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

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Date:	Tue, 13 Jan 2015 09:42:41 +0100
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	linux-leds@...r.kernel.org,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Pavel Machek <pavel@....cz>, Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>, sakari.ailus@....fi,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH/RFC v10 03/19] DT: leds: Add led-sources property

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@...sung.com> wrote:
>> On 01/12/2015 02:52 PM, Rob Herring wrote:
>>>
>>> On Mon, Jan 12, 2015 at 2:32 AM, Jacek Anaszewski
>>> <j.anaszewski@...sung.com> wrote:
>>>> On 01/09/2015 07:33 PM, Rob Herring wrote:
>>>>> On Fri, Jan 9, 2015 at 9:22 AM, Jacek Anaszewski
>>>>> <j.anaszewski@...sung.com> 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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