[RFC 4/8] drm/i915: Add a wait_for routine with more exact timeout

[jeff.mcgee@intel.com]

From: Jeff McGee <jeff.mcgee@intel.com>

The current non-atomic wait_for routines convert the provided usec
timeout into jiffies with upward rounding. This can increase the
actual timeout several msecs which is fine in most cases. In the next
patch we need the timeout to conform more exactly to what is
requested.

This patch is required to support the force preemption feature.

Change-Id: I20372ccd4db6609c42a6b41000ca7ff7700358fc
Signed-off-by: Jeff McGee <jeff.mcgee@intel.com>
---
 drivers/gpu/drm/i915/intel_drv.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index fa800918f67d..7cb6619129c3 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -75,6 +75,28 @@
 						   (Wmax))
 #define wait_for(COND, MS)		_wait_for((COND), (MS) * 1000, 10, 1000)
 
+#define _wait_for_exact(COND, US, Wmin, Wmax) ({ \
+	u64 timeout__ = local_clock() + (US) * 1000;			\
+	long wait__ = (Wmin); /* recommended min for usleep is 10 us */	\
+	int ret__;							\
+	might_sleep();							\
+	for (;;) {							\
+		bool expired__ = (local_clock() > timeout__);		\
+		if (COND) {						\
+			ret__ = 0;					\
+			break;						\
+		}							\
+		if (expired__) {					\
+			ret__ = -ETIMEDOUT;				\
+			break;						\
+		}							\
+		usleep_range(wait__, wait__ * 2);			\
+		if (wait__ < (Wmax))					\
+			wait__ <<= 1;					\
+	}								\
+	ret__;								\
+})
+
 /* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
 #if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
 # define _WAIT_FOR_ATOMIC_CHECK(ATOMIC) WARN_ON_ONCE((ATOMIC) && !in_atomic())
-- 
2.16.2

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