linux-kernel - [PATCH] drm/amdgpu: Use local64_try_cmpxchg in amdgpu_perf

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Date:   Mon,  3 Jul 2023 17:00:59 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: [PATCH] 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@....com>
Cc: "Christian König" <christian.koenig@....com>
Cc: "Pan, Xinhui" <Xinhui.Pan@....com>
Cc: David Airlie <airlied@...il.com>
Cc: Daniel Vetter <daniel@...ll.ch>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_pmu.c
index 71ee361d0972..6e91ea1de5aa 100644
--- 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);
 }
-- 
2.41.0