[Bug 107835] DisplayPort audio stopped working in a kernel upgrade

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=107835

            Bug ID: 107835
           Summary: DisplayPort audio stopped working in a kernel upgrade
           Product: DRI
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: bugs@benmorris.org.uk

In a recent kernel upgrade, my DisplayPort audio output stopped working.
PulseAudio showed no available profiles for the device, while aplay appeared to
work normally but produced no sound. I'm running Debian Unstable, and I have a
Radeon RX 470.

I have bisected (in linux-stable), and narrowed down the start of the problem
to this commit:

commit 9168c089a2122606e2c01f6fbeda85ff950ac189 (HEAD, refs/bisect/bad)
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Tue Jul 10 12:56:45 2018 -0500

    Revert "drm/amd/display: Don't return ddc result and read_bytes in same
return value"

    commit 5292221d6ddfed75e5b46cd42237a677094b99f3 upstream.

    This reverts commit 018d82e5f02ef3583411bcaa4e00c69786f46f19.

    This breaks DDC in certain cases.  Revert for 4.18 and previous kernels.
    For 4.19, this is fixed with the following more extensive patches:
    drm/amd/display: Serialize is_dp_sink_present
    drm/amd/display: Break out function to simply read aux reply
    drm/amd/display: Return aux replies directly to DRM
    drm/amd/display: Right shift AUX reply value sooner than later
    drm/amd/display: Read AUX channel even if only status byte is returned

    Link: https://lists.freedesktop.org/archives/amd-gfx/2018-July/023788.html
    Acked-by: Harry Wentland <harry.wentland@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


I have tried kernel 4.19-rc2, and, as suggested in the commit message, the
audio problem does indeed seem to be resolved in 4.19.

I have never seen the stacktrace associated with the same commit at
https://bugzilla.kernel.org/show_bug.cgi?id=200667

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