[Bug 92448] New: i915 driver tracks relative_constants_mode incorrectly when using execlists

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92448

            Bug ID: 92448
           Summary: i915 driver tracks relative_constants_mode incorrectly
                    when using execlists
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: minor
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: david.s.gordon at intel.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

'relative_constants_mode' has always been tracked per-device, but whereas this
was adequate in legacy ringbuffer mode, it's wrong in execlists (or GuC) mode,
as the INSTPM register is saved and restored with the logical context, and the
per-context value could therefore get out of sync with the tracked value.

The test case (if anyone wants to write it) would be to create two separate
logical contexts, submit a render batch with a non-default mode in the first
context, submit a render batch with the default mode in the other context, then
submit a second batch in the first context, but this time selecting the default
mode. The driver will fail to insert the instructions to reset INSTPM into the
first context's ringbuffer, as it will compare the mode required for the new
batch with the last mode set (which will be in a different context). The batch
will then be executed with the incorrect mode setting.

The bug has been there since the first implementation of execlists:

ba8b7ccb1 drm/i915/bdw: Workload submission mechanism for Execlists

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