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

[Mark Rutland <mark.rutland@arm.com>]

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.

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.

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
>