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

[Rob Herring <robh@kernel.org>]

On Wed, Apr 13, 2022 at 6:55 AM Jason A. Donenfeld <Jason@zx2c4.com> 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);
> +
>  #include <asm/timex.h>
>
>  #ifndef random_get_entropy
> @@ -74,8 +76,14 @@
>   *
>   * 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

'does not have a usable get_cycles(), ...' as clearly some arches have
get_cycles() and yet still need a fallback.

Why not handle the 'if get_cycles() returns 0 do the fallback' within
a weak random_get_entropy() function? Then more arches don't need any
random_get_entropy() implementation.

Rob