Support a waited variant of polling on a conditional variable
via smp_cond_relaxed_timeout().

This uses the __cmpwait_relaxed() primitive to do the actual
waiting, when the wait can be guaranteed to not block forever
(in case there are no stores to the waited for cacheline.)
For this we depend on the availability of the event-stream.

For cases when the event-stream is unavailable, we fallback to
a spin-waited implementation which is identical to the generic
variant.

Signed-off-by: Ankur Arora <redacted>
---
 arch/arm64/include/asm/barrier.h | 54 ++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 1ca947d5c939..ab2515ecd6ca 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h

@@ -216,6 +216,60 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+#define __smp_cond_load_timeout_spin(ptr, cond_expr,			\
+				     time_expr_ns, time_limit_ns)	\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	unsigned int __count = 0;					\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		cpu_relax();						\
+		if (__count++ < smp_cond_time_check_count)		\
+			continue;					\
+		if ((time_expr_ns) >= time_limit_ns)			\
+			break;						\
+		__count = 0;						\
+	}								\
+	(typeof(*ptr))VAL;						\
+})