[Intel-gfx] [PATCH 1/3] drm/i915: Fix eviction when the GGTT is idle but full

[Chris Wilson]

Quoting Chris Wilson (2017-10-11 15:06:05)
> In the full-ppgtt world, we can fill the GGTT full of context objects.
> These context objects are currently implicitly tracked by the requests
> that pin them i.e. they are only unpinned when the request is completed
> and retired, but we do not have the link from the vma to the request
> (anymore). In order to unpin those contexts, we have to issue another
> request and wait upon the switch to the kernel context.
> 
> The bug during eviction was that we assumed that a full GGTT meant we
> would have requests on the GGTT timeline, and so we missed situations
> where those requests where merely in flight (and when even they have not
> yet been submitted to hw yet). The fix employed here is to change the
> already-is-idle test to no look at the execution timeline, but count the
> outstanding requests and then check that we have switched to the kernel
> context. Erring on the side of overkill here just means that we stall a
> little longer than may be strictly required, but we only expect to hit
> this path in extreme corner cases where returning an erroneous error is
> worse than the delay.
> 
> v2: Logical inversion when swapping over branches.
> 
> Fixes: 80b204bce8f2 ("drm/i915: Enable multiple timelines")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
-Chris