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

[gareth.yu at intel.com]

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.

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,
-- 
2.25.1