[Bug 103025] [GM45] loss of driver acceleration after some time (bisected)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=103025

--- Comment #73 from Adric Blake <promarbler14 at gmail.com> ---
Getting back on track, I made a decent attempt at debugging the (underlying)
bug that is responsible for these issues.

Tested on kernel 4.18rc6+218. I reproduced the bug with full debug logging and
drm.debug=0x1f. When the bug would occur, it would hit an assert that I have
ignored in the xorg driver. Using the time of the assert, it is easy to line it
up with the ioctl result listed in the kernel logs. I determined the execution
of the ioctl by setting a logged global to certain values depending of the part
of the code that gets run.

Here is the trace of I915_GEM_SET_TILING ioctl:
deepest/latest call last
ioctl(I915_GEM_SET_TILING...) -> ret = -512
drm_ioctl_kernel(...)
i915_gem_set_tiling_ioctl(...)
i915_gem_object_set_tiling(...)
i915_gem_object_fence_prepare(...)
i915_vma_unbind(...)
i915_gem_active_retire(...)
i915_request_wait(...) -> ret = -ERESTARTSYS (512?)
signal_pending_state(...) -> ret != 0
<uncertain core kernel code follows...>

This is probably not the only kernel code path which causes the bug in the xorg
driver. My bug replication method tends to only hit the bug on one path in the
xorg driver, while a few other known paths get triggered by other uncertain
uses (see the asserts that I ignore).

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