drm/i915: Use a hybrid scheme for fast register waits
Ville Syrjälä reported that in the majority of wait_for(I915_READ()) he
inspect, most completed within the first couple of reads and that the
delay between those wait_for() reads was the ratelimiting step for many
code paths. For example, __gen6_update_ring_freq() was blamed for
slowing down boot by many milliseconds, but under Ville's scrutiny the
issue was just excessive delay waiting for sandybridge_pcode_write().
We can eliminate the wait by initially using a busyspin upon the register
read and only fallback to the sleeping loop in cases where the hardware
is indeed too slow. A threshold of 2 microseconds is used as the initial
ballpark.
To avoid excessive code bloating from converting every wait_for() into a
hybrid busy/sleep loop, we extend wait_for_register_fw() and export it
for use by other callers.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467297225-21379-1-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 82b20e1..48d3067 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2968,6 +2968,17 @@
void assert_forcewakes_inactive(struct drm_i915_private *dev_priv);
+int intel_wait_for_register(struct drm_i915_private *dev_priv,
+ i915_reg_t reg,
+ const u32 mask,
+ const u32 value,
+ const unsigned long timeout_ms);
+int intel_wait_for_register_fw(struct drm_i915_private *dev_priv,
+ i915_reg_t reg,
+ const u32 mask,
+ const u32 value,
+ const unsigned long timeout_ms);
+
static inline bool intel_gvt_active(struct drm_i915_private *dev_priv)
{
return dev_priv->gvt.initialized;
