Re: [PATCH v4 05/10] powerpc/dt_cpu_ftrs: Add feature for 2nd DAWR

[Michael Ellerman <mpe@ellerman.id.au>]

Ravi Bangoria <ravi.bangoria@linux.ibm.com> writes:
> On 7/21/20 4:59 PM, Michael Ellerman wrote:
>> Ravi Bangoria <ravi.bangoria@linux.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@linux.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@linux.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.

When we use DT CPU features we don't use CPU_FTRS_POWER10 at all.

We construct a cpu_spec from scratch with just the base set of features:

static struct cpu_spec __initdata base_cpu_spec = {
	.cpu_name		= NULL,
	.cpu_features		= CPU_FTRS_DT_CPU_BASE,


And then individual features are enabled via the device tree flags.

cheers