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

[Linu Cherian <linu.cherian@arm.com>]

Hi Mark,

On Thu, Jul 02, 2026 at 11:45:14AM +0100, 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.
> 
> 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.

Okay, agree.

--
Thanks,
Linu Cherian.