As Thomas Gleixner spotted, it's rather horrible racy:
- We can miss almost a full tick, so need to compensate by 1 jiffy.
- We need to re-check the condition when having timed-out, since a
the last check could have been before the timeout expired. E.g. when
we've been preempted or a long irq happened.
Cc: Thomas Gleixner <tglx at linutronix.de>
Reported-by: Jack Winter <jbh at alchemy.lu>
Cc: Jack Winter <jbh at alchemy.lu>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
drivers/gpu/drm/i915/intel_drv.h | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index c8c1979..9dcae4e 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -33,12 +33,21 @@
#include <drm/drm_fb_helper.h>
#include <drm/drm_dp_helper.h>
+/**
+ * _wait_for - magic (register) wait macro
+ *
+ * Does the right thing for modeset paths when run under kdgb or similar atomic
+ * contexts. Note that it's important that we check the condition again after
+ * 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, MS, W) ({ \
- unsigned long timeout__ = jiffies + msecs_to_jiffies(MS); \
+ unsigned long timeout__ = jiffies + msecs_to_jiffies(MS) + 1; \
int ret__ = 0; \
while (!(COND)) { \
if (time_after(jiffies, timeout__)) { \
- ret__ = -ETIMEDOUT; \
+ if (!(COND)) \
+ ret__ = -ETIMEDOUT; \
break; \
} \
if (W && drm_can_sleep()) { \
--
1.7.11.7
