Re: [patch 35/38] s390: Select ARCH_HAS_RANDOM_ENTROPY

[Heiko Carstens <hca@linux.ibm.com>]

On Fri, Apr 10, 2026 at 02:21:19PM +0200, Thomas Gleixner wrote:
> The only remaining non-architecture usage of get_cycles() is to provide
> random_get_entropy().
> 
> Switch s390 over to the new scheme of selecting ARCH_HAS_RANDOM_ENTROPY and
> providing random_get_entropy() in asm/random.h.
> 
> Add 'asm/timex.h' includes to the relevant files, so the global include can
> be removed once all architectures are converted over.
> 
> Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: linux-s390@vger.kernel.org
> ---
>  arch/s390/Kconfig              |    1 +
>  arch/s390/include/asm/random.h |   12 ++++++++++++
>  arch/s390/include/asm/timex.h  |    6 ------
>  arch/s390/kernel/time.c        |    1 +
>  arch/s390/kernel/vtime.c       |    1 +
>  5 files changed, 15 insertions(+), 6 deletions(-)

Acked-by: Heiko Carstens <hca@linux.ibm.com>

Thomas, would you mind adding the below as minor improvement to this
series?

From 7072e5d66b99a7fa666d17c6ad8cb254f2d8f473 Mon Sep 17 00:00:00 2001
From: Heiko Carstens <hca@linux.ibm.com>
Date: Thu, 16 Apr 2026 15:08:15 +0200
Subject: [PATCH] s390: Use get_tod_clock_fast() for random_get_entropy()

Use get_tod_clock_fast() instead of get_tod_clock_monotonic() to implement
random_get_entropy().

There is no need for random_get_entropy() to provide monotonic increasing
values, nor is there any need to provide (close to) nanosecond granularity
timestamps by shifting the result.

This slightly reduces the execution time of random_get_entropy() and adds
two bits of randomness.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
 arch/s390/include/asm/random.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/random.h b/arch/s390/include/asm/random.h
index 7daf42dbed32..f6d9312efdbf 100644
--- a/arch/s390/include/asm/random.h
+++ b/arch/s390/include/asm/random.h
@@ -6,7 +6,7 @@
 
 static inline unsigned long random_get_entropy(void)
 {
-	return (unsigned long)get_tod_clock_monotonic() >> 2;
+	return get_tod_clock_fast();
 }
 
 #endif
-- 
2.51.0