[PATCH RFC v1 01/10] random: use sched_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 sched_clock() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though sched_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@xxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 include/linux/timex.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 5745c90c8800..bd78f784762e 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -61,6 +61,7 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <linux/param.h>
+#include <linux/sched/clock.h>
 
 #include <asm/timex.h>
 
@@ -74,8 +75,13 @@
  *
  * 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)sched_clock())
 #endif
 
 /*
-- 
2.35.1