linux-kernel - Re: [PATCH] powerpc/time: use feature fixup in __USE

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Message-ID: <60da7620a43dc29317a062f1d58dcfde8d32b258.camel@kernel.crashing.org>
Date:   Mon, 26 Aug 2019 23:25:14 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/time: use feature fixup in __USE_RTC() instead
 of cpu feature.

On Mon, 2019-08-26 at 21:41 +1000, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@....fr> writes:
> > sched_clock(), used by printk(), calls __USE_RTC() to know
> > whether to use realtime clock or timebase.
> > 
> > __USE_RTC() uses cpu_has_feature() which is initialised by
> > machine_init(). Before machine_init(), __USE_RTC() returns true,
> > leading to a program check exception on CPUs not having realtime
> > clock.
> > 
> > In order to be able to use printk() earlier, use feature fixup.
> > Feature fixups are applies in early_init(), enabling the use of
> > printk() earlier.
> > 
> > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > ---
> >  arch/powerpc/include/asm/time.h | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> The other option would be just to make this a compile time decision, eg.
> add CONFIG_PPC_601 and use that to gate whether we use RTC.
> 
> Given how many 601 users there are, maybe 1?, I think that would be a
> simpler option and avoids complicating the code / binary for everyone
> else.

Didn't we ditch 601 support years ago anyway ? We had workaround we
threw out I think...

Cheers,
Ben.

> cheers
> 
> > diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
> > index 54f4ec1f9fab..3455cb54c333 100644
> > --- a/arch/powerpc/include/asm/time.h
> > +++ b/arch/powerpc/include/asm/time.h
> > @@ -42,7 +42,14 @@ struct div_result {
> >  /* Accessor functions for the timebase (RTC on 601) registers. */
> >  /* If one day CONFIG_POWER is added just define __USE_RTC as 1 */
> >  #ifdef CONFIG_PPC_BOOK3S_32
> > -#define __USE_RTC()	(cpu_has_feature(CPU_FTR_USE_RTC))
> > +static inline bool __USE_RTC(void)
> > +{
> > +	asm_volatile_goto(ASM_FTR_IFCLR("nop;", "b %1;", %0) ::
> > +			  "i" (CPU_FTR_USE_RTC) :: l_use_rtc);
> > +	return false;
> > +l_use_rtc:
> > +	return true;
> > +}
> >  #else
> >  #define __USE_RTC()	0
> >  #endif
> > -- 
> > 2.13.3