On 26.05.2021 08:42, Matthew Brost wrote: > Ensure H2G buffer updates are visible before descriptor tail updates by > inserting a barrier between the H2G buffer update and the tail. The > barrier is simple wmb() for SMEM and is register write for LMEM. This is > needed if more than 1 H2G can be inflight at once. > > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx> > Cc: Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx> > --- > drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c > index fb875d257536..42063e1c355d 100644 > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c > @@ -328,6 +328,18 @@ static u32 ct_get_next_fence(struct intel_guc_ct *ct) > return ++ct->requests.last_fence; > } > > +static void write_barrier(struct intel_guc_ct *ct) { > + struct intel_guc *guc = ct_to_guc(ct); > + struct intel_gt *gt = guc_to_gt(guc); > + > + if (i915_gem_object_is_lmem(guc->ct.vma->obj)) { > + GEM_BUG_ON(guc->send_regs.fw_domains); > + intel_uncore_write_fw(gt->uncore, GEN11_SOFT_SCRATCH(0), 0); hmm, as this is one of the GuC scratch registers used for H2G MMIO communication, writing 0 there might be interpreted by the GuC as new request with action=0 and might results in extra processing/logging on GuC side, and, since from here we don't protect access to this register by send_mutex, we can corrupt other MMIO message being prepared from different thread, ... can't we use other register ? > + } else { > + wmb(); > + } > +} > + > /** > * DOC: CTB Host to GuC request > * > @@ -411,6 +423,12 @@ static int ct_write(struct intel_guc_ct *ct, > } > GEM_BUG_ON(tail > size); > > + /* > + * make sure H2G buffer update and LRC tail update (if this triggering a > + * submission) are visible before updating the descriptor tail > + */ > + write_barrier(ct); > + > /* now update desc tail (back in bytes) */ > desc->tail = tail * 4; > return 0; >