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

[David Weinehall]

On 2017-07-20 12:33, Jani Nikula wrote:
> On Wed, 19 Jul 2017, "Pandiyan, Dhinakaran" <dhinakaran.pandiyan at intel.com> wrote:
>> On Wed, 2017-07-19 at 15:59 +0530, Jenny TC wrote:
>>> With older panels, AUX pin for backlight doesn't work.
> What evidence do you have to back up that claim?
Chapter 10.1 does, at least according to my reading, state that AUX CH 
backlight control
requires eDP v1.4.

>>> On some panels, this causes backlight issues on S3 resume.
>> What is it that we are missing in the resume path?
I suspect this isn't so much a resume issue, as a matter of some devices 
by sheer luck (read by using the backlight set already on bootup) 
keeping the right backlight level until after a suspend/resume cycle.
>>
>>>   Enable the
>>> feature only for eDP1.4 or later.
>> I can't find the eDP 1.4 requirement in the spec. Regional brightness
>> control was added in eDP 1.4, but we don't enable that. My concern is we
>> might be missing a real fix by ignoring < eDP 1.4 panels.
> Agreed.
>
> This has been going on too long now, with no real effort to get at the
> roots of the problem. The right approach is to revert the regressing
> commits now, and start over. Reverts posted [1].
>
> BR,
> Jani.
>
> [1] https://patchwork.freedesktop.org/series/27623/
>
>>
>>> Fix-suggested-by: Puthikorn Voravootivat <puthik at chromium.org>
>> 1. Please use the "Fixes" tag to refer to the commit that introduced the
>> code you are fixing.
>> 2. The "Suggested-by" tag is more common to give credits to the person
>> who suggested the fix.
>> 3. Please use the "Bugzilla" tag to point to the bug that David
>> reported.
>>
>>
>>> Signed-off-by: Jenny TC <jenny.tc at intel.com>
>>> ---
>>>   drivers/gpu/drm/i915/intel_dp_aux_backlight.c | 6 +++++-
>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>>> index b25cd88..421f31f 100644
>>> --- a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>>> +++ b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>>> @@ -292,7 +292,7 @@ static int intel_dp_aux_setup_backlight(struct intel_connector *connector,
>>>    * via PWM pin or using AUX is better than using PWM pin.
>>>    *
>>>    * The heuristic to determine that using AUX pin is better than using PWM pin is
>>> - * that the panel support any of the feature list here.
>>> + * that the panel is eDP 1.4 or later and support any of the feature list here
>>>    * - Regional backlight brightness adjustment
>>>    * - Backlight PWM frequency set
>>>    * - More than 8 bits resolution of brightness level
>>> @@ -310,6 +310,10 @@ static int intel_dp_aux_setup_backlight(struct intel_connector *connector,
>>>   	if (!(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_PWM_PIN_CAP))
>>>   		return true;
>>>   
>>> +	/* Enable this for eDP 1.4 panel or later. */
>>> +	if (intel_dp->edp_dpcd[0] < DP_EDP_14)
>>> +		return false;
>>> +
>>>   	/* Panel supports regional backlight brightness adjustment */
>>>   	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_GENERAL_CAP_3,
>>>   			      &reg_val) != 1) {

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