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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/time: use feature fixup in __USE_RTC() instead of cpu feature.
In-Reply-To: <ba2a7a1f-8dcc-ec44-81d4-ef11a9e29377@c-s.fr>
References: <55c267ac6e0cd289970accfafbf9dda11a324c2e.1566802736.git.christophe.leroy@c-s.fr> <87blwc40i4.fsf@concordia.ellerman.id.au> <ba2a7a1f-8dcc-ec44-81d4-ef11a9e29377@c-s.fr>
Date:   Tue, 27 Aug 2019 12:13:51 +1000
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Christophe Leroy <christophe.leroy@c-s.fr> writes:
> Le 26/08/2019 à 13:41, Michael Ellerman a écrit :
>> Christophe Leroy <christophe.leroy@c-s.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@c-s.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.
>
> 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.

Thanks.

>> 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 ?

It's more complicated in that it's not using cpu_has_feature() it's
doing some custom thing that is not used anywhere else. But yeah I guess
it's not much extra complication.

cheers