[Intel-gfx] [PATCH] drm/i915: Use AUX for backlight only if eDP 1.4 or later

[David Weinehall]

On 2017-08-02 02:15, Pandiyan, Dhinakaran wrote:
>
>
> On Mon, 2017-07-31 at 15:41 -0700, Puthikorn Voravootivat wrote:
>>> But now you're suggesting another arbitrary narrow selection of panels
>>> based on limited evidence.
>> I understand your point that the panel I observe is not the
>> representative of the real world.
>>
>> My point is that we don't know that the panel will work or not unless
>> we test all panel in the world.
>> And blacklist would be too much work to maintain and whitelist would
>> make this code too limited.
>> As standard adoption should be better over time, I suggest that the
>> newer panel should have
>> better implement of the standard than older panel. And I suggest that
>> eDP 1.4 should be a good
>> heuristic for the "newer panel" based on these 2 reasons
>>
>> 1. Even though it is a limited evident, David and I independently saw
>> unrelated eDP 1.3 panel that
>> implement this feature incorrectly.
>> 2. eDP 1.4 is the first version that support AUX backlight enablement.
>> TCON vendor probably also
>> make sure the AUX backlight brightness ajustment works when testing
>> that feature.
>>
>> Is this make sense?
>>
>> Thanks.
>>
> I tried to investigate this a little bit and found a device that
> reproduces the issue. The backlight does not come back up after a
> suspend-resume cycle because the PWM controller does not get enabled at
> resume. However, things just work at boot because the BIOS happens to
> enable PCH PWM at boot and the panel lights up via the BL_PWM_PIN. Like
> you said, this could be because some eDP 1.3 panels have a broken
> implementation and eDP 1.4 panels are better. Or, with the BL_PWM_PIN
> wired to the board, it simply overrides the DPCD settings. I decided to
> not disconnect the PWM pin and test this theory since this was a
> development laptop. In summary, I am not really sure blacklisting all
> eDP 1.3 panels is the best idea. Also, I don't know how many eDP 1.4
> panels this has been tested to correctly work on.
>
> Anyway, since we have four panels that do not work, we could check if
> these are the same model/make etc. and blacklist them if there's a
> common thread.
>
The panel in my system is made by LG, model LP140QH1-SPF1.


Kind regards, David
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