Re: [PATCH/RFC v10 03/19] DT: leds: Add led-sources property

[Rob Herring <robherring2@gmail.com>]

On Thu, Jan 15, 2015 at 6:33 AM, Sylwester Nawrocki
<s.nawrocki@samsung.com> wrote:
> On 12/01/15 18:06, Mark Brown wrote:
>> On Mon, Jan 12, 2015 at 10:55:29AM -0600, Rob Herring wrote:
>>> > On Mon, Jan 12, 2015 at 10:10 AM, Jacek Anaszewski
>>>> > > There are however devices that don't fall into this category, i.e. they
>>>> > > have many outputs, that can be connected to a single LED or to many LEDs
>>>> > > and the driver has to know what is the actual arrangement.
>>> >
>>> > We may need to extend the regulator binding slightly and allow for
>>> > multiple phandles on a supply property, but wouldn't something like
>>> > this work:
>>> > led-supply = <&led-reg0>, <&led-reg1>, <&led-reg2>, <&led-reg3>;
>>> > The shared source is already supported by the regulator binding.
>>
>> What is the reasoning for this?  If a single supply is being supplied by
>> multiple regulators then in general those regulators will all know about
>> each other at a hardware level and so from a functional and software
>> point of view will effectively be one regulator.  If they don't/aren't
>> then they tend to interfere with each other.
>
> For LED current regulators like this one [1] we want to be able to
> communicate to the software the hardware wiring, e.g. if a single LED is
> connected to only one or both the current regulators.  The device needs
> to be programmed differently for each configuration, as shown on page 36
> of the datasheet [2].
>
> Now, the LED DT binding describes the LEDs (current consumers) as child
> nodes of the LED driver IC (current supplier), e.g. (from [3]):
>
> pca9632@62 {
>         compatible = "nxp,pca9632";
>         #address-cells = <1>;
>         #size-cells = <0>;
>         reg = <0x62>;
>
>         red@0 {
>                 label = "red";
>                 reg = <0>;

This only works if you don't have sub blocks or different functions to
describe. I suppose you could add yet another level of nodes. This
feels like abuse of the reg property even though to use the reg
property is a frequent review comment.

OTOH, we don't need 2 ways to describe this.

>                 linux,default-trigger = "none";
>         };
>         green@1 {
>                 label = "green";
>                 reg = <1>;
>                 linux,default-trigger = "none";
>         };
>         ...
> };
>
> What is missing in this binding is the ability to tell that a single LED
> is connected to more than one current source.
>
> We could, for example adopt the multiple phandle in the supply property
> scheme, but not use the kernel regulator API, e.g.
>
> flash-led {
>          compatible = "maxim,max77387";
>
>          current-reg1 { // FLED1
>                  led-output-id = <0>;
>          };
>
>          current-reg2 { // FLED2
>                  led-output-id = <1>;
>          };
>
>          red_led {
>                  led-supply = <&current-reg1>, <&current-reg2>;
>          };
> };
>
> However my feeling is that it is unnecessarily complicated that way.

This example is not so complicated, but I already agreed on not using
regulators on the basis there are other properties of the driver
unique to LEDs.

> Perhaps we could use the 'reg' property to describe actual connections,
> I'm not sure if it's better than a LED specific property, e.g.
>
> max77387@52 {
>         compatible = "nxp,max77387";
>         #address-cells = <2>;
>         #size-cells = <0>;
>         reg = <0x52>;
>
>         flash_led {
>                 reg = <1 1>;

Don't you mean <0 1> as the values are the "address" which in this
case are the LED driver output indexes.

Rob

>                 ...
>         };
> };
>
> [1] http://www.maximintegrated.com/en/products/power/led-drivers/MAX77387.html
> [2] http://datasheets.maximintegrated.com/en/ds/MAX77387.pdf
> [3] Documentation/devicetree/bindings/leds/pca963x.txt