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A lot of modern laptops use the Parade PS8461E MUX for eDP switching. The MUX can operate in jitter cleaning mode or redriver mode, the first one resulting in higher link quality. The jitter cleaning mode needs to know the link rate used and the MUX achieves this by snooping the LINK_BW_SET, LINK_RATE_SELECT and SUPPORTED_LINK_RATES DPCD accesses. When the MUX is powered down (seems this can happen whenever the display is turned off) it loses track of the snooped link rates so when we do the LINK_RATE_SELECT write it no longer knowns which link rate we're selecting, and thus it falls back to the lower quality redriver mode. This results in unstable high link rates (eg. usually 8.1Gbps link rate no longer works correctly). In order to avoid all that let's re-snoop SUPPORTED_LINK_RATES from the sink at the start of every link training. Unfortunately we don't have a way to detect the presence of the MUX. It looks like the set of laptops equipped with this MUX is fairly large and contains devices from multiple manufacturers. It may also still be growing with new models. So a quirk doesn't seem like a very easily maintainable option, thus we shall attempt to do this unconditionally on all machines that use LINK_RATE_SELECT. Hopefully this extra DPCD read doesn't cause issues for any unaffected machine. If that turns out to be the case we'll need to convert this into a quirk in the future. Cc: stable@vger.kernel.org Cc: Jason A. Donenfeld <Jason@zx2c4.com> Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com> Cc: Jani Nikula <jani.nikula@intel.com> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6205 Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20220902070319.15395-1-ville.syrjala@linux.intel.com Tested-by: Aaron Ma <aaron.ma@canonical.com> Tested-by: Jason A. Donenfeld <Jason@zx2c4.com> Reviewed-by: Jani Nikula <jani.nikula@intel.com> |
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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.