'd expect
>> many of them will be block functions so you'd have a binding with an
>> array for things like "databus" and "addrbus".
>
> But please name them like "databus-gpio", so that it is obvious what it
> is.  Also have to think about how this will work with multiple GPIO
> controllers: do you require the GPIO controller node to be part of every
> GPIO description, or do you do some "gpio-parent" scheme as well, how
> does that interact with not having a single array of GPIOs?
>
> Better write this down as a binding, before committing to it :-)

Here's my thinking: Exactly the same binding for "gpios" as is now,
except can use different property names, like "chipsel-gpios" or
"wp-gpio".  Here is a draft patch to the binding:

diff --git a/Documentation/devicetree/bindings/gpio/gpio.txt
b/Documentation/devicetree/bindings/gpio/gpio.txt
index edaa84d..21dd4f9 100644
--- a/Documentation/devicetree/bindings/gpio/gpio.txt
+++ b/Documentation/devicetree/bindings/gpio/gpio.txt
@@ -4,17 +4,45 @@ Specifying GPIO information for devices
 1) gpios property
 -----------------

-Nodes that makes use of GPIOs should define them using `gpios' property,
-format of which is: <&gpio-controller1-phandle gpio1-specifier
-		     &gpio-controller2-phandle gpio2-specifier
-		     0 /* holes are permitted, means no GPIO 3 */
-		     &gpio-controller4-phandle gpio4-specifier
-		     ...>;
+Nodes that makes use of GPIOs should specify them using one or more
+properties, each containing a 'gpio-list':

-Note that gpio-specifier length is controller dependent.
+	gpio-list ::= <single-gpio> [gpio-list]
+	single-gpio ::= <gpio-phandle> <gpio-specifier>
+	gpio-phandle : phandle to gpio controller node
+	gpio-specifier : Array of #gpio-cells specifying specific gpio
+			 (controller specific)
+
+GPIO properties should be named either "gpios" or "*-gpio".  Exact
+meaning of each gpio property must be documented in the device tree
+binding for each device.
+
+For example, the following could be used to describe gpios pins to use
+as chip select lines; with chip selects 0, 1 and 3 populated, and chip
+select 2 left empty:
+
+	gpio1: gpio1 {
+		gpio-controller
+		 #gpio-cells = <2>;
+	};
+	gpio2: gpio2 {
+		gpio-controller
+		 #gpio-cells = <1>;
+	};
+	[...]
+	 chipsel-gpio = <&gpio1 12 0>,
+			<&gpio1 13 0>,
+			<0>, /* holes are permitted, means no GPIO 2 */
+			<&gpio2 2>;
+
+Note that gpio-specifier length is controller dependent.  In the
+above example, &gpio1 uses 2 cells to specify a gpio, while &gpio2
+only uses one.

 gpio-specifier may encode: bank, pin position inside the bank,
 whether pin is open-drain and whether pin is logically inverted.
+Exact meaning of each specifier cell is controller specific, and must
+be documented in the device tree binding for the device.

 Example of the node using GPIOs:

@@ -28,8 +56,8 @@ and empty GPIO flags as accepted by the "qe_pio_e"
gpio-controller.
 2) gpio-controller nodes
 ------------------------

-Every GPIO controller node must have #gpio-cells property defined,
-this information will be used to translate gpio-specifiers.
+Every GPIO controller node must both an empty "gpio-controller"
+property, and have #gpio-cells contain the size of the gpio-specifier.

 Example of two SOC GPIO banks defined as gpio-controller nodes: