Re: [PATCH] drm/i915/dp: add a delay before setting panel brightness after power on

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 13 Sep 2021, Vasily Khoruzhick <anarsoul@gmail.com> wrote:
> Panel in my Dell XPS 7590, that uses Intel's HDR backlight interface to
> control brightness, apparently needs a delay before setting brightness
> after power on. Without this delay the panel does accept the setting
> and may come up with some arbitrary brightness (sometimes it's too dark,
> sometimes it's too bright, I wasn't able to find a system).
>
> I don't have access to the spec, so I'm not sure if it's expected
> behavior or a quirk for particular device.
>
> Delay was chosen by experiment: it works with 100ms, but fails with
> anything lower than 75ms.

Looks like we don't respect the panel delays for DPCD backlight. The
values are used for setting up the panel power sequencer, and thus PWM
based backlight, but we should probably use the delays in DPCD backlight
code too.

BR,
Jani.


>
> Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> index 4f8337c7fd2e..c4f35e1b5870 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -210,6 +210,10 @@ intel_dp_aux_hdr_enable_backlight(const struct intel_crtc_state *crtc_state,
>  
>  	ctrl = old_ctrl;
>  	if (panel->backlight.edp.intel.sdr_uses_aux) {
> +		/* Wait 100ms to ensure that panel is ready otherwise it may not
> +		 * set chosen backlight level
> +		 */
> +		msleep(100);
>  		ctrl |= INTEL_EDP_HDR_TCON_BRIGHTNESS_AUX_ENABLE;
>  		intel_dp_aux_hdr_set_aux_backlight(conn_state, level);
>  	} else {

-- 
Jani Nikula, Intel Open Source Graphics Center