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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:2227816e64("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 (cherry picked from commit9e2eb6d538) Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.