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

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Message-ID: <YmyRATzJBQX+wnIP@zx2c4.com>
Date:   Sat, 30 Apr 2022 03:29:37 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Stafford Horne <shorne@...il.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 Stafford,

On Sat, Apr 30, 2022 at 10:19:03AM +0900, Stafford Horne wrote:
> 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..."

Nice catch, will do.

Jason

> 
> > 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