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

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

On Tue, 14 Sep 2021, Vasily Khoruzhick <anarsoul@gmail.com> wrote:
> On Tue, Sep 14, 2021 at 3:31 PM Jani Nikula <jani.nikula@linux.intel.com> wrote:
>>
>> On Tue, 14 Sep 2021, Lyude Paul <lyude@redhat.com> wrote:
>> > On Tue, 2021-09-14 at 12:09 +0300, Jani Nikula wrote:
>> >> 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.
>> >
>> > This makes sense to me, you're referring to the panel delays in the VBT
>> > correct?
>>
>> Yes. See pps_init_delays() and intel_pps_backlight_on(). The delays
>> aren't applied to DPCD backlight, I think it would make sense if they
>> were.
>
> I guess it explains why it usually stops working after suspend.
> Probably BIOS doesn't re-init the power sequencer on resume.

The point is, the DPCD backlight isn't driven via the power sequencer,
while the PWM pin would be.

Please file a bug at [1], and attach /sys/kernel/debug/dri/0/i915_vbt as
well as dmesg from boot with drm.debug=14 module parameter set.

Thanks,
Jani.


[1] https://gitlab.freedesktop.org/drm/intel/issues/new



>
>> BR,
>> Jani.
>>
>> >
>> >>
>> >> 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

-- 
Jani Nikula, Intel Open Source Graphics Center