drm/i915: Order OP vs. timeout correctly in __wait_for()

Put the barrier() before the OP so that anything we read out in
OP and check in COND will actually be read out after the timeout
has been evaluated.

Currently the only place where we use OP is __intel_wait_for_register(),
but the use there is precisely susceptible to this reordering, assuming
the ktime_*() stuff itself doesn't act as a sufficient barrier:

__intel_wait_for_register(...)
{
	...
	ret = __wait_for(reg_value = intel_uncore_read_notrace(...),
 			 (reg_value & mask) == value, ...);
	...
}

Cc: [email protected]
Fixes: 1c3c1dc ("drm/i915: Add compiler barrier to wait_for")
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patch.msgid.link/[email protected]
Reviewed-by: Jani Nikula <[email protected]>
(cherry picked from commit a464bac)
Signed-off-by: Joonas Lahtinen <[email protected]>

Author: vsyrjala

drivers/gpu/drm/i915/i915_wait_util.h

@@ -25,9 +25,9 @@
 	might_sleep();							\
 	for (;;) {							\
 		const bool expired__ = ktime_after(ktime_get_raw(), end__); \
-		OP;							\
 		/* Guarantee COND check prior to timeout */		\
 		barrier();						\
+		OP;							\
 		if (COND) {						\
 			ret__ = 0;					\
 			break;						\