[PATCH 1/5] asm-generic: add smp_vcond_load_relaxed()

[Haris Okanovic <harisokn@amazon.com>]

Relaxed poll until desired mask/value is observed at the specified
address or timeout.

This macro is a specialization of the generic smp_cond_load_relaxed(),
which takes a simple mask/value condition (vcond) instead of an
arbitrary expression. It allows architectures to better specialize the
implementation, e.g. to enable wfe() polling of the address on arm.

Signed-off-by: Haris Okanovic <harisokn@amazon.com>
---
 include/asm-generic/barrier.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..112027eabbfc 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -256,6 +256,31 @@ do {									\
 })
 #endif
 
+/**
+ * smp_vcond_load_relaxed() - (Spin) wait until an expected value at address
+ * with no ordering guarantees. Spins until `(*addr & mask) == val` or
+ * `nsecs` elapse, and returns the last observed `*addr` value.
+ *
+ * @nsecs: timeout in nanoseconds
+ * @addr: pointer to an integer
+ * @mask: a bit mask applied to read values
+ * @val: Expected value with mask
+ */
+#ifndef smp_vcond_load_relaxed
+#define smp_vcond_load_relaxed(nsecs, addr, mask, val) ({	\
+	const u64 __start = local_clock_noinstr();		\
+	u64 __nsecs = (nsecs);					\
+	typeof(addr) __addr = (addr);				\
+	typeof(*__addr) __mask = (mask);			\
+	typeof(*__addr) __val = (val);				\
+	typeof(*__addr) __cur;					\
+	smp_cond_load_relaxed(__addr, (				\
+		(VAL & __mask) == __val ||			\
+		local_clock_noinstr() - __start > __nsecs	\
+	));							\
+})
+#endif
+
 /**
  * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
  * @ptr: pointer to the variable to wait on
-- 
2.34.1