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

[Tvrtko Ursulin]

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 ?

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Regards,

Tvrtko