[PATCH] drm/i915/display: Fixed a screen flickering when turning on display from off

[Jani Nikula]

On Thu, 21 Mar 2024, gareth.yu at intel.com wrote:
> From: Gareth Yu <gareth.yu at intel.com>
>
> Turn on the panel from zero brightness of the last state, the panel was
> set a maximum PWM in the flow. Once the panel initialization is completed,
> the backlight is restored to xero brightness. There is a flckering
> generated. This flicker happens in "Screen dimming and power off" of
> Google's design and resume from sleep. The sample of DMESG is below.
>
> (suspend)
> [53949.248875] i915 0000:00:02.0: [drm:intel_edp_backlight_off]
> [53949.452046] i915 0000:00:02.0: [drm:intel_backlight_set_pwm_level] set backlight PWM = 0
>
> (wakeup)
> [53986.067356] i915 0000:00:02.0: [drm:intel_edp_backlight_on]
> [53986.067367] i915 0000:00:02.0: [drm:intel_backlight_enable] pipe A
> [53986.067476] i915 0000:00:02.0: [drm:intel_backlight_set_pwm_level] set backlight PWM = 96000
> [53986.119766] backlight intel_backlight: PM: calling backlight_resume+0x0/0x7a @ 4916, parent: card0-eDP-1
> [53986.119781] backlight intel_backlight: PM: backlight_resume+0x0/0x7a returned 0 after 0 usecs
> [53986.220068] [drm:intel_backlight_device_update_status] updating intel_backlight, brightness=26321/96000
> [53986.220086] i915 0000:00:02.0: [drm:intel_panel_actually_set_backlight] set backlight level = 27961
>
> Set the brightness to the minimum value when the new brightness is less
> or equal to the minimum value to mitigate this flickering.

Thanks for the patch. I've pushed this to drm-intel-next, but took the
liberty to rewrite the commit message:

commit 92714006eb4d10ddfaa0eca41c81e6b483e02f93
Author: Gareth Yu <gareth.yu at intel.com>
Date:   Thu Mar 21 12:53:11 2024 +0800

    drm/i915/backlight: Do not bump min brightness to max on enable
    
    Historically the expectation was to set brightness to max on enable, if
    it was zero. However, the policy should be to preserve brightness across
    disable/enable, for example the userspace might want to dim the
    brightness before disable (e.g. on suspend) and gradually bring it back
    up after enable (e.g. on resume). If the brightness gets bumped to max
    at enable, this causes flicker as the userspace expects the brightness
    to have been preserved across disable/enable.
    
    For example:
    
    (suspend)
    [53949.248875] i915 0000:00:02.0: [drm:intel_edp_backlight_off]
    [53949.452046] i915 0000:00:02.0: [drm:intel_backlight_set_pwm_level] set backlight PWM = 0
    
    (wakeup)
    [53986.067356] i915 0000:00:02.0: [drm:intel_edp_backlight_on]
    [53986.067367] i915 0000:00:02.0: [drm:intel_backlight_enable] pipe A
    [53986.067476] i915 0000:00:02.0: [drm:intel_backlight_set_pwm_level] set backlight PWM = 96000
    [53986.119766] backlight intel_backlight: PM: calling backlight_resume+0x0/0x7a @ 4916, parent: card0-eDP-1
    [53986.119781] backlight intel_backlight: PM: backlight_resume+0x0/0x7a returned 0 after 0 usecs
    [53986.220068] [drm:intel_backlight_device_update_status] updating intel_backlight, brightness=26321/96000
    [53986.220086] i915 0000:00:02.0: [drm:intel_panel_actually_set_backlight] set backlight level = 27961
    
    Reduce the check to respecting the minimum brightness, and avoid bumping
    min brightness to max on enable.
    
    Note: It's possible there's still userspace out there that relies on the
    old behaviour. It would be unfortunate, but there's really only one way
    to find out.

Fingers crossed, let's hope nothing regresses.

BR,
Jani.


>
> Cc : Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay at intel.com>
> Cc : Matt Roper <matthew.d.roper at intel.com>
> Cc : Ville Syrjälä <ville.syrjala at linux.intel.com>
> Signed-off-by: Gareth Yu <gareth.yu at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_backlight.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
> index 3f3cd944a1c5..23fd011b6bfb 100644
> --- a/drivers/gpu/drm/i915/display/intel_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_backlight.c
> @@ -761,8 +761,8 @@ static void __intel_backlight_enable(const struct intel_crtc_state *crtc_state,
>  
>  	WARN_ON(panel->backlight.max == 0);
>  
> -	if (panel->backlight.level <= panel->backlight.min) {
> -		panel->backlight.level = panel->backlight.max;
> +	if (panel->backlight.level < panel->backlight.min) {
> +		panel->backlight.level = panel->backlight.min;
>  		if (panel->backlight.device)
>  			panel->backlight.device->props.brightness =
>  				scale_hw_to_user(connector,

-- 
Jani Nikula, Intel