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

[Pandiyan, Dhinakaran]

On Wed, 2017-07-19 at 15:59 +0530, Jenny TC wrote:
> With older panels, AUX pin for backlight doesn't work. On some
> panels, this causes backlight issues on S3 resume.

What is it that we are missing in the resume path?

>  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. 


> 
> 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) {