[PATCH v5] drm/i915: Use atomic waits for short non-atomic ones

Tvrtko Ursulin
2016-06-29T11:27:26Z

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)

v3: Fixed in_atomic check due rebase error.
v4: Build bug on non-constant timeouts.
v5: Compile away cpu migration code in atomic paths.

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 | 62 ++++++++++++++++++++++++++++------------
1 file changed, 43 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 3156d8df7921..98a5be4ec8c5 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -69,39 +69,63 @@
})

#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)
-# define _WAIT_FOR_ATOMIC_CHECK WARN_ON_ONCE(!in_atomic())
+# define _WAIT_FOR_ATOMIC_CHECK(ATOMIC) WARN_ON_ONCE((ATOMIC) && !in_atomic())
#else
-# define _WAIT_FOR_ATOMIC_CHECK do { } while (0)
+# define _WAIT_FOR_ATOMIC_CHECK(ATOMIC) do { } while (0)
#endif

-#define _wait_for_atomic(COND, US) ({ \
- unsigned long end__; \
- int ret__ = 0; \
- _WAIT_FOR_ATOMIC_CHECK; \
+#define _wait_for_atomic(COND, US, ATOMIC) \
+({ \
+ int cpu, ret, timeout = (US) * 1000; \
+ u64 base; \
+ _WAIT_FOR_ATOMIC_CHECK(ATOMIC); \
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; \
+ if (!(ATOMIC)) { \
+ preempt_disable(); \
+ cpu = smp_processor_id(); \
+ } \
+ base = local_clock(); \
+ for (;;) { \
+ u64 now = local_clock(); \
+ if (!(ATOMIC)) \
+ preempt_enable(); \
+ if (COND) { \
+ ret = 0; \
+ break; \
+ } \
+ if (now - base >= timeout) { \
+ ret = -ETIMEDOUT; \
break; \
} \
cpu_relax(); \
+ if (!(ATOMIC)) { \
+ 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__; \
+ BUILD_BUG_ON(!__builtin_constant_p(US)); \
+ if ((US) > 10) \
+ ret__ = _wait_for((COND), (US), 10); \
+ else \
+ ret__ = _wait_for_atomic((COND), (US), 0); \
ret__; \
})

-#define wait_for_atomic(COND, MS) _wait_for_atomic((COND), (MS) * 1000)
-#define wait_for_atomic_us(COND, US) _wait_for_atomic((COND), (US))
+#define wait_for_atomic(COND, MS) _wait_for_atomic((COND), (MS) * 1000, 1)
+#define wait_for_atomic_us(COND, US) _wait_for_atomic((COND), (US), 1)

#define KHz(x) (1000 * (x))
#define MHz(x) KHz(1000 * (x))
--
1.9.1