linux-kernel - Re: [PATCH v2 2/2] leds: Add Spreadtrum SC27xx breathing light controller driver

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Date:   Sat, 12 May 2018 22:44:14 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Baolin Wang <baolin.wang@...aro.org>, robh+dt@...nel.org,
        mark.rutland@....com, xiaotong.lu@...eadtrum.com,
        broonie@...nel.org, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] leds: Add Spreadtrum SC27xx breathing light
 controller driver

Hi Pavel,

On 05/12/2018 10:35 AM, Pavel Machek wrote:
> Hi!
>
>>>> I disagree here. We already had the same discussion at the occasion
>>>> of the patch [0] and it turned out to be a dead-end [1]. Now we have
>>>> neither the driver nor the generic pattern interface.
>>>>
>>>> We also already have some older LED class drivers that implement custom
>>>> pattern interfaces (e.g. drivers/leds/leds-lm3533.c) and the same
>>>> approach can be applied in this case.
>>>
>>> Please don't. It was mistake to implement custom pattern interfaces
>>> back then, it is still mistake now.
>>
>> It turned out to be really hard to cover all known pattern generator
>> implementations with generic interface. Sure, it would be nice to have
>> one, but the whole discussion around [0] only unveiled the diversity of
>> parameters to cover. And still new devices appear on the market.
>>
>> We would have to propose a set of pattern schemes and allow to
>> add new ones to it.
>
> I believe that what I'm proposing below is close enough to universal.
>
>>> If we really need solution now, I'd recommend "pattern" file with
>>>
>>> "<delta time> <brightness> <delta time> <brightness>".
>>>
>>> In this specific case, hardware only supports patterns in this format:
>>>
>>> low_time 0 rise_time 255 high_time 255 fall_time 0
>>>
>>> so driver would simply -EINVAL on anything else.
>>
>> I'm fine with the pattern file, but the pattern format would have
>> to be defined in the per-driver ABI documentation. It wouldn't much
>> differ from the custom pattern approach though, unless I'm missing some
>> gain of having pattern setting in a uniformly named single sysfs file
>> (with semantics differing from driver to driver).
>
> I'm proposing "<delta time> <brightness> ..." sysfs file. It certainly
> covers this hardware, it would be enough to cover the Qualcomm Pulse
> generator (IIRC), and it would cover most uses cases of Nokia N900's
> LED.
>
> Yes, we would need to document limitations of each chip. But it should
> be easily possible to run pattern designed for Spreadtrum on N900,
> even if it would not work the other way around.
>
> (If someone really wants to run complex patterns on simple hardware,
> we can provide software emulation using same file format. I believe I
> still have that patch somewhere.)

OK, I've revised the discussion under Qualcomm LPG patch set and
it seems that we have almost ready solution in [0], except the
pattern_repeat file you mention in [1]. So probably Baolin could
address your remarks from [1] and add pattern_repeat file to the
patch that begins thread [0].

[0] https://lkml.org/lkml/2017/11/15/27
[1] https://lkml.org/lkml/2017/12/8/470

-- 
Best regards,
Jacek Anaszewski