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@chris-wilson.co.uk Cc: Mika Kuoppala mika.kuoppala@linux.intel.com Cc: Tvrtko Ursulin tvrtko.ursulin@intel.com Cc: Joonas Lahtinen joonas.lahtinen@linux.intel.com Cc: stable@vger.kernel.org --- 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(); return ce->lrc_desc; }
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@chris-wilson.co.uk Cc: Mika Kuoppala mika.kuoppala@linux.intel.com Cc: Tvrtko Ursulin tvrtko.ursulin@intel.com Cc: Joonas Lahtinen joonas.lahtinen@linux.intel.com Cc: stable@vger.kernel.org
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
linux-stable-mirror@lists.linaro.org
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Chris Wilson