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

[Chris Wilson]

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);
 	}
 }
-- 
2.24.0