Re: [PATCH 6/8] drm/i915: Disable semaphores on Sandybridge

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

On Tue, 2017-11-14 at 21:30 +0000, Chris Wilson wrote:
> I should have admitted defeat long ago as there has been a rare but
> persistent error on Sandybridge where semaphore signaling did not
> propagate to the waiter, leading to a GPU hang.
> 
> With the work on fence signaling for v4.9, the impact of using CPU driven
> signaling was greatly reduced wrt to the latency of GPU semaphores,
> though without logical rings support, the benefit of reordering work to
> avoid bubbles is not realised (i.e. as it stands fence signaling is just
> a slower, more costly version of HW semaphores; but works more
> consistently). As a rough indicator of the difference,
> 
> with semaphores:
> Sequential (3 engines, 1 processes): average 5.470us per cycle [expected 4.988us]
> 
> w/o semaphores:
> Sequential (3 engines, 1 processes): average 15.771us per cycle [expected 4.923us]
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54226
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

Might be worth mentioning that the bug casually has 110 duplicates...

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

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