On 03/21/2014 12:00 PM, Chris Wilson wrote:
On Fri, Mar 21, 2014 at 10:50:05AM +0000, Tvrtko Ursulin wrote:
No, think you misunderstood me. I said "slightly more defensive"
just in the sense that in case of weird hardware failures you have a
potentially infinite loop now, where you don't really need a loop -
probabilities strongly suggest you cannot get two upper dword wraps
between the reads. So it is enough to read the upper dword twice,
without the loop. Same effect, slightly more defensive in reality.
Yup, misunderstood what you wanted. If in doubt, C is much more
concise ;-)
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 45d8011..8c82316 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -425,12 +425,14 @@ u64 intel_ring_get_active_head(struct intel_ring_buffer *ring)
if (INTEL_INFO(ring->dev)->gen >= 8) {
u32 upper, lower, tmp;
+ upper = I915_READ(RING_ACTHD_UDW(ring->mmio_base));
+ lower = I915_READ(RING_ACTHD(ring->mmio_base));
tmp = I915_READ(RING_ACTHD_UDW(ring->mmio_base));
- do {
+ if (upper != tmp) {
upper = tmp;
lower = I915_READ(RING_ACTHD(ring->mmio_base));
- tmp = I915_READ(RING_ACTHD_UDW(ring->mmio_base));
- } while (upper != tmp);
+ WARN_ON(I915_READ(RING_ACTHD_UDW(ring->mmio_base) != upper);
+ }
acthd = (u64)upper << 32 | lower;
} else if (INTEL_INFO(ring->dev)->gen >= 4)
Yes, I was just uneasy with the loop. Also Ben's suggestion in case of
wrap was I think:
WARN_ON(I915_READ(RING_ACTHD(ring->mmio_base) >= lower);
Or in other words, if we have observed the upper wrap, check that the
lower matches with that observation. But I feel bad now that we are
over-engineering this. Perhaps these WARNs are just silly.
Tvrtko
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