4.9-stable review patch. If anyone has any objections, please let me know.
------------------ From: Chris Wilson <[email protected]> commit cf66b8a0ba142fbd1bf10ac8f3ae92d1b0cb7b8f upstream. 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 <[email protected]> Cc: Mika Kuoppala <[email protected]> Cc: Tvrtko Ursulin <[email protected]> Cc: Joonas Lahtinen <[email protected]> Cc: [email protected] Reviewed-by: Tvrtko Ursulin <[email protected]> Link: https://patchwork.freedesktop.org/patch/msgid/[email protected] (cherry picked from commit 490b8c65b9db45896769e1095e78725775f47b3e) Signed-off-by: Joonas Lahtinen <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- drivers/gpu/drm/i915/intel_lrc.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -386,8 +386,13 @@ static u64 execlists_update_context(stru * 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, at least, 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; }
