Add a timed variant of smp_cond_load_relaxed().

This is useful because arm64 supports polling on a conditional variable
by directly waiting on the cacheline instead of spin waiting for the
condition to change.

However, an implementation such as this has a problem that it can block
forever -- unless there's an explicit timeout or another out-of-band
mechanism which allows it to come out of the wait state periodically.

smp_cond_load_relaxed_timeout() supports these semantics by specifying
a time-check expression and an associated time-limit.

However, note that for the generic spin-wait implementation we want to
minimize the numbers of instructions executed in each iteration. So,
limit how often we evaluate the time-check expression by doing it once
every smp_cond_time_check_count.

The inner loop in poll_idle() has a substantially similar structure
and constraints as smp_cond_load_relaxed_timeout(), so define
smp_cond_time_check_count to the same value used in poll_idle().

Signed-off-by: Ankur Arora <redacted>
---
 include/asm-generic/barrier.h | 42 +++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..77726ef807e4 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h

@@ -273,6 +273,48 @@ do {									\
 })
 #endif

+#ifndef smp_cond_time_check_count
+/*
+ * Limit how often smp_cond_load_relaxed_timeout() evaluates time_expr_ns.
+ * This helps reduce the number of instructions executed while spin-waiting.
+ */
+#define smp_cond_time_check_count	200
+#endif
+
+/**
+ * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
+ * guarantees until a timeout expires.
+ * @ptr: pointer to the variable to wait on
+ * @cond: boolean expression to wait for
+ * @time_expr_ns: evaluates to the current time
+ * @time_limit_ns: compared against time_expr_ns
+ *
+ * Equivalent to using READ_ONCE() on the condition variable.
+ *
+ * Due to C lacking lambda expressions we load the value of *ptr into a
+ * pre-named variable @VAL to be used in @cond.