Re: [PATCH 2/5] arm64: cpufeature: Detect BBML3 based on MMFR2 ID

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 02/07/26 4:15 PM, Mark Rutland wrote:
> On Wed, Jul 01, 2026 at 03:11:28PM +0530, Linu Cherian wrote:
>> Add MMFR2 ID based BBML3 feature detection, so
>> that compliant cpus doesn't need to be added to the
>> midr list.
>>
>> Signed-off-by: Linu Cherian <linu.cherian@arm.com>
>> ---
>>  arch/arm64/kernel/cpufeature.c | 14 +++++++-------
>>  arch/arm64/tools/sysreg        |  1 +
>>  2 files changed, 8 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 9986eb7b379c..d754b1b7da77 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -2133,6 +2133,7 @@ static bool hvhe_possible(const struct arm64_cpu_capabilities *entry,
>>  
>>  bool cpu_supports_bbml3(void)
>>  {
>> +	u64 mmfr2;
>>  	/* CPUs that support BBML3 but dont advertise through MMFR2 ID */
>>  	static const struct midr_range supports_bbml3_list[] = {
>>  		MIDR_REV_RANGE(MIDR_CORTEX_X4, 0, 3, 0xf),
>> @@ -2144,15 +2145,14 @@ bool cpu_supports_bbml3(void)
>>  		{}
>>  	};
>>  
>> -	if (!is_midr_in_range_list(supports_bbml3_list))
>> -		return false;
>> +	if (is_midr_in_range_list(supports_bbml3_list))
>> +		return true;
>>  
>> -	/*
>> -	 * We currently ignore the ID_AA64MMFR2_EL1 register, and only care
>> -	 * about whether the MIDR check passes.
>> -	 */
>> +	mmfr2 = __read_sysreg_by_encoding(SYS_ID_AA64MMFR2_EL1);
>> +	if (SYS_FIELD_GET(ID_AA64MMFR2_EL1, BBM, mmfr2) == ID_AA64MMFR2_EL1_BBM_3)
>> +		return true;
> 
> This needs to be '>=', so that if there's a future BBML4, we correctly
> detect that CPUs with BBML4 also have the BBML3 behaviour.

That's right.
> 
> It would also be better to check the ID field first, before falling back
> to the MIDR check. That way a reader can more clearly see that
> supports_bbml3_list catches older parts that don't advertised BBML3, and
> the comment above supports_bbml3_list would be clearer.

Flipping the check order makes things clearer.
> 
> With those changes, this looks sane to me.
> 
> Mark.
> 
>>  
>> -	return true;
>> +	return false;
>>  }
>>  
>>  static bool has_bbml3(const struct arm64_cpu_capabilities *caps, int scope)
>> diff --git a/arch/arm64/tools/sysreg b/arch/arm64/tools/sysreg
>> index bc1788b1662b..082256ec3bf9 100644
>> --- a/arch/arm64/tools/sysreg
>> +++ b/arch/arm64/tools/sysreg
>> @@ -2259,6 +2259,7 @@ UnsignedEnum	55:52	BBM
>>  	0b0000	0
>>  	0b0001	1
>>  	0b0010	2
>> +	0b0011	3
>>  EndEnum
>>  UnsignedEnum	51:48	TTL
>>  	0b0000	NI
>> -- 
>> 2.43.0
>>

Please move above tools/sysreg change into a separate patch
as an update for existing ID_AA64MMFR2_EL1 definition.