Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> Quoting Chris Wilson (2017-09-29 11:45:57)
>> We coalesce onto an active port[0], but not onto an active port[1]
>> despite it being the same mechanism. So explain the complications that
>> make skipping the first active port and coalescing onto the second
>> active port intractable (at least while keeping some resemblance of
>> sanity).
>>
>> Suggested-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
>> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
>> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
>> ---
>> drivers/gpu/drm/i915/intel_lrc.c | 20 ++++++++++++++++++++
>> 1 file changed, 20 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>> index bc3fc4cd039e..cc5574ff56c8 100644
>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>> @@ -579,6 +579,26 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>> execlists->preempt = true;
>> goto unlock;
>> } else {
>> + /*
>> + * In theory, we could coalesce more requests onto
>> + * the second port (the first port is active, with
>> + * no preemptions pending). However, that means we
>> + * then have to deal with the possible lite-restore
>> + * of the second port (as we submit the ELSP, there
>> + * may be a context-switch) but also we may complete
>> + * the resubmission before the context-switch. Ergo,
>> + * coalescing onto the second port will cause a
>> + * preemption event, but we cannot predict whether
>> + * that will affect port[0] or port[1].
>> + *
>> + * If the second port is already active, we can wait
>> + * until the next context-switch before contemplating
>> + * new requests. The GPU will be busy and we should be
>> + * able to resubmit the new ELSP before it idles,
>> + * avoiding pipeline bubbles (momentary pauses where
>> + * the driver is unable to keep up the supply of new
>> + * work).
>> + */
>> if (port_count(&port[1]))
>> goto unlock;
>
> I think I'll squash this into the "Preemption!" patch if we're happy
> with the explanation?
Yup please do.
-Mika
> -Chris
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