linux-kernel - Re: [PATCH v7 11/17] openrisc: account for 0 starting value in random_get

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Date:   Sat, 30 Apr 2022 10:19:03 +0900
From:   Stafford Horne <shorne@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH v7 11/17] openrisc: account for 0 starting value in
 random_get_entropy()

Hi Jason,

On Fri, Apr 29, 2022 at 02:16:48AM +0200, Jason A. Donenfeld wrote:
> As a sanity check, this series makes sure that during early boot, the
> cycle counter isn't returning all zeros. However, OpenRISC's TTCR timer
> can be rather slow and starts out as zero during stages of early boot.
> We know it works, however. So just always add 1 to random_get_entropy()
> so that it doesn't trigger these checks.

Just one nit, you might want to qualify that this is related to simulators/qemu:
 * "However, in simulators OpenRISC's TTCR timer can be rather slow..."

> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Jonas Bonn <jonas@...thpole.se>
> Cc: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
> Acked-by: Stafford Horne <shorne@...il.com>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> Changes v6->v7:
> - Add 1 to cycle counter to account for functional but slow-to-begin
>   counter on QEMU.
> 
>  arch/openrisc/include/asm/timex.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/openrisc/include/asm/timex.h b/arch/openrisc/include/asm/timex.h
> index d52b4e536e3f..a78a5807c927 100644
> --- a/arch/openrisc/include/asm/timex.h
> +++ b/arch/openrisc/include/asm/timex.h
> @@ -23,6 +23,9 @@ static inline cycles_t get_cycles(void)
>  {
>  	return mfspr(SPR_TTCR);
>  }
> +#define get_cycles get_cycles
> +
> +#define random_get_entropy() ((unsigned long)get_cycles() + 1)
>  
>  /* This isn't really used any more */
>  #define CLOCK_TICK_RATE 1000

Thanks,

-Stafford