On Thu, Jun 03, 2021 at 11:44:57AM +0200, Michal Wajdeczko wrote:
>
>
> On 03.06.2021 07:16, 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.
> >
> > If this barrier is not inserted it is possible the descriptor tail
> > update is scene by the GuC before H2G buffer update which results in the
> > GuC reading a corrupt H2G value. This can bring down the H2G channel
> > among other bad things.
> >
> > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > Cc: Michal Wajdeczko <michal.wajdeczko@xxxxxxxxx>
> > Reviewed-by: John Harrison <John.C.Harrison@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 28 +++++++++++++++++++++++
> > 1 file changed, 28 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 80976fe40fbf..31f83956bfc3 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 +328,28 @@ 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);
> > + /*
> > + * This register is used by the i915 and GuC for MMIO based
> > + * communication. Once we are in this code CTBs are the only
> > + * method the i915 uses to communicate with the GuC so it is
> > + * safe to write to this register (a value of 0 is NOP for MMIO
> > + * communication). If we ever start mixing CTBs and MMIOs a new
> > + * register will have to be chosen.
> > + */
> > + intel_uncore_write_fw(gt->uncore, GEN11_SOFT_SCRATCH(0), 0);
>
> can't we at least start with SOFT_SCRATCH register that is not used for
> GuC MMIO based communication on Gen12 LMEM platforms? see [1]
>
We likely can use this but I really don't feel comfortable switching the
register without some more testing first (e.g. let's change in this in
internal, let it soak for bit, then make the change upstream).
> I really don't feel comfortable that we are touching a register that
> elsewhere is protected with the mutex. And mixing CTBs and MMIO is not
> far away.
>
The only code that mixes CTBs and MMIOs is SRIOV which is a ways away
from landing.
Matt
> Michal
>
> [1]
> https://lore.kernel.org/intel-gfx/51b9bd05-7d6f-29f1-de0f-3a14bade6c9c@xxxxxxxxx/
>
> > + } else {
> > + /* wmb() sufficient for a barrier if in smem */
> > + wmb();
> > + }
> > +}
> > +
> > /**
> > * DOC: CTB Host to GuC request
> > *
> > @@ -411,6 +433,12 @@ 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;
> >
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