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 <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c
index 71ee361d097261f7e0e27e30175d3d955103eff0..6e91ea1de5aafa951b832cb460709d97388ec507 100644 (file)
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c
+++ b/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);
 }