Quoting Chris Wilson (2018-12-01 09:52:31)
> Braswell is really picky about having our writes posted to memory before
> we execute or else the GPU may see stale values. A wmb() is insufficient
> as it only ensures the writes are visible to other cores, we need a full
> mb() to ensure the writes are in memory and visible to the GPU.
>
> The most frequent failure in flushing before execution is that we see
> stale PTE values and execute the wrong pages.
>
> References: 987abd5c62f9 ("drm/i915/execlists: Force write serialisation into context image vs execution")
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> drivers/gpu/drm/i915/intel_lrc.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 0a690c557113..cad1228f6503 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -379,8 +379,12 @@ static u64 execlists_update_context(struct i915_request *rq)
> * may not be visible to the HW prior to the completion of the UC
> * register write and that we may begin execution from the context
> * before its image is complete leading to invalid PD chasing.
> + *
> + * Furthermore, Braswell wants a full mb to be sure that the writes
> + * are coherent in memory (visible to the GPU) prior to execution,
> + * and not just visible to other CPUs (as is the result of wmb).
> */
> - wmb();
> + mb();
Scratch that. Just took ages to hit this time. It seemed so promising!
-Chris
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