[Bug 82781] New: Option to disable mclk reclocking with AMD R9 280X (TAHITI) to avoid screen flickering on VGA/CRT

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=82781

            Bug ID: 82781
           Summary: Option to disable mclk reclocking with AMD R9 280X
                    (TAHITI) to avoid screen flickering on VGA/CRT
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.16.1
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: joker@netswarm.net
        Regression: No

I would like an option to forcefully disable mclk reclocking. Either as a
module parameter
or of course prefered at runtime as additional sysfs option in
/sys/class/drm/card[n]/device
probably with "power_<something>". The reason for this follows.

I switched from a AMD 5570 to a R9 280X card. Both have working and stable DPM.

With the new AMD R9 280X i noticed ocasiuonal screen flickering and narrowed it
down
to the mclk reclocking between "15000" level 0 and "160000" level 1-3.

By connecting my TV over HDMI i noticed this would stop as multi head is known
to
disable mclk reclocking (I've checked si_dpm.c)

I now hardcoded "disable_mclk_switching = true;" there and have a perfectly
working
flicker free screen.

The sclk still changes fine and while probably not as power saving as full
reclocking
of course a lot better than totaly disabling DPM or forcing it to "low" and
"high" based
on the tasks i'm doing.

I've also noticed this effect on Windows using proprietary drivers even tough a
lot less
than on Linux. Therefor i expect this to be a hardware related issue probably
only visible
on VGA analog out to CRT screens, so i fear it's not fixable in an automatic
way.

An option to manualy disable this feature in sysfs would help a lot. At runtime
mclk reclocking
could be reenabled while DPMS is on (aka. the screen is turned off). If this
could be detected
automaticaly it would be even better.

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