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

[Christophe Leroy]

Le 26/08/2019 Ã 15:25, Benjamin Herrenschmidt a ÃcritÂ:
> On Mon, 2019-08-26 at 21:41 +1000, Michael Ellerman wrote:
> > 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.
> >
> > 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...

There are still workarounds for 601 in the kernel, for exemple (in 
arch/powerpc/include/asm/ppc_asm.h)

/* various errata or part fixups */
#ifdef CONFIG_PPC601_SYNC_FIX
#define SYNC				\
BEGIN_FTR_SECTION			\
	sync;				\
	isync;				\
END_FTR_SECTION_IFSET(CPU_FTR_601)
#define SYNC_601			\
BEGIN_FTR_SECTION			\
	sync;				\
END_FTR_SECTION_IFSET(CPU_FTR_601)
#define ISYNC_601			\
BEGIN_FTR_SECTION			\
	isync;				\
END_FTR_SECTION_IFSET(CPU_FTR_601)
#else
#define	SYNC
#define SYNC_601
#define ISYNC_601
#endif

But if you think we can get rid of 601 completely, I'm happy with that.

Christophe


> 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