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

[Lyude Paul]

On Tue, 2021-09-14 at 12:09 +0300, Jani Nikula wrote:
> On Mon, 13 Sep 2021, Vasily Khoruzhick <anarsoul at 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.

This makes sense to me, you're referring to the panel delays in the VBT
correct?

> 
> BR,
> Jani.
> 
> 
> > 
> > Signed-off-by: Vasily Khoruzhick <anarsoul at 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 {
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat