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?
-Chris
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