[PATCH RFC v1 07/10] arm64: use sched_clock() for random_get_entropy() instead of zero

[Jason A. Donenfeld]

In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 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.

If CONFIG_ARM_ARCH_TIMER=n, then get_cycles() will return 0, so we only
need the fallback code for that case.

Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 arch/arm64/include/asm/timex.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/include/asm/timex.h b/arch/arm64/include/asm/timex.h
index cf59ce91b22d..bfebd2e1ce45 100644
--- a/arch/arm64/include/asm/timex.h
+++ b/arch/arm64/include/asm/timex.h
@@ -13,6 +13,15 @@
  */
 #define get_cycles()	arch_timer_read_counter()
 
+#ifndef CONFIG_ARM_ARCH_TIMER
+/*
+ * The default implementation of random_get_entropy() calls get_cycles(),
+ * which will return 0 if CONFIG_ARM_ARCH_TIMER=n, so we fall back to
+ * sched_clock() here. Not a great situation, but better than nothing.
+ */
+#define random_get_entropy() ((unsigned long)sched_clock())
+#endif
+
 #include <asm-generic/timex.h>
 
 #endif
-- 
2.35.1