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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: 2227816e ("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>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318170204.513000-1-lyude@redhat.com