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Subject[PATCH v2 02/11] timekeeping: use ktime_read_raw_clock() for random_get_entropy() if no get_cycles()
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 <>
Cc: Arnd Bergmann <>
Cc: Theodore Ts'o <>
Signed-off-by: Jason A. Donenfeld <>
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())
+#define random_get_entropy() ((unsigned long)ktime_read_raw_clock())

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