Re: [Intel-gfx] [PATCH 25/27] drm/i915/guc: Handle errors in multi-lrc requests

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Sep 29, 2021 at 01:44:10PM -0700, John Harrison wrote:
> On 8/20/2021 15:44, Matthew Brost wrote:
> > If an error occurs in the front end when multi-lrc requests are getting
> > generated we need to skip these in the backend but we still need to
> > emit the breadcrumbs seqno. An issues arrises because with multi-lrc
> arrises -> arises
> 

Yep.

> > breadcrumbs there is a handshake between the parent and children to make
> > forwad progress. If all the requests are not present this handshake
> forwad -> forward
> 

Yep.

> > doesn't work. To work around this, if multi-lrc request has an error we
> > skip the handshake but still emit the breadcrumbs seqno.
> > 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 61 ++++++++++++++++++-
> >   1 file changed, 58 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > index 2ef38557b0f0..61e737fd1eee 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -3546,8 +3546,8 @@ static int emit_bb_start_child_no_preempt_mid_batch(struct i915_request *rq,
> >   }
> >   static u32 *
> > -emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> > -						 u32 *cs)
> > +__emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> > +						   u32 *cs)
> >   {
> >   	struct intel_context *ce = rq->context;
> >   	u8 i;
> > @@ -3575,6 +3575,41 @@ emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> >   				  get_children_go_addr(ce),
> >   				  0);
> > +	return cs;
> > +}
> > +
> > +/*
> > + * If this true, a submission of multi-lrc requests had an error and the
> > + * requests need to be skipped. The front end (execuf IOCTL) should've called
> > + * i915_request_skip which squashes the BB but we still need to emit the fini
> > + * breadrcrumbs seqno write. At this point we don't know how many of the
> > + * requests in the multi-lrc submission were generated so we can't do the
> > + * handshake between the parent and children (e.g. if 4 requests should be
> > + * generated but 2nd hit an error only 1 would be seen by the GuC backend).
> > + * Simply skip the handshake, but still emit the breadcrumbd seqno, if an error
> > + * has occurred on any of the requests in submission / relationship.
> > + */
> > +static inline bool skip_handshake(struct i915_request *rq)
> > +{
> > +	return test_bit(I915_FENCE_FLAG_SKIP_PARALLEL, &rq->fence.flags);
> > +}
> > +
> > +static u32 *
> > +emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> > +						 u32 *cs)
> > +{
> > +	struct intel_context *ce = rq->context;
> > +
> > +	GEM_BUG_ON(!intel_context_is_parent(ce));
> > +
> > +	if (unlikely(skip_handshake(rq))) {
> > +		memset(cs, 0, sizeof(u32) *
> > +		       (ce->engine->emit_fini_breadcrumb_dw - 6));
> > +		cs += ce->engine->emit_fini_breadcrumb_dw - 6;
> Why -6? There are 12 words about to be written. Indeed the value of
> emit_..._dw is '12 + 4*num_children'. This should only be skipping over the
> 4*children, right? As it stands, it will skip all but the last six words,
> then write an extra twelve words and thus overflow the reservation by six.
> Unless I am totally confused?
> 

Let me decode the length:

'Wait on children' (in __emit_fini_breadcrumb_parent_no_preempt_mid_batch) = 4 * num_children
'Turn on preemption' (in __emit_fini_breadcrumb_parent_no_preempt_mid_batch) = 2
'Tell children go' (in __emit_fini_breadcrumb_parent_no_preempt_mid_batch) = 4
'Emit fini breadcrumb' (in emit_fini_breadcrumb_child_no_preempt_mid_batch) = 4
'User interrupt' (in emit_fini_breadcrumb_child_no_preempt_mid_batch) = 2

So for a total (emit_fini_breadcrumb_dw) we have '12 + 4 * num_children'

We want skip everything in __emit_fini_breadcrumb_parent_no_preempt_mid_batch, so that is
'6 + 4 * num_children' or 'emit_fini_breadcrumb_dw - 6'

Make sense?

> I assume there is some reason why the amount of data written must exactly
> match the space reserved? It's a while since I've looked at the ring buffer
> code!
> 

I think it because the ring space is reserved at request creation time
but the fini breadcrumbs are not written until submission time.

> Seems like it would be clearer to not split the semaphore writes out but
> have them right next to the skip code that is meant to replicate them but
> with no-ops.
>

I guess that works too, I personally like the way it is but if you
insist I can change it.

> > +	} else {
> > +		cs = __emit_fini_breadcrumb_parent_no_preempt_mid_batch(rq, cs);
> > +	}
> > +
> >   	/* Emit fini breadcrumb */
> >   	cs = gen8_emit_ggtt_write(cs,
> >   				  rq->fence.seqno,
> > @@ -3591,7 +3626,8 @@ emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> >   }
> >   static u32 *
> > -emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq, u32 *cs)
> > +__emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
> > +						  u32 *cs)
> >   {
> >   	struct intel_context *ce = rq->context;
> > @@ -3617,6 +3653,25 @@ emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq, u32 *cs
> >   	*cs++ = get_children_go_addr(ce->parent);
> >   	*cs++ = 0;
> > +	return cs;
> > +}
> > +
> > +static u32 *
> > +emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
> > +						u32 *cs)
> > +{
> > +	struct intel_context *ce = rq->context;
> > +
> > +	GEM_BUG_ON(!intel_context_is_child(ce));
> > +
> > +	if (unlikely(skip_handshake(rq))) {
> > +		memset(cs, 0, sizeof(u32) *
> > +		       (ce->engine->emit_fini_breadcrumb_dw - 6));
> > +		cs += ce->engine->emit_fini_breadcrumb_dw - 6;
> > +	} else {
> > +		cs = __emit_fini_breadcrumb_child_no_preempt_mid_batch(rq, cs);
> > +	}
> Same points as above - why -6 not -12 and would be clearer to keep the
> no-ops and the writes adjacent.
>

Same as above we are NOP the length of
__emit_fini_breadcrumb_child_no_preempt_mid_batch and still want the
emit breadcrumbs below.

Matt

 
> John.
> 
> > +
> >   	/* Emit fini breadcrumb */
> >   	cs = gen8_emit_ggtt_write(cs,
> >   				  rq->fence.seqno,
>