drm/i915: fix up _wait_for macro

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@linutronix.de>
Reported-by: Jack Winter <jbh@alchemy.lu>
Cc: Jack Winter <jbh@alchemy.lu>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 11 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 8720a67395f8..18bba6e25e1e 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())  {                            \