Re: [PATCH v4 01/11] timekeeping: add raw clock fallback for random_get_entropy()

Russell King (Oracle)
2022-04-14T10:12:50Z

On Wed, Apr 13, 2022 at 01:54:01PM +0200, Jason A. Donenfeld wrote:
> The addition of random_get_entropy_fallback() provides access to
> whichever time source has the highest frequency, which is useful for
> gathering entropy on platforms without available cycle counters. It's
> not necessarily as good as being able to quickly access a cycle counter
> that the CPU has, but it's still something, even when it falls back to
> being jiffies-based.
>
> 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
> random_get_entropy_fallback() would be preferable, because that always
> needs to return _something_, even falling back to jiffies eventually.
> It's not as though random_get_entropy_fallback() 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@linutronix.de>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> include/linux/timex.h | 8 ++++++++
> kernel/time/timekeeping.c | 10 ++++++++++
> 2 files changed, 18 insertions(+)
>
> diff --git a/include/linux/timex.h b/include/linux/timex.h
> index 5745c90c8800..fbbe34226044 100644
> --- a/include/linux/timex.h
> +++ b/include/linux/timex.h
> @@ -62,6 +62,8 @@
> #include <linux/types.h>
> #include <linux/param.h>
>
> +extern unsigned long random_get_entropy_fallback(void);

Hi

I'm surprised this didn't trigger checkpatch to warn. From
coding-style:

6.1) Function prototypes
Do not use the ``extern`` keyword with function declarations as this makes
lines longer and isn't strictly necessary.

Thanks!

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