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

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Date:   Mon, 26 Aug 2019 21:41:39 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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.

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.

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