arm64: barrier: support smp_cond_load_relaxed_timeout()

Support waiting in smp_cond_load_relaxed_timeout() via
__cmpwait_relaxed(). Limit this to when the event-stream is enabled,
to ensure that we wake from WFE periodically and don't block forever
if there are no stores to the cacheline.

In the unlikely event that the event-stream is unavailable, fallback
to spin-waiting.

Also set SMP_TIMEOUT_POLL_COUNT to 1 so we do the time-check for each
iteration in smp_cond_load_relaxed_timeout().

Cc: [email protected]
Cc: Catalin Marinas <[email protected]>
Suggested-by: Will Deacon <[email protected]>
Signed-off-by: Ankur Arora <[email protected]>

Author: terminus

arch/arm64/include/asm/barrier.h

@@ -219,6 +219,19 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+#define SMP_TIMEOUT_POLL_COUNT	1
+
+/* Re-declared here to avoid include dependency. */
+extern bool arch_timer_evtstrm_available(void);
+
+#define cpu_poll_relax(ptr, val)					\
+do {									\
+	if (arch_timer_evtstrm_available())				\
+		__cmpwait_relaxed(ptr, val);				\
+	else								\
+		cpu_relax();						\
+} while (0)
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLY__ */