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

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Date:   Tue, 21 Jul 2020 19:12:23 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        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



On 7/21/20 4:59 PM, Michael Ellerman wrote:
> 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.

Ah ok. scan_features() clears the feature if the bit is not set in
pa-features. So it should work find for powervm. I'll verify the same
thing happens in case of baremetal where we use cpu-features not
pa-features. If it works in baremetal as well, will put it in
CPU_FTRS_POWER10.

Thanks for the clarification,
Ravi