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From:   Baolin Wang <baolin.wang@linaro.org>
Date:   Sun, 13 May 2018 10:19:37 +0800
Message-ID: <CAMz4kuLJbq0G4TGoRp_if1mwe_4pa69haJvmT9SoRJ-UwGDa_g@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] leds: Add Spreadtrum SC27xx breathing light
 controller driver
To:     Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc:     Pavel Machek <pavel@ucw.cz>, Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        xiaotong.lu@spreadtrum.com, Mark Brown <broonie@kernel.org>,
        linux-leds@vger.kernel.org, DTML <devicetree@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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Hi Jacek and Pavel,

On 13 May 2018 at 04:44, Jacek Anaszewski <jacek.anaszewski@gmail.com> wrote:
> 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

Thanks for your suggestion. So I will remove the sysfs part from the
new driver, then send incremental patches when introducing some common
LED interfaces.

-- 
Baolin.wang
Best Regards