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

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Date:   Tue, 27 Aug 2019 12:12:20 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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.

Benjamin Herrenschmidt <benh@...nel.crashing.org> writes:

> 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...

Paul said his still booted recently.

cheers