drm/amdgpu: Use local64_try_cmpxchg in amdgpu_perf_read

Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
in amdgpu_perf_read.  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc: Alex Deucher <[email protected]>
Cc: "Christian König" <[email protected]>
Cc: "Pan, Xinhui" <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: ubizjak

drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c

@@ -276,9 +276,8 @@ static void amdgpu_perf_read(struct perf_event *event)
 	    (!pe->adev->df.funcs->pmc_get_count))
 		return;
 
+	prev = local64_read(&hwc->prev_count);
 	do {
-		prev = local64_read(&hwc->prev_count);
-
 		switch (hwc->config_base) {
 		case AMDGPU_PMU_EVENT_CONFIG_TYPE_DF:
 		case AMDGPU_PMU_EVENT_CONFIG_TYPE_XGMI:
@@ -289,7 +288,7 @@ static void amdgpu_perf_read(struct perf_event *event)
 			count = 0;
 			break;
 		}
-	} while (local64_cmpxchg(&hwc->prev_count, prev, count) != prev);
+	} while (!local64_try_cmpxchg(&hwc->prev_count, &prev, count));
 
 	local64_add(count - prev, &event->count);
 }