[Bug 107691] New: Invalid data in error state

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107691

            Bug ID: 107691
           Summary: Invalid data in error state
           Product: DRI
           Version: XOrg git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: lionel.g.landwerlin at linux.intel.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

As part of debugging hangs we're starting to look more at the error states
generated by i915.
Together with Jason we've noticed that the data produced is incorrect.
Chunks of 32 bytes appear to just go missing (replaced by 0s).

For example this bug report :
https://bugs.freedesktop.org/show_bug.cgi?id=107586
has 2 error state from which you can see that the context image has its first
32bytes at 0s.
What we should be finding at that location looks more like this :

https://gitlab.freedesktop.org/mesa/mesa/blob/master/src/intel/tools/gen8_context.h#L27

MI_NOOP followed by MI_LRI.

We've noticed this issue on both Skylake & Kabylake (I believe this affects at
least all big cores).

To workaround this issue I came up with this patch :
https://github.com/djdeath/linux/commit/c18d4e1ee66cf587c484a60bba64f3dc4f35fc2e
This is probably wrong as pointed by Chris on IRC, but it gets us correct data
in the error state.

This issue can be easily reproduced by running the IGT drv_hangman test on the
render ring and checking the content of the "rcs0 --- HW context" BO in the
error state. At offset 8092 we should find the MI_NOOP following by MI_LRI I
pointed above, but instead of 32 bytes of 0s.

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