Re: [PATCH v4] drm/i915: Advance ring->head fully when idle

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On to, 2017-04-06 at 18:00 +0100, Chris Wilson wrote:
> When we retire the last request on the ring, before we ever access that
> ring again we know it will be completely idle and so we can advance the
> ring->head fully to the end (i.e. ring->tail) and not just to the start
> of the breadcrumb. This allows us to skip re-emitting the breadcrumb
> after resetting the GPU if the ring was entirely idle. This prevents us
> from overwriting a seqno wraparound by re-executing a stale breadcrumb,
> i.e.
> 	submit_request(1)
> 	intel_engine_init_global_seqno(0)
> 	i915_reset()
> would then leave 1 in the HWS, but the next request to execute would
> also be with seqno 1. The sanity checks upon submission detect this as a
> timewarp and explode. By setting the ring as empty, upon reset the HWS
> is left as 0, leaving it consistent with the timeline.
> 
> v2: Fix check for deleting last element of list. We know that this
> request is always the first element of the ring, so only if next
> points back to the start will this be the only request in flight.
> v3: Remove opencoding of list_is_last()
> v4: Move the block to its own function for some clarity.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100144
> Testcase: igt/gem_exec_whisper/hang-*
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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