drm/i915: Use ktime on wait_for

We use jiffies to determine when wait expires. However
Imre did find out that jiffies can and will do a >1
increments on certain situations [1]. When this happens
in a wait_for loop, we return timeout errorneously
much earlier than what the real wallclock would say.

We can't afford our waits to timeout prematurely.
Discard jiffies and change to ktime to detect timeouts.

v2: added bugzilla entry (Imre), added stable (Chris)

Reported-by: Imre Deak <imre.deak@intel.com>
References: https://lkml.org/lkml/2018/4/18/798 [1]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105771
Cc: Imre Deak <imre.deak@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423113754.28424-1-mika.kuoppala@linux.intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -49,12 +49,12 @@
  * check the condition before the timeout.
  */
 #define __wait_for(OP, COND, US, Wmin, Wmax) ({ \
-	unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1;	\
+	const ktime_t end__ = ktime_add_ns(ktime_get_raw(), 1000ll * (US)); \
 	long wait__ = (Wmin); /* recommended min for usleep is 10 us */	\
 	int ret__;							\
 	might_sleep();							\
 	for (;;) {							\
-		bool expired__ = time_after(jiffies, timeout__);	\
+		const bool expired__ = ktime_after(ktime_get_raw(), end__); \
 		OP;							\
 		if (COND) {						\
 			ret__ = 0;					\