Re: [PATCH v3 1/2] arm64: errata: Factor out broken AMU const counter cap

[Beata Michalska <beata.michalska@arm.com>]

On Wed, Sep 30, 2026 at 10:21:28AM +0100, Vladimir Murzin wrote:
> Hi Beata,
Hi Vladimir,
> 
> On 9/29/26 16:42, Beata Michalska wrote:
> > Move the workaround from the erratum 2457168-specific cpucap to a generic
> > broken AMU constant-counter one. This keeps the existing Cortex-A510
> > handling unchanged while allowing other errata with similar AMU constant
> > counter issue to share the capability bit and call sites.
> > 
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> > ---
> >  arch/arm64/Kconfig             |  8 ++++++++
> >  arch/arm64/kernel/cpu_errata.c | 23 +++++++++++++++++------
> >  arch/arm64/kernel/cpufeature.c |  2 +-
> >  arch/arm64/kernel/topology.c   |  7 ++++---
> >  arch/arm64/tools/cpucaps       |  2 +-
> >  5 files changed, 31 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index b5a51b0ef9440..3b6c052d46a18 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -1040,9 +1040,17 @@ config ARM64_ERRATUM_1902691
> >  
> >  	  If unsure, say Y.
> >  
> > +config ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
> > +	bool
> > +	# Targeting errata affecting AMEVCNTR01, the AMU constant counter.
> > +	# On affected CPUs the counter may not increment at the expected rate,
> > +	# making it unreliable. As a result, AMU users should treat it as
> > +	# unavailable rather than relying on misleading counter values.
> > +
> >  config ARM64_ERRATUM_2457168
> >  	bool "Cortex-A510: 2457168: workaround for AMEVCNTR01 incrementing incorrectly"
> >  	depends on ARM64_AMU_EXTN
> > +	select ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
> >  	default y
> >  	help
> >  	  This option adds the workaround for ARM Cortex-A510 erratum 2457168.
> > diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> > index 5db8f0619e4bc..f18683e56184b 100644
> > --- a/arch/arm64/kernel/cpu_errata.c
> > +++ b/arch/arm64/kernel/cpu_errata.c
> > @@ -381,6 +381,18 @@ static const struct arm64_cpu_capabilities arm64_repeat_tlbi_list[] = {
> >  };
> >  #endif
> >  
> > +#ifdef CONFIG_ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
> > +static const struct arm64_cpu_capabilities arm64_repeat_amu_constcnt_list[] = {
>                                                     ^^^^^^
>                                            There is nothing to repeat :)
> 
> > +#ifdef CONFIG_ARM64_ERRATUM_2457168
> > +	{
> > +		/* Cortex-A510 r0p0-r1p1 */
> > +		CAP_MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1)
> > +	},
> > +#endif
> > +	{}
> > +};
> > +#endif /* CONFIG_ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT */
> > +
> >  #ifdef CONFIG_CAVIUM_ERRATUM_23154
> >  static const struct midr_range cavium_erratum_23154_cpus[] = {
> >  	MIDR_ALL_VERSIONS(MIDR_THUNDERX),
> > @@ -916,14 +928,13 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
> >  		ERRATA_MIDR_REV_RANGE(MIDR_CORTEX_A510, 0, 0, 2)
> >  	},
> >  #endif
> > -#ifdef CONFIG_ARM64_ERRATUM_2457168
> > +#ifdef CONFIG_ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
> >  	{
> > -		.desc = "ARM erratum 2457168",
> > -		.capability = ARM64_WORKAROUND_2457168,
> > +		.desc = "Broken AMU AMEVCNTR01 (const counter)",
> > +		.capability = ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT,
> >  		.type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
> > -
> > -		/* Cortex-A510 r0p0-r1p1 */
> > -		CAP_MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1)
> 
> I think it should be
> 
>                 CAP_MIDR_RANGE_LIST(amu_broken_constcnt_cpus)
True, that's more appropriate here.
Thanks

---
BR
Beata
> 
> where amu_broken_constcnt_cpus is
> 
> static const struct midr_range amu_broken_constcnt_cpus[] = {
> #ifdef CONFIG_ARM64_ERRATUM_2457168
>        /* Cortex-A510 r0p0-r1p1 */
>        MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1),
> #endif
>        {}
> };
> 
> so in the next patch you would add new entry
> 
> #ifdef CONFIG_ARM64_ERRATUM_3821522
>        /* Cortex-A725 r0p0 - r0p2 */
>        MIDR_RANGE(MIDR_CORTEX_A725, 0, 0, 0, 2),
> #endif
> 
> 
> Cheers
> Vladimir
>