[Intel-gfx] [PATCH v2] drm/i915: Use exponential backoff for wait_for()

[Chris Wilson]

Instead of sleeping for a fixed 1ms (roughly, depending on timer slack),
start with a small sleep and exponentially increase the sleep on each
cycle.

A good example of a beneficiary is the guc mmio communication channel.
Typically we expect (and so spin) for 10us for a quick response, but this
doesn't cover everything and so sometimes we fallback to the millisecond+
sleep. This incurs a significant delay in time-critical operations like
preemption (igt/gem_exec_latency), which can be improved significantly by
using a small sleep after the spin fails.

We've made this suggestion many times, but had little experimental data
to support adding the complexity.

v2: Bump the minimum usleep to 10us on advice of
Documentation/timers/timers-howto.txt (Tvrko)

References: 1758b90e38f5 ("drm/i915: Use a hybrid scheme for fast register waits")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: John Harrison <John.C.Harrison at intel.com>
Cc: Michał Winiarski <michal.winiarski at intel.com>
Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_drv.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 69aab324aaa1..3e115599c2e6 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -50,6 +50,7 @@
  */
 #define _wait_for(COND, US, W) ({ \
 	unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1;	\
+	long wait__ = 10; /* recommended minimum for usleep */		\
 	int ret__;							\
 	might_sleep();							\
 	for (;;) {							\
@@ -62,7 +63,9 @@
 			ret__ = -ETIMEDOUT;				\
 			break;						\
 		}							\
-		usleep_range((W), (W) * 2);				\
+		usleep_range(wait__, wait__ * 2);			\
+		if (wait__ < (W))					\
+			wait__ <<= 1;					\
 	}								\
 	ret__;								\
 })
-- 
2.15.0