drm/i915/pmu: Only use exclusive mmio access for gen7

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@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191108103511.20951-2-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index 8e74f40..4804775 100644 (file)
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -292,6 +292,16 @@ add_sample(struct i915_pmu_sample *sample, u32 val)
        sample->cur += val;
 }
 
+static bool exclusive_mmio_access(const struct drm_i915_private *i915)
+{
+       /*
+        * We have to avoid concurrent mmio cache line access on gen7 or
+        * risk a machine hang. For a fun history lesson dig out the old
+        * userspace intel_gpu_top and run it on Ivybridge or Haswell!
+        */
+       return IS_GEN(i915, 7);
+}
+
 static void
 engines_sample(struct intel_gt *gt, unsigned int period_ns)
 {
@@ -304,6 +314,7 @@ engines_sample(struct intel_gt *gt, unsigned int period_ns)
 
        for_each_engine(engine, gt, id) {
                struct intel_engine_pmu *pmu = &engine->pmu;
+               spinlock_t *mmio_lock;
                unsigned long flags;
                bool busy;
                u32 val;
@@ -311,7 +322,12 @@ 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);
+               mmio_lock = NULL;
+               if (exclusive_mmio_access(i915))
+                       mmio_lock = &engine->uncore->lock;
+
+               if (unlikely(mmio_lock))
+                       spin_lock_irqsave(mmio_lock, flags);
 
                val = ENGINE_READ_FW(engine, RING_CTL);
                if (val == 0) /* powerwell off => engine idle */
@@ -342,7 +358,8 @@ 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 (unlikely(mmio_lock))
+                       spin_unlock_irqrestore(mmio_lock, flags);
                intel_engine_pm_put(engine);
        }
 }