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Subject: Re: [PATCH] powerpc/time: use feature fixup in __USE_RTC() instead of
 cpu feature.
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <55c267ac6e0cd289970accfafbf9dda11a324c2e.1566802736.git.christophe.leroy@c-s.fr>
 <87blwc40i4.fsf@concordia.ellerman.id.au>
 <60da7620a43dc29317a062f1d58dcfde8d32b258.camel@kernel.crashing.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <bbfb63db-4d73-b638-79f3-9bee3b6749f9@c-s.fr>
Date:   Mon, 26 Aug 2019 15:40:07 +0200
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Le 26/08/2019 à 15:25, Benjamin Herrenschmidt a écrit :
> On Mon, 2019-08-26 at 21:41 +1000, Michael Ellerman wrote:
>> 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.
>>
>> 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