[PATCH] drm/i915/gt: Do not consider preemption during execlists_dequeue for gen8

[Gote, Nitin R]

> -----Original Message-----
> From: Tvrtko Ursulin <tursulin at ursulin.net>
> Sent: Wednesday, July 10, 2024 4:22 PM
> To: Gote, Nitin R <nitin.r.gote at intel.com>; intel-gfx at lists.freedesktop.org
> Cc: dri-devel at lists.freedesktop.org; Shyti, Andi <andi.shyti at intel.com>;
> chris.p.wilson at linux.intel.com; Das, Nirmoy <nirmoy.das at intel.com>;
> janusz.krzysztofik at linux.intel.com; stable at vger.kernel.org
> Subject: Re: [PATCH] drm/i915/gt: Do not consider preemption during
> execlists_dequeue for gen8
> 
> 
> On 09/07/2024 15:02, Tvrtko Ursulin wrote:
> >
> > On 09/07/2024 13:53, Nitin Gote wrote:
> >> We're seeing a GPU HANG issue on a CHV platform, which was caused by
> >> bac24f59f454 ("drm/i915/execlists: Enable coarse preemption
> >> boundaries for gen8").
> >>
> >> Gen8 platform has only timeslice and doesn't support a preemption
> >> mechanism as engines do not have a preemption timer and doesn't send
> >> an irq if the preemption timeout expires. So, add a fix to not
> >> consider preemption during dequeuing for gen8 platforms.
> >>
> >> Also move can_preemt() above need_preempt() function to resolve
> >> implicit declaration of function ‘can_preempt' error and make
> >> can_preempt() function param as const to resolve error: passing
> >> argument 1 of ‘can_preempt’ discards ‘const’ qualifier from the pointer
> target type.
> >>
> >> Fixes: bac24f59f454 ("drm/i915/execlists: Enable coarse preemption
> >> boundaries for gen8")
> >> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11396
> >> Suggested-by: Andi Shyti <andi.shyti at intel.com>
> >> Signed-off-by: Nitin Gote <nitin.r.gote at intel.com>
> >> Cc: Chris Wilson <chris.p.wilson at linux.intel.com>
> >> CC: <stable at vger.kernel.org> # v5.2+
> >> ---
> >>   .../drm/i915/gt/intel_execlists_submission.c  | 24
> >> ++++++++++++-------
> >>   1 file changed, 15 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> >> b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> >> index 21829439e686..30631cc690f2 100644
> >> --- a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> >> +++ b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
> >> @@ -294,11 +294,26 @@ static int virtual_prio(const struct
> >> intel_engine_execlists *el)
> >>       return rb ? rb_entry(rb, struct ve_node, rb)->prio : INT_MIN;
> >>   }
> >> +static bool can_preempt(const struct intel_engine_cs *engine) {
> >> +    if (GRAPHICS_VER(engine->i915) > 8)
> >> +        return true;
> >> +
> >> +    if (IS_CHERRYVIEW(engine->i915) || IS_BROADWELL(engine->i915))
> >> +        return false;
> >> +
> >> +    /* GPGPU on bdw requires extra w/a; not implemented */
> >> +    return engine->class != RENDER_CLASS;
> >
> > Aren't BDW and CHV the only Gen8 platforms, in which case this
> > function can be simplifies as:
> >
> > ...
> > {
> >      return GRAPHICS_VER(engine->i915) > 8; }
> >
> > ?
> >
> >> +}

Yes. I will simply this function.

> >> +
> >>   static bool need_preempt(const struct intel_engine_cs *engine,
> >>                const struct i915_request *rq)
> >>   {
> >>       int last_prio;
> >> +    if ((GRAPHICS_VER(engine->i915) <= 8) && can_preempt(engine))
> >
> > The GRAPHICS_VER check here looks redundant with the one inside
> > can_preempt().
True. I will update the condition.
> 
> One more thing - I think gen8_emit_bb_start() becomes dead code after this
> and can be removed.

I think gen8_emit_bb_start() still require for graphics version < 12 as it is
used in else part of if (GRAPHICS_VER_FULL(engine->i915) >= IP_VER(12, 55)) condition.
So, it will not be dead code.

Thanks and Regards,
Nitin
> 
> Regards,
> 
> Tvrtko
> 
> >> +        return false;
> >> +
> >>       if (!intel_engine_has_semaphores(engine))
> >>           return false;
> >> @@ -3313,15 +3328,6 @@ static void remove_from_engine(struct
> >> i915_request *rq)
> >>       i915_request_notify_execute_cb_imm(rq);
> >>   }
> >> -static bool can_preempt(struct intel_engine_cs *engine) -{
> >> -    if (GRAPHICS_VER(engine->i915) > 8)
> >> -        return true;
> >> -
> >> -    /* GPGPU on bdw requires extra w/a; not implemented */
> >> -    return engine->class != RENDER_CLASS; -}
> >> -
> >>   static void kick_execlists(const struct i915_request *rq, int prio)
> >>   {
> >>       struct intel_engine_cs *engine = rq->engine;