drm/i915/active: Use try_cmpxchg64() in __active_lookup()

Replace this pattern in __active_lookup():

    cmpxchg64(*ptr, old, new) == old

... with the simpler and faster:

    try_cmpxchg64(*ptr, &old, new)

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after the CMPXCHG.

The patch also improves the explanation of what the code really
does. cmpxchg64() will *succeed* for the winner of the race and
try_cmpxchg64() nicely documents this fact.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20250814064326.95519-1-tvrtko.ursulin@igalia.com

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index 0dbc4e2893004a428b19b7a87f898b903b759ca8..6b0c1162505ac2d830db3a7d94576f12e999e282 100644 (file)
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -257,10 +257,9 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
                 * claimed the cache and we know that is does not match our
                 * idx. If, and only if, the timeline is currently zero is it
                 * worth competing to claim it atomically for ourselves (for
-                * only the winner of that race will cmpxchg return the old
-                * value of 0).
+                * only the winner of that race will cmpxchg succeed).
                 */
-               if (!cached && !cmpxchg64(&it->timeline, 0, idx))
+               if (!cached && try_cmpxchg64(&it->timeline, &cached, idx))
                        return it;
        }