[Intel-gfx] [PATCH 1/2] drm/i915: Wait for completion of pending flips when starved of fences

[Chris Wilson]

On Mon, Jan 20, 2014 at 10:17:36AM +0000, Chris Wilson wrote:
> On older generations (gen2, gen3) the GPU requires fences for many
> operations, such as blits. The display hardware also requires fences for
> scanouts and this leads to a situation where an arbitrary number of
> fences may be pinned by old scanouts following a pageflip but before we
> have executed the unpin workqueue. This is unpredictable by userspace
> and leads to random EDEADLK when submitting an otherwise benign
> execbuffer. However, we can detect when we have an outstanding flip and
> so cause userspace to wait upon their completion before finally
> declaring that the system is starved of fences. This is really no worse
> than forcing the GPU to stall waiting for older execbuffer to retire and
> release their fences before we can reallocate them for the next
> execbuffer.
> 
> v2: move the test for a pending fb unpin to a common routine for
> later reuse during eviction
> 
> Reported-and-tested-by: dimon at gmx.net
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73696
Testcase: i-g-t/kms_flip/flip-vs-fences
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

The testcase is only for the trivial reproduction scenario, not the more
sporadic situation involving a slightly hungry workqueue, but it is
enough to demonstrate the issue and the effectiveness of the simple
workaround (for typical userspace).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre