[PATCH v2 02/11] timekeeping: use ktime_read_raw_clock() for random_get_entropy() if no get_cycles()

[Jason A. Donenfeld]

In the event that a given arch does not define get_cycles(), falling
back to the get_cycles() default implementation that returns 0 is really
not the best we can do. Instead, at least calling ktime_read_raw_clock()
would be preferable, because that always needs to return _something_,
even falling back to jiffies eventually. It's not as though
ktime_read_raw_clock() is super high precision or guaranteed to be
entropic, but basically anything that's not zero all the time is better
than returning zero all the time.

Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Theodore Ts'o <tytso at mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
---
 include/linux/timex.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 56502b338287..4050f68c34cb 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -76,8 +76,14 @@ extern u64 ktime_read_raw_clock(void);
  *
  * By default we use get_cycles() for this purpose, but individual
  * architectures may override this in their asm/timex.h header file.
+ * If a given arch does not have get_cycles(), then we fallback to
+ * using sched_clock().
  */
+#ifdef get_cycles
 #define random_get_entropy()	((unsigned long)get_cycles())
+#else
+#define random_get_entropy()	((unsigned long)ktime_read_raw_clock())
+#endif
 #endif
 
 /*
-- 
2.35.1