[Intel-gfx] [PATCH 4/4] drm/i915/pmu: Only use exclusive mmio access for gen7

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-11-08 10:21:22)
> 
> On 08/11/2019 08:56, Chris Wilson wrote:
> > On gen7, we have to avoid concurrent access to the same mmio cacheline,
> > and so coordinate all mmio access with the uncore->lock. However, for
> > pmu, we want to avoid perturbing the system and disabling interrupts
> > unnecessarily, so restrict the w/a to gen7 where it is requied to
> > prevent machine hangs.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_pmu.c | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> > index dbde80a376cb..9e1627782284 100644
> > --- a/drivers/gpu/drm/i915/i915_pmu.c
> > +++ b/drivers/gpu/drm/i915/i915_pmu.c
> > @@ -292,6 +292,11 @@ add_sample(struct i915_pmu_sample *sample, u32 val)
> >       sample->cur += val;
> >   }
> >   
> > +static bool exclusive_mmio_access(const struct drm_i915_private *i915)
> > +{
> > +     return IS_GEN(i915, 7);
> > +}
> > +
> >   static void
> >   engines_sample(struct intel_gt *gt, unsigned int period_ns)
> >   {
> > @@ -311,7 +316,8 @@ engines_sample(struct intel_gt *gt, unsigned int period_ns)
> >               if (!intel_engine_pm_get_if_awake(engine))
> >                       continue;
> >   
> > -             spin_lock_irqsave(&engine->uncore->lock, flags);
> > +             if (exclusive_mmio_access(i915))
> > +                     spin_lock_irqsave(&engine->uncore->lock, flags);
> >   
> >               val = ENGINE_READ_FW(engine, RING_CTL);
> >               if (val == 0) /* powerwell off => engine idle */
> > @@ -342,7 +348,8 @@ engines_sample(struct intel_gt *gt, unsigned int period_ns)
> >                       add_sample(&pmu->sample[I915_SAMPLE_BUSY], period_ns);
> >   
> >   skip:
> > -             spin_unlock_irqrestore(&engine->uncore->lock, flags);
> > +             if (exclusive_mmio_access(i915))
> > +                     spin_unlock_irqrestore(&engine->uncore->lock, flags);
> >               intel_engine_pm_put(engine);
> >       }
> >   }
> > 
> 
> For bonus points cache the check in a local? Or cache the lock in a 
> local for even more win, spinlock_t *lock = exclusive_mmio_access(..) ? 
> &.. : NULL ?

In the back of my mind, I was assuming that IS_GEN() gets compiled out
for single sku builds.

lock = exclusive_mmio_access() ...

if (lock)
	spin_lock_irqsave(lock, flags);
...
if (lock)
	spin_unlock_irqsave(lock, flags);

Ok.
-Chris