Re: [PATCH v4 03/11] riscv: use fallback for random_get_entropy() instead of zero

[Rob Herring <robh@kernel.org>]

On Wed, Apr 13, 2022 at 6:56 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> 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 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: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  arch/riscv/include/asm/timex.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> index 507cae273bc6..d6a7428f6248 100644
> --- a/arch/riscv/include/asm/timex.h
> +++ b/arch/riscv/include/asm/timex.h
> @@ -41,7 +41,7 @@ static inline u32 get_cycles_hi(void)
>  static inline unsigned long random_get_entropy(void)
>  {
>         if (unlikely(clint_time_val == NULL))

Moving this check to get_cycles() implementation would eliminate the
RiscV implementation of random_get_entropy() if you follow my other
suggestion.

I guess there's some advantage to skipping a NULL check every time for
get_cycles(), but really the register read time will be much slower
than an added check.

> -               return 0;
> +               return random_get_entropy_fallback();
>         return get_cycles();
>  }
>  #define random_get_entropy()   random_get_entropy()