linux-kernel - Re: [PATCH v4 05/10] powerpc/dt_cpu

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Date:   Tue, 21 Jul 2020 21:29:21 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Jordan Niethe <jniethe5@...il.com>
Cc:     mikey@...ling.org, apopple@...ux.ibm.com,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        naveen.n.rao@...ux.vnet.ibm.com, peterz@...radead.org,
        jolsa@...nel.org, oleg@...hat.com, fweisbec@...il.com,
        mingo@...nel.org, pedromfc@...ibm.com, miltonm@...ibm.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v4 05/10] powerpc/dt_cpu_ftrs: Add feature for 2nd DAWR

Ravi Bangoria <ravi.bangoria@...ux.ibm.com> writes:
> On 7/17/20 11:14 AM, Jordan Niethe wrote:
>> On Fri, Jul 17, 2020 at 2:10 PM Ravi Bangoria
>> <ravi.bangoria@...ux.ibm.com> wrote:
>>>
>>> Add new device-tree feature for 2nd DAWR. If this feature is present,
>>> 2nd DAWR is supported, otherwise not.
>>>
>>> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
>>> ---
>>>   arch/powerpc/include/asm/cputable.h | 7 +++++--
>>>   arch/powerpc/kernel/dt_cpu_ftrs.c   | 7 +++++++
>>>   2 files changed, 12 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
>>> index e506d429b1af..3445c86e1f6f 100644
>>> --- a/arch/powerpc/include/asm/cputable.h
>>> +++ b/arch/powerpc/include/asm/cputable.h
>>> @@ -214,6 +214,7 @@ static inline void cpu_feature_keys_init(void) { }
>>>   #define CPU_FTR_P9_TLBIE_ERAT_BUG      LONG_ASM_CONST(0x0001000000000000)
>>>   #define CPU_FTR_P9_RADIX_PREFETCH_BUG  LONG_ASM_CONST(0x0002000000000000)
>>>   #define CPU_FTR_ARCH_31                        LONG_ASM_CONST(0x0004000000000000)
>>> +#define CPU_FTR_DAWR1                  LONG_ASM_CONST(0x0008000000000000)
>>>
>>>   #ifndef __ASSEMBLY__
>>>
>>> @@ -497,14 +498,16 @@ static inline void cpu_feature_keys_init(void) { }
>>>   #define CPU_FTRS_POSSIBLE      \
>>>              (CPU_FTRS_POWER7 | CPU_FTRS_POWER8E | CPU_FTRS_POWER8 | \
>>>               CPU_FTR_ALTIVEC_COMP | CPU_FTR_VSX_COMP | CPU_FTRS_POWER9 | \
>>> -            CPU_FTRS_POWER9_DD2_1 | CPU_FTRS_POWER9_DD2_2 | CPU_FTRS_POWER10)
>>> +            CPU_FTRS_POWER9_DD2_1 | CPU_FTRS_POWER9_DD2_2 | CPU_FTRS_POWER10 | \
>>> +            CPU_FTR_DAWR1)
>>>   #else
>>>   #define CPU_FTRS_POSSIBLE      \
>>>              (CPU_FTRS_PPC970 | CPU_FTRS_POWER5 | \
>>>               CPU_FTRS_POWER6 | CPU_FTRS_POWER7 | CPU_FTRS_POWER8E | \
>>>               CPU_FTRS_POWER8 | CPU_FTRS_CELL | CPU_FTRS_PA6T | \
>>>               CPU_FTR_VSX_COMP | CPU_FTR_ALTIVEC_COMP | CPU_FTRS_POWER9 | \
>>> -            CPU_FTRS_POWER9_DD2_1 | CPU_FTRS_POWER9_DD2_2 | CPU_FTRS_POWER10)
>>> +            CPU_FTRS_POWER9_DD2_1 | CPU_FTRS_POWER9_DD2_2 | CPU_FTRS_POWER10 | \
>>> +            CPU_FTR_DAWR1)

>> Instead of putting CPU_FTR_DAWR1 into CPU_FTRS_POSSIBLE should it go
>> into CPU_FTRS_POWER10?
>> Then it will be picked up by CPU_FTRS_POSSIBLE.
>
> I remember a discussion about this with Mikey and we decided to do it
> this way. Obviously, the purpose is to make CPU_FTR_DAWR1 independent of
> CPU_FTRS_POWER10 because DAWR1 is an optional feature in p10. I fear
> including CPU_FTR_DAWR1 in CPU_FTRS_POWER10 can make it forcefully enabled
> even when device-tree property is not present or pa-feature bit it not set,
> because we do:
>
>        {       /* 3.1-compliant processor, i.e. Power10 "architected" mode */
>                .pvr_mask               = 0xffffffff,
>                .pvr_value              = 0x0f000006,
>                .cpu_name               = "POWER10 (architected)",
>                .cpu_features           = CPU_FTRS_POWER10,

The pa-features logic will turn it off if the feature bit is not set.

So you should be able to put it in CPU_FTRS_POWER10.

See for example CPU_FTR_NOEXECUTE.

cheers