drm/i915: Use exponential backoff for wait_for()

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)
v3: Specify min, max range for usleep intervals -- some code may
crucially depend upon and so want to specify the sleep pattern.

References: 1758b90e38f5 ("drm/i915: Use a hybrid scheme for fast register waits")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: John Harrison <John.C.Harrison@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171124130031.20761-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_drv.h

@@ -48,8 +48,9 @@
  * having timed out, since the timeout could be due to preemption or similar and
  * we've never had a chance to check the condition before the timeout.
  */
-#define _wait_for(COND, US, W) ({ \
+#define _wait_for(COND, US, Wmin, Wmax) ({ \
 	unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1;	\
+	long wait__ = (Wmin); /* recommended min for usleep is 10 us */	\
 	int ret__;							\
 	might_sleep();							\
 	for (;;) {							\
@@ -62,12 +63,14 @@
 			ret__ = -ETIMEDOUT;				\
 			break;						\
 		}							\
-		usleep_range((W), (W) * 2);				\
+		usleep_range(wait__, wait__ * 2);			\
+		if (wait__ < (Wmax))					\
+			wait__ <<= 1;					\
 	}								\
 	ret__;								\
 })
 
-#define wait_for(COND, MS)	  	_wait_for((COND), (MS) * 1000, 1000)
+#define wait_for(COND, MS)	_wait_for((COND), (MS) * 1000, 10, 1000)
 
 /* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
 #if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
@@ -116,7 +119,7 @@
 	int ret__; \
 	BUILD_BUG_ON(!__builtin_constant_p(US)); \
 	if ((US) > 10) \
-		ret__ = _wait_for((COND), (US), 10); \
+		ret__ = _wait_for((COND), (US), 10, 10); \
 	else \
 		ret__ = _wait_for_atomic((COND), (US), 0); \
 	ret__; \

drivers/gpu/drm/i915/intel_pm.c

@@ -9294,7 +9294,7 @@ int skl_pcode_request(struct drm_i915_private *dev_priv, u32 mbox, u32 request,
 		ret = 0;
 		goto out;
 	}
-	ret = _wait_for(COND, timeout_base_ms * 1000, 10);
+	ret = _wait_for(COND, timeout_base_ms * 1000, 10, 10);
 	if (!ret)
 		goto out;