[Bug 97940] New: Short desktop freezes with Radeon 7850

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97940

            Bug ID: 97940
           Summary: Short desktop freezes with Radeon 7850
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/Radeon
          Assignee: xorg-driver-ati at lists.x.org
          Reporter: daniel.lichtenberger at gmail.com
        QA Contact: xorg-team at lists.x.org

Created attachment 126797
  --> https://bugs.freedesktop.org/attachment.cgi?id=126797&action=edit
test program to list all connectors

Kernel: openSUSE Tumbleweed 4.7.4-2-default
HW: Radeon 7850 2GB, dual DVI monitors

I have an annoying issue on my Radeon 7850. At some point in
the kernel 4.x timeframe, the entire desktop started to freeze for
short periods (< 1 second), almost exclusively during startup when
the desktop applets/widgets were loading. It's very
noticeable/annoying because the mouse cursor freezes as well.

Aside from the startup freezes that happen every time, the issue 
is hard to reproduce (I get also short freezes when opening a PDF 
in a new instance of Okular - the KDE PDF viewer).

With the help of perf I stumbled over suspicious traces of DRM
getconnector calls. I hacked together a small program that reproduces
the issue on my system: when run in a loop, I can barely move the
mouse cursor and I get perf traces like the following:


   - drm_helper_probe_single_connector_modes
-   45,15%     0,00%  drm-getconnecto  [kernel.kallsyms]
[k] drm_helper_probe_single_connector_modes
      - 41,70% radeon_dvi_detect
         - 40,19% radeon_connector_get_edid
            - drm_get_edid
               - 39,30% drm_do_get_edid
                    drm_do_probe_ddc_edid
                    i2c_transfer
                    __i2c_transfer
                  - bit_xfer
                     + 16,45% sclhi
                       15,74% delay_tsc
                     + 4,39% acknak
                       1,14% set_clock
                       0,76% get_data
                     + 0,54% try_address
               + 0,89% drm_do_probe_ddc_edid
         + 1,51% radeon_ddc_probe
      + 3,37% radeon_dp_detect

The program enumerates the VGA connectors with DRM_IOCTL_MODE_GETRESOURCES and
DRM_IOCTL_MODE_GETCONNECTOR syscalls. It's the first call to
DRM_IOCTL_MODE_GETCONNECTOR that seems to cause the issues on my system.

After I stop the program all is well again, the system is stable and I
experience no other issues. 

When I ran the program on another system with a Radeon 5750 and a
recent kernel (4.5 or 4.6), it did not cause the intermittent freezes and
worked faster.

I'm running a dual DVI monitor setup and KDE 5. The issue also happens
under other window managers/DEs (IceWM, XFCE), so I don't think it is
related to KDE (but the pattern of connector lookups may have changed in a
recent Qt or xorg version so that the issue became more noticeable).

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