gpu: host1x: Use HW-equivalent syncpoint expiration check

Make syncpoint expiration checks always use the same logic used by
the hardware. This ensures that there are no race conditions that
could occur because of the hardware triggering a syncpoint interrupt
and then the driver disagreeing.

One situation where this could occur is if a job incremented a
syncpoint too many times -- then the hardware would trigger an
interrupt, but the driver would assume that a syncpoint value
greater than the syncpoint's max value is in the future, and not
clean up the job.
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/host1x/syncpt.c

@@ -321,59 +321,12 @@ EXPORT_SYMBOL(host1x_syncpt_wait);
 bool host1x_syncpt_is_expired(struct host1x_syncpt *sp, u32 thresh)
 {
 	u32 current_val;
-	u32 future_val;
 
 	smp_rmb();
 
 	current_val = (u32)atomic_read(&sp->min_val);
-	future_val = (u32)atomic_read(&sp->max_val);
-
-	/* Note the use of unsigned arithmetic here (mod 1<<32).
-	 *
-	 * c = current_val = min_val	= the current value of the syncpoint.
-	 * t = thresh			= the value we are checking
-	 * f = future_val  = max_val	= the value c will reach when all
-	 *				  outstanding increments have completed.
-	 *
-	 * Note that c always chases f until it reaches f.
-	 *
-	 * Dtf = (f - t)
-	 * Dtc = (c - t)
-	 *
-	 *  Consider all cases:
-	 *
-	 *	A) .....c..t..f.....	Dtf < Dtc	need to wait
-	 *	B) .....c.....f..t..	Dtf > Dtc	expired
-	 *	C) ..t..c.....f.....	Dtf > Dtc	expired	   (Dct very large)
-	 *
-	 *  Any case where f==c: always expired (for any t).	Dtf == Dcf
-	 *  Any case where t==c: always expired (for any f).	Dtf >= Dtc (because Dtc==0)
-	 *  Any case where t==f!=c: always wait.		Dtf <  Dtc (because Dtf==0,
-	 *							Dtc!=0)
-	 *
-	 *  Other cases:
-	 *
-	 *	A) .....t..f..c.....	Dtf < Dtc	need to wait
-	 *	A) .....f..c..t.....	Dtf < Dtc	need to wait
-	 *	A) .....f..t..c.....	Dtf > Dtc	expired
-	 *
-	 *   So:
-	 *	   Dtf >= Dtc implies EXPIRED	(return true)
-	 *	   Dtf <  Dtc implies WAIT	(return false)
-	 *
-	 * Note: If t is expired then we *cannot* wait on it. We would wait
-	 * forever (hang the system).
-	 *
-	 * Note: do NOT get clever and remove the -thresh from both sides. It
-	 * is NOT the same.
-	 *
-	 * If future valueis zero, we have a client managed sync point. In that
-	 * case we do a direct comparison.
-	 */
-	if (!host1x_syncpt_client_managed(sp))
-		return future_val - thresh >= current_val - thresh;
-	else
-		return (s32)(current_val - thresh) >= 0;
+
+	return ((current_val - thresh) & 0x80000000U) == 0U;
 }
 
 int host1x_syncpt_init(struct host1x *host)