[PATCH v2] drm/i915: Use atomic waits for short non-atomic ones

Tvrtko Ursulin
2016-06-28T16:16:15Z

From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

usleep_range is not recommended for waits shorten than 10us.

Make the wait_for_us use the atomic variant for such waits.

To do so we need to reimplement the _wait_for_atomic macro to
be safe with regards to preemption and interrupts.

v2: Reimplement _wait_for_atomic to be irq and preemption safe.
(Chris Wilson and Imre Deak)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
---
drivers/gpu/drm/i915/intel_drv.h | 46 ++++++++++++++++++++++++++++------------
1 file changed, 32 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 3156d8df7921..a200fd2be908 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -69,7 +69,6 @@
})

#define wait_for(COND, MS) _wait_for((COND), (MS) * 1000, 1000)
-#define wait_for_us(COND, US) _wait_for((COND), (US), 1)

/* If CONFIG_PREEMPT_COUNT is disabled, in_atomic() always reports false. */
#if defined(CONFIG_DRM_I915_DEBUG) && defined(CONFIG_PREEMPT_COUNT)
@@ -78,25 +77,44 @@
# define _WAIT_FOR_ATOMIC_CHECK do { } while (0)
#endif

-#define _wait_for_atomic(COND, US) ({ \
- unsigned long end__; \
- int ret__ = 0; \
+#define _wait_for_atomic(COND, US) \
+({ \
+ int cpu, ret, timeout = (US) * 1000; \
+ u64 base; \
_WAIT_FOR_ATOMIC_CHECK; \
BUILD_BUG_ON((US) > 50000); \
- end__ = (local_clock() >> 10) + (US) + 1; \
- while (!(COND)) { \
- if (time_after((unsigned long)(local_clock() >> 10), end__)) { \
- /* Unlike the regular wait_for(), this atomic variant \
- * cannot be preempted (and we'll just ignore the issue\
- * of irq interruptions) and so we know that no time \
- * has passed since the last check of COND and can \
- * immediately report the timeout. \
- */ \
- ret__ = -ETIMEDOUT; \
+ preempt_disable(); \
+ cpu = smp_processor_id(); \
+ base = local_clock(); \
+ for (;;) { \
+ u64 now = local_clock(); \
+ preempt_enable(); \
+ if (COND) { \
+ ret = 0; \
+ break; \
+ } \
+ if (now - base >= timeout) { \
+ ret = -ETIMEDOUT; \
break; \
} \
cpu_relax(); \
+ preempt_disable(); \
+ if (unlikely(cpu != smp_processor_id())) { \
+ timeout -= now - base; \
+ cpu = smp_processor_id(); \
+ base = local_clock(); \
+ } \
} \
+ ret; \
+})
+
+#define wait_for_us(COND, US) \
+({ \
+ int ret__; \
+ if ((US) > 10) \
+ ret__ = _wait_for((COND), (US), 10); \
+ else \
+ ret__ = _wait_for_atomic((COND), (US)); \
ret__; \
})

--
1.9.1