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

[Lionel Landwerlin <lionel.g.landwerlin@intel.com>]

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 :)
>   
>> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@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@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?


-Lionel

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