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

[Jacek Anaszewski]

On 01/12/2015 05:55 PM, Rob Herring wrote:
> Adding Mark B and Liam...
>
> On Mon, Jan 12, 2015 at 10:10 AM, Jacek Anaszewski
> <j.anaszewski@xxxxxxxxxxx> wrote:
> > On 01/12/2015 02:52 PM, Rob Herring wrote:
> > > On Mon, Jan 12, 2015 at 2:32 AM, Jacek Anaszewski
> > > <j.anaszewski@xxxxxxxxxxx> wrote:
> > > > On 01/09/2015 07:33 PM, Rob Herring wrote:
> > > > > On Fri, Jan 9, 2015 at 9:22 AM, Jacek Anaszewski
> > > > > <j.anaszewski@xxxxxxxxxxx> wrote:
> > > > > > Add a property for defining the device outputs the LED
> > > > > > represented by the DT child node is connected to.
>
> [...]
>
> > > > > > b/Documentation/devicetree/bindings/leds/common.txt
> > > > > > index a2c3f7a..29295bf 100644
> > > > > > --- a/Documentation/devicetree/bindings/leds/common.txt
> > > > > > +++ b/Documentation/devicetree/bindings/leds/common.txt
> > > > > > @@ -1,6 +1,10 @@
> > > > > >     Common leds properties.
> > > > > >
> > > > > >     Optional properties for child nodes:
> > > > > > +- led-sources : Array of bits signifying the LED current regulator
> > > > > > outputs the
> > > > > > +               LED represented by the child node is connected to (1 -
> > > > > > the LED
> > > > > > +               is connected to the output, 0 - the LED isn't connected
> > > > > > to the
> > > > > > +               output).
> > > > >
> > > > >
> > > > >
> > > > > Sorry, I just don't understand this.
> > > >
> > > >
> > > >
> > > > In some Flash LED devices one LED can be connected to one or more
> > > > electric current outputs, which allows for multiplying the maximum
> > > > current allowed for the LED. Each sub-LED is represented by a child
> > > > node in the DT binding of the Flash LED device and it needs to declare
> > > > which outputs it is connected to. In the example below the led-sources
> > > > property is a two element array, which means that the flash LED device
> > > > has two current outputs, and the bits signify if the LED is connected
> > > > to the output.
> > >
> > >
> > > Sounds like a regulator for which we already have bindings for and we
> > > have a driver for regulator based LEDs (but no binding for it).
> >
> >
> > Do you think of drivers/leds/leds-regulator.c driver? This driver just
> > allows for registering an arbitrary regulator device as a LED subsystem
> > device.
> >
> > 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.

I think that we shouldn't split the LED devices into power supply
providers and consumers as in case of generic regulators. From this
point of view a LED device current output is a provider and a discrete
LED element is a consumer. In this approach each discrete LED element
should have a related driver which is not how LED devices are being
handled in the LED subsystem, where there is a single binding for a LED
device and there is a single driver for it which creates separate LED
class devices for each LED connected to the LED device output. Each
discrete LED is represented by a child node in the LED device binding.

I am aware that it may be tempting to treat LED devices as common
regulators, but they have their specific features which gave a
reason for introducing LED class for them. Besides, there is already
drivers/leds/leds-regulator.c driver for LED devices which support only
turning on/off and setting brightness level.

In your proposition a separate regulator provider binding would have
to be created for each current output and a separate binding for
each discrete LED connected to the LED device. It would create
unnecessary noise in a dts file.

Moreover, using regulator binding implies that we want to treat it
as a sheer power supply for our device (which would be a discrete LED
element in this case), whereas LED devices provide more features like
blinking pattern and for flash LED devices - flash timeout, external
strobe and flash faults.

--
Best Regards,
Jacek Anaszewski
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