Re: [PATCH 1/8] i2c: mux-pinctrl: Rework to honor disabled child nodes

From: Stephen Warren
Date: Tue Feb 17 2015 - 16:14:53 EST

On 02/17/2015 02:08 PM, Sebastian Hesselbarth wrote:
On 17.02.2015 21:46, Stephen Warren wrote:
On 02/17/2015 11:52 AM, Sebastian Hesselbarth wrote:
I2C mux pinctrl driver currently determines the number of sub-busses by
counting available pinctrl-names. Unfortunately, this requires each
incarnation of the devicetree node with different available sub-busses
to be rewritten.

Can you be more explicit about the problem here? Why does anything need
to be re-written if a child node is disabled; presumably there's no need
for the child bus numbers to be contiguous. In other words, with the
example in the existing DT binding doc:

i2cmux {
compatible = "i2c-mux-pinctrl";
pinctrl-names = "ddc", "pta", "idle";
pinctrl-0 = <&state_i2cmux_ddc>;
pinctrl-1 = <&state_i2cmux_pta>;
pinctrl-2 = <&state_i2cmux_idle>;

i2c@0 {
reg = <0>;
i2c@1 {
reg = <1>;

That would generate child busses 0 and 1. If I was to disable the i2c@0
node, then there would still be definitions for child busses 0 and 1 in
the DT, it's just that child bus 0 wouldn't actually exist at run-time.
I don't see what part of DT needs to be re-written to accomodate this?

The way the current driver works, to disable i2c@0 you'd have to remove
the pinctrl-0 state, pinctrl-names string at position 0, and the node

So, on Dove SoC there is three sub-busses, now consider one board A with
i2c0 and i2c1 enabled but board B with i2c0 and i2c2 enabled:


i2cmux {
pinctrl-names = "i2c0", "i2c1", "idle";
pinctrl-0 = <&state_for_i2c0>;
pinctrl-1 = <&state_for_i2c1>;



i2cmux {
pinctrl-names = "i2c0", "i2c2", "idle";
pinctrl-0 = <&state_for_i2c0>;
pinctrl-1 = <&state_for_i2c2>;
/* Note that this ^^^ is state_for_i2c2 */

while the approach with status = "disabled" allows all properties for
both board remain the same - except you'll enable either i2c1 or i2c2
sub-node on board level:

i2cmux {
pinctrl-names = "i2c0", "i2c1", "i2c2", "idle";
pinctrl-0 = <&state_for_i2c0>;
pinctrl-1 = <&state_for_i2c1>;
pinctrl-2 = <&state_for_i2c2>;


i2cmux {
i2c@0 { status = "okay"; };
i2c@1 { status = "okay"; };



i2cmux {
i2c@0 { status = "okay"; };
i2c@2 { status = "okay"; };

OK, that all makes sense, but I don't think there's any change at all to the binding; this can all be fixed in the driver without affecting the definition of the binding at all. At most all that's needed in the binding is a note to the effect that if a particular child node is disabled, then this has no effect at all on the requirements for the pinctrl properties.

In general, it is less about the binding but how the driver is written:
Number of sub-busses is determined by elements in pinctrl-names not
available (enabled) sub-nodes.

This patch reworks i2c-mux-pinctrl driver to count the number of
available sub-nodes instead. The rework should be compatible to the old
way of probing for sub-busses and additionally allows to disable unused
sub-busses with standard DT property status = "disabled".

This also amends the corresponding devicetree binding documentation to
reflect the new functionality to disable unused sub-nodes. While at it,
also fix two references to binding documentation files that miss an

diff --git a/Documentation/devicetree/bindings/i2c/i2c-mux-pinctrl.txt

-For each named state defined in the pinctrl-names property, an I2C
child bus
-will be created. I2C child bus numbers are assigned based on the
index into
-the pinctrl-names property.
+For each child node that is not disabled by a status != "okay", an I2C
+child bus will be created. I2C child bus numbers are assigned based
on the
+order of child nodes.

I would have assumed that disabled sub-nodes was a global concept within
DT, and so wouldn't be mentioned in the binding. It would just be a bug
in the driver if it didn't ignore disabled sub-nodes.

Yep, the concept is very global. It is about the current driver and this
binding changes are just to make it a little more clear that the driver
should behave different, i.e. get rid of anything that implies that
pinctrl-names has any effect on the number of sub-busses registered.

-The only exception is that no bus will be created for a state named
"idle". If
-such a state is defined, it must be the last entry in pinctrl-names.
- pinctrl-names = "ddc", "pta", "idle" -> ddc = bus 0, pta = bus 1
- pinctrl-names = "ddc", "idle", "pta" -> Invalid ("idle" not last)
- pinctrl-names = "idle", "ddc", "pta" -> Invalid ("idle" not last)
+There must be a corresponding pinctrl-names entry for each enabled
+node at the position of the child node's "reg" property.

The addition there seems fine, but the existing text re: the idle state
seems clearer in the original text.

Ok, I'll have a look at how to preserve this section better.

Do you still have one of the current boards available for testing?

Yes, I have both Seaboard and Ventana still (the two Tegra boards that use this driver). I haven't used them in a while; I hope they still work:-)
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