Re: [PATCH] powerpc/time: use feature fixup in __USE_RTC() instead of cpu feature.

[Christophe Leroy]

Le 26/08/2019 Ã 13:41, Michael Ellerman a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> 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@xxxxxx>
> > ---
> >   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.

Euh ... yes OK, why not. And that would help simplify many places in the 
code. I can propose something in that direction, but it will be a bigger 
change.


> 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.

However this patch doesn't complicate things more than it was with 
cpu_has_feature() which does exactly the same but using static keys, 
does it ?

Christophe


> 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