[Intel-gfx] [PATCH] drm/i915/perf: don't forget noa wait after oa config

[Chris Wilson]

Quoting Lionel Landwerlin (2019-11-13 18:33:59)
> On 13/11/2019 20:11, Chris Wilson wrote:
> > Quoting Chris Wilson (2019-11-13 18:10:22)
> >> Quoting Lionel Landwerlin (2019-11-13 18:07:59)
> >>> On 13/11/2019 18:35, Chris Wilson wrote:
> >>>> Quoting Lionel Landwerlin (2019-11-13 15:46:39)
> >>>>> I'm observing incoherence metric values, changing from run to run.
> >>>>>
> >>>>> It appears the patches introducing noa wait & reconfiguration from
> >>>>> command stream switched places in the series multiple times during the
> >>>>> review. This lead to the dependency of one onto the order to go
> >>>>> missing...
> >>>> I don't think I dropped it; if I did my apologies. I do feel the
> >>>> egg-on-face for writing a selftest to verify that noa_wait does what you
> >>>> said it did, but completely missing that it went unused :)
> 
> 
> It was probably my mistake :)
> 
> 
> >>>>    
> >>>>> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> >>>>> Fixes: 15d0ace1f876 ("drm/i915/perf: execute OA configuration from command stream")
> >>>>> ---
> >>>>>    drivers/gpu/drm/i915/i915_perf.c | 9 +++++++--
> >>>>>    1 file changed, 7 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> >>>>> index 507236bd41ae..31e47ee23357 100644
> >>>>> --- a/drivers/gpu/drm/i915/i915_perf.c
> >>>>> +++ b/drivers/gpu/drm/i915/i915_perf.c
> >>>>> @@ -1870,7 +1870,7 @@ alloc_oa_config_buffer(struct i915_perf_stream *stream,
> >>>>>           config_length += num_lri_dwords(oa_config->mux_regs_len);
> >>>>>           config_length += num_lri_dwords(oa_config->b_counter_regs_len);
> >>>>>           config_length += num_lri_dwords(oa_config->flex_regs_len);
> >>>>> -       config_length++; /* MI_BATCH_BUFFER_END */
> >>>>> +       config_length += 3; /* MI_BATCH_BUFFER_START */
> >>>>>           config_length = ALIGN(sizeof(u32) * config_length, I915_GTT_PAGE_SIZE);
> >>>>>    
> >>>>>           obj = i915_gem_object_create_shmem(stream->perf->i915, config_length);
> >>>>> @@ -1895,7 +1895,12 @@ alloc_oa_config_buffer(struct i915_perf_stream *stream,
> >>>>>                                oa_config->flex_regs,
> >>>>>                                oa_config->flex_regs_len);
> >>>>>    
> >>>>> -       *cs++ = MI_BATCH_BUFFER_END;
> >>>>> +       /* Jump into the active wait. */
> >>>>> +       *cs++ = (INTEL_GEN(stream->perf->i915) < 8 ?
> >>>>> +                MI_BATCH_BUFFER_START :
> >>>>> +                MI_BATCH_BUFFER_START_GEN8);
> >>>>> +       *cs++ = i915_ggtt_offset(stream->noa_wait);
> >>>>> +       *cs++ = 0;
> >>>> Yikes, stream->noa_wait is unused.
> >>>>
> >>>> Hmm, the noa_wait doesn't have any arbitration points internally, so we
> >>>> probably do need to make it non-preemptable as well?
> >>>>
> >>>> With a rq->flags |= I915_REQUEST_NOPREEMPT in emit_oa_config,
> >>>> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> >>>>
> >>>> We need to wrap emit_oa_config() in a similar selftest and verify
> >>>> that a read of the oa regs are correct and that the TIMESTAMP indicates
> >>>> the appropriate delay before the read. If you feel bored.
> >>>> -Chris
> >>>
> >>> As long as we wait long enough, it should be okay.
> >>>
> >>> Why making it nopreempt?
> >> Aiui, the batch buffer has no arbitration points so the delay may incur
> >> the wrath of the forced preemption. That is another request (of higher
> >> priority) wishing to run, but not being able to.
> > The alternative would be adding a MI_ARB_CHECK at the start of the loop
> > if you happy with being preempted out.
> > -Chris
> 
> I guess I'll do that :)

Ok, then have an 
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
on the understanding you'll look at adding an MI_ARB_CHECK next :)
-Chris