Subject: [PATCH] powerpc: Use hardware RNG for arch_get_random_seed_* not arch_get_random_*

From: Paul Mackerras <[email-masked]>
Date: Fri, 17 Jul 2015 19:15:58 +1000

The hardware RNG on POWER8 and POWER7+ can be relatively slow, since
it can only supply one 64-bit value per microsecond.  Currently we
read it in arch_get_random_long(), but that slows down reading from
/dev/urandom since the code in random.c calls arch_get_random_long()
for every longword read from /dev/urandom.

Since the hardware RNG supplies high-quality entropy on every read, it
matches the semantics of arch_get_random_seed_long() better than those
of arch_get_random_long().  Therefore this commit makes the code use
the POWER8/7+ hardware RNG only for arch_get_random_seed_{long,int}
and not for arch_get_random_{long,int}.

This won't affect any other PowerPC-based platforms because none of
them currently support a hardware RNG.

Signed-off-by: Paul Mackerras <[email-masked]>
---
 arch/powerpc/include/asm/archrandom.h | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/arch/powerpc/include/asm/archrandom.h b/arch/powerpc/include/asm/archrandom.h
index 0cc6eed..a4c3f54 100644
--- a/arch/powerpc/include/asm/archrandom.h
+++ b/arch/powerpc/include/asm/archrandom.h
@@ -7,13 +7,22 @@
 
 static inline int arch_get_random_long(unsigned long *v)
 {
+	return 0;
+}
+
+static inline int arch_get_random_int(unsigned int *v)
+{
+	return 0;
+}
+
+static inline int arch_get_random_seed_long(unsigned long *v)
+{
 	if (ppc_md.get_random_long)
 		return ppc_md.get_random_long(v);
 
 	return 0;
 }
-
-static inline int arch_get_random_int(unsigned int *v)
+static inline int arch_get_random_seed_int(unsigned int *v)
 {
 	unsigned long val;
 	int rc;
@@ -27,22 +36,13 @@ static inline int arch_get_random_int(unsigned int *v)
 
 static inline int arch_has_random(void)
 {
-	return !!ppc_md.get_random_long;
-}
-
-static inline int arch_get_random_seed_long(unsigned long *v)
-{
-	return 0;
-}
-static inline int arch_get_random_seed_int(unsigned int *v)
-{
 	return 0;
 }
+
 static inline int arch_has_random_seed(void)
 {
-	return 0;
+	return !!ppc_md.get_random_long;
 }
-
 #endif /* CONFIG_ARCH_RANDOM */
 
 #ifdef CONFIG_PPC_POWERNV
-- 
2.1.4