[Intel-gfx] [PATCH 01/28] drm/i915: Adjust the sentinel assert to match implementation

[Chris Wilson]

Quoting Tvrtko Ursulin (2020-06-09 11:39:11)
> 
> On 09/06/2020 11:29, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2020-06-09 07:59:27)
> >> 666
> >> On 08/06/2020 10:33, Chris Wilson wrote:
> >>> Quoting Tvrtko Ursulin (2020-06-08 08:44:01)
> >>>>
> >>>> On 07/06/2020 23:20, Chris Wilson wrote:
> >>>>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>>>>
> >>>>> Sentinels are supposed to be last reqeusts in the elsp queue, not the
> >>>>> only one, so adjust the assert accordingly.
> >>>>>
> >>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>>>> ---
> >>>>>     drivers/gpu/drm/i915/gt/intel_lrc.c | 14 +++-----------
> >>>>>     1 file changed, 3 insertions(+), 11 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>>>> index d55a5e0466e5..db8a170b0e5c 100644
> >>>>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>>>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> >>>>> @@ -1635,9 +1635,9 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
> >>>>>                 ccid = ce->lrc.ccid;
> >>>>>     
> >>>>>                 /*
> >>>>> -              * Sentinels are supposed to be lonely so they flush the
> >>>>> -              * current exection off the HW. Check that they are the
> >>>>> -              * only request in the pending submission.
> >>>>> +              * Sentinels are supposed to be the last request so they flush
> >>>>> +              * the current exection off the HW. Check that they are the only
> >>>>> +              * request in the pending submission.
> >>>>>                  */
> >>>>>                 if (sentinel) {
> >>>>>                         GEM_TRACE_ERR("%s: context:%llx after sentinel in pending[%zd]\n",
> >>>>> @@ -1646,15 +1646,7 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
> >>>>>                                       port - execlists->pending);
> >>>>>                         return false;
> >>>>>                 }
> >>>>> -
> >>>>>                 sentinel = i915_request_has_sentinel(rq);
> >>>>
> >>>> FWIW I was changing it to "sentinel |= ..." so it keeps working if we
> >>>> decide to use more than 2 elsp ports on Icelake one day.
> >>>
> >>> But it will always fail on the next port...
> >>
> >> I don't follow. Sentinel has to be last so if it fails on the next port
> >> it is correct to do so, no?
> > 
> > Exactly. We only check the first port after setting sentinel, if that
> > port is occupied we fail. Hence why we don't need |=, since there is no
> > continuation.
> 
> But if more than two ports we also overwrite the bools so: sentinel, 
> non-sentinel, sentinel would not catch. I was just future proofing it. :)

[0] -> sentinel
[1] != NULL -> ERROR

[0] -> not sentinel
[1] -> sentinel
[2] != NULL -> ERROR

We fail if anything comes after a sentinel.
-Chris