Quoting Mika Kuoppala (2020-03-27 20:19:15)
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
>
> > Quoting Tvrtko Ursulin (2020-03-27 15:59:45)
> >>
> >> On 27/03/2020 11:26, Chris Wilson wrote:
> >> > In what seems remarkably similar to the w/a required to not reload an
> >> > idle context with HEAD==TAIL, it appears we must prevent the HW from
> >> > switching to an idle context in ELSP[1], while simultaneously trying to
> >> > preempt the HW to run another context and a continuation of the idle
> >> > context (which is no longer idle).
> >> >
> >> > process_csb: vecs0: cs-irq head=0, tail=1
> >> > process_csb: vecs0: csb[1]: status=0x00000882:0x00000020
> >> > trace_ports: vecs0: preempted { 8c0:30!, 0:0 }
> >> > trace_ports: vecs0: promote { 8b2:32!, 8c0:30 }
> >> > trace_ports: vecs0: submit { 8b8:32, 8c0:32 }
> >> > process_csb: vecs0: cs-irq head=1, tail=2
> >> > process_csb: vecs0: csb[2]: status=0x00000814:0x00000040
> >> > trace_ports: vecs0: completed { 8b2:32!, 8c0:30 }
> >> > process_csb: vecs0: cs-irq head=2, tail=5
> >> > process_csb: vecs0: csb[3]: status=0x00000812:0x00000020
> >> > trace_ports: vecs0: preempted { 8c0:30!, 0:0 }
> >> > trace_ports: vecs0: promote { 8b8:32!, 8c0:32 }
> >> > process_csb: vecs0: csb[4]: status=0x00000814:0x00000060
> >> > trace_ports: vecs0: completed { 8b8:32!, 8c0:32 }
> >> > process_csb: vecs0: csb[5]: status=0x00000818:0x00000020
> >> > trace_ports: vecs0: completed { 8c0:32, 0:0 }
> >> > process_csb: vecs0: ring:{start:0x00021000, head:03f8, tail:03f8, ctl:00000000, mode:00000200}
> >> > process_csb: vecs0: rq:{start:00021000, head:03c0, tail:0400, seqno:8c0:32, hwsp:30},
> >> > process_csb: vecs0: ctx:{start:00021000, head:03f8, tail:03f8},
> >> > process_csb: GEM_BUG_ON("context completed before request")
> >> >
> >> > Fortunately, we just so happen to have a semaphore in place to prevent
> >> > the ring HEAD from proceeding past the end of a request that we can use
> >> > to fix the HEAD in position as we reprogram ELSP.
> >> >
> >> > Closes: https://gitlab.freedesktop.org/drm/intel/issues/1501
> >> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> >> > Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> >> > ---
> >> > drivers/gpu/drm/i915/gt/intel_gt_irq.c | 2 +
> >> > drivers/gpu/drm/i915/gt/intel_lrc.c | 72 +++++++++++++-------------
> >> > 2 files changed, 38 insertions(+), 36 deletions(-)
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/gt/intel_gt_irq.c b/drivers/gpu/drm/i915/gt/intel_gt_irq.c
> >> > index f0e7fd95165a..be3817d99908 100644
> >> > --- a/drivers/gpu/drm/i915/gt/intel_gt_irq.c
> >> > +++ b/drivers/gpu/drm/i915/gt/intel_gt_irq.c
> >> > @@ -24,6 +24,8 @@ cs_irq_handler(struct intel_engine_cs *engine, u32 iir)
> >> > {
> >> > bool tasklet = false;
> >> >
> >> > + ENGINE_TRACE(engine, "iir: %04x\n", iir);
> >> > +
> >> > if (unlikely(iir & GT_CS_MASTER_ERROR_INTERRUPT)) {
> >> > u32 eir;
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >> > index b12355048501..53650b452bc9 100644
> >> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> >> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >> > @@ -1854,7 +1854,7 @@ static inline void clear_ports(struct i915_request **ports, int count)
> >> > memset_p((void **)ports, NULL, count);
> >> > }
> >> >
> >> > -static void execlists_dequeue(struct intel_engine_cs *engine)
> >> > +static bool execlists_dequeue(struct intel_engine_cs *engine)
> >> > {
> >> > struct intel_engine_execlists * const execlists = &engine->execlists;
> >> > struct i915_request **port = execlists->pending;
> >> > @@ -1928,13 +1928,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >> > execlists->queue_priority_hint);
> >> > record_preemption(execlists);
> >> >
> >> > - /*
> >> > - * Don't let the RING_HEAD advance past the breadcrumb
> >> > - * as we unwind (and until we resubmit) so that we do
> >> > - * not accidentally tell it to go backwards.
> >> > - */
> >> > - ring_set_paused(engine, 1);
> >> > -
> >> > /*
> >> > * Note that we have not stopped the GPU at this point,
> >> > * so we are unwinding the incomplete requests as they
> >> > @@ -1954,7 +1947,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >> > last->sched.attr.priority,
> >> > execlists->queue_priority_hint);
> >> >
> >> > - ring_set_paused(engine, 1);
> >> > defer_active(engine);
> >> >
> >> > /*
> >> > @@ -1988,7 +1980,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >> > * of timeslices, our queue might be.
> >> > */
> >> > start_timeslice(engine);
> >> > - return;
> >> > + return false;
> >> > }
> >> > }
> >> > }
> >> > @@ -2021,9 +2013,10 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >> > }
> >> >
> >> > if (last && !can_merge_rq(last, rq)) {
> >> > + /* leave this for another sibling */
> >> > spin_unlock(&ve->base.active.lock);
> >> > start_timeslice(engine);
> >> > - return; /* leave this for another sibling */
> >> > + return false;
> >> > }
> >> >
> >> > ENGINE_TRACE(engine,
> >> > @@ -2193,32 +2186,31 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> >> > * interrupt for secondary ports).
> >> > */
> >> > execlists->queue_priority_hint = queue_prio(execlists);
> >> > + if (!submit)
> >> > + return false;
> >> >
> >> > - if (submit) {
> >> > - *port = execlists_schedule_in(last, port - execlists->pending);
> >> > - execlists->switch_priority_hint =
> >> > - switch_prio(engine, *execlists->pending);
> >> > + *port = execlists_schedule_in(last, port - execlists->pending);
> >> > + execlists->switch_priority_hint =
> >> > + switch_prio(engine, *execlists->pending);
> >> >
> >> > - /*
> >> > - * Skip if we ended up with exactly the same set of requests,
> >> > - * e.g. trying to timeslice a pair of ordered contexts
> >> > - */
> >> > - if (!memcmp(active, execlists->pending,
> >> > - (port - execlists->pending + 1) * sizeof(*port))) {
> >> > - do
> >> > - execlists_schedule_out(fetch_and_zero(port));
> >> > - while (port-- != execlists->pending);
> >> > -
> >> > - goto skip_submit;
> >> > - }
> >> > - clear_ports(port + 1, last_port - port);
> >> > + /*
> >> > + * Skip if we ended up with exactly the same set of requests,
> >> > + * e.g. trying to timeslice a pair of ordered contexts
> >> > + */
> >> > + if (!memcmp(active, execlists->pending,
> >> > + (port - execlists->pending + 1) * sizeof(*port))) {
> >> > + do
> >> > + execlists_schedule_out(fetch_and_zero(port));
> >> > + while (port-- != execlists->pending);
> >> >
> >> > - execlists_submit_ports(engine);
> >> > - set_preempt_timeout(engine, *active);
> >> > - } else {
> >> > -skip_submit:
> >> > - ring_set_paused(engine, 0);
> >> > + return false;
> >> > }
> >> > + clear_ports(port + 1, last_port - port);
> >> > +
> >> > + execlists_submit_ports(engine);
> >> > + set_preempt_timeout(engine, *active);
> >> > + tasklet_hi_schedule(&execlists->tasklet); /* lost interrupt */
> >> > + return true;
> >> > }
> >> >
> >> > static void
> >> > @@ -2478,7 +2470,16 @@ static void __execlists_submission_tasklet(struct intel_engine_cs *const engine)
> >> > lockdep_assert_held(&engine->active.lock);
> >> > if (!READ_ONCE(engine->execlists.pending[0])) {
> >> > rcu_read_lock(); /* protect peeking at execlists->active */
> >> > - execlists_dequeue(engine);
> >> > +
> >> > + /*
> >> > + * Don't let the RING_HEAD advance past the breadcrumb
> >> > + * as we unwind (and until we resubmit) so that we do
> >> > + * not accidentally tell it to go backwards.
> >> > + */
> >> > + ring_set_paused(engine, 1);
> >> > + if (!execlists_dequeue(engine))
> >> > + ring_set_paused(engine, 0);
> >> > +
> >> > rcu_read_unlock();
> >> > }
> >> > }
> >> > @@ -2816,8 +2817,7 @@ static void execlists_reset(struct intel_engine_cs *engine, const char *msg)
> >> > ring_set_paused(engine, 1); /* Freeze the current request in place */
> >> > if (execlists_capture(engine))
> >> > intel_engine_reset(engine, msg);
> >> > - else
> >> > - ring_set_paused(engine, 0);
> >> > + ring_set_paused(engine, 0);
> >> >
> >> > tasklet_enable(&engine->execlists.tasklet);
> >> > clear_and_wake_up_bit(bit, lock);
> >> >
> >>
> >> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> >
> > The problem appears to be that if we lite-restore into a context inside
> > the semaphore, it doesn't yield an arbitration point and we do not raise
> > a CSB event.
>
> Trying to make senses of it all...
>
> we do not raise == the hardware does not raise?
HW.
> So if it is about lite restoring, we can't workaround by always making
> sure elsp[1] head != tail?
No. The context we enter into ELSP[1] is still executing on the GPU and
hits HEAD==TAIL after we write the register, but before the GPU
processes the register.
-Chris
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