drm/i915: Clear the vblank status bit before polling for the next vblank
The vblank status bit is a sticky bit that must be cleared with a write
of '1' prior to polling for the next vblank.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
jbarnes: I'd still rather see a lock, but I think you're right that
we don't generally wait in code that needs not to miss an interrupt.
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 11a3394..3fc767b 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -990,6 +990,22 @@
struct drm_i915_private *dev_priv = dev->dev_private;
int pipestat_reg = (pipe == 0 ? PIPEASTAT : PIPEBSTAT);
+ /* Clear existing vblank status. Note this will clear any other
+ * sticky status fields as well.
+ *
+ * This races with i915_driver_irq_handler() with the result
+ * that either function could miss a vblank event. Here it is not
+ * fatal, as we will either wait upon the next vblank interrupt or
+ * timeout. Generally speaking intel_wait_for_vblank() is only
+ * called during modeset at which time the GPU should be idle and
+ * should *not* be performing page flips and thus not waiting on
+ * vblanks...
+ * Currently, the result of us stealing a vblank from the irq
+ * handler is that a single frame will be skipped during swapbuffers.
+ */
+ I915_WRITE(pipestat_reg,
+ I915_READ(pipestat_reg) | PIPE_VBLANK_INTERRUPT_STATUS);
+
/* Wait for vblank interrupt bit to set */
if (wait_for((I915_READ(pipestat_reg) &
PIPE_VBLANK_INTERRUPT_STATUS),