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    SubjectRe: [PATCH] drm/i915/dpcd_bl: Don't try vesa interface unless specified by VBT
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    Actually-NAK this. I just realized I've been misreading the bug and that this
    doesn't actually seem to be fixed. Will resend once I figure out what's going on

    On Thu, 2021-03-18 at 13:02 -0400, Lyude Paul wrote:
    > Looks like that there actually are another subset of laptops on the market
    > that don't support the Intel HDR backlight interface, but do advertise
    > support for the VESA DPCD backlight interface despite the fact it doesn't
    > seem to work.
    >
    > Note though I'm not entirely clear on this - on one of the machines where
    > this issue was observed, I also noticed that we appeared to be rejecting
    > the VBT defined backlight frequency in
    > intel_dp_aux_vesa_calc_max_backlight(). It's noted in this function that:
    >
    > /* Use highest possible value of Pn for more granularity of brightness
    >  * adjustment while satifying the conditions below.
    >  * ...
    >  * - FxP is within 25% of desired value.
    >  *   Note: 25% is arbitrary value and may need some tweak.
    >  */
    >
    > So it's possible that this value might just need to be tweaked, but for now
    > let's just disable the VESA backlight interface unless it's specified in
    > the VBT just to be safe. We might be able to try enabling this again by
    > default in the future.
    >
    > Fixes: 2227816e647a ("drm/i915/dp: Allow forcing specific interfaces through
    > enable_dpcd_backlight")
    > Cc: Jani Nikula <jani.nikula@intel.com>
    > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
    > Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3169
    > Signed-off-by: Lyude Paul <lyude@redhat.com>
    > ---
    >  drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 1 -
    >  1 file changed, 1 deletion(-)
    >
    > 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 651884390137..4f8337c7fd2e 100644
    > --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
    > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
    > @@ -646,7 +646,6 @@ int intel_dp_aux_init_backlight_funcs(struct
    > intel_connector *connector)
    >                         break;
    >                 case INTEL_BACKLIGHT_DISPLAY_DDI:
    >                         try_intel_interface = true;
    > -                       try_vesa_interface = true;
    >                         break;
    >                 default:
    >                         return -ENODEV;

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

    Note: I deal with a lot of emails and have a lot of bugs on my plate. If you've
    asked me a question, are waiting for a review/merge on a patch, etc. and I
    haven't responded in a while, please feel free to send me another email to check
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