[PATCH v3 0/2] arm64: errata: Share broken AMU constant-counter handling

[PATCH v3 0/2] arm64: errata: Share broken AMU constant-counter handling

[Beata Michalska]

This series factors out the existing Cortex-A510 erratum 2457168 AMU constant
counter workaround into a generic broken AMU constant-counter capability,
then wires Cortex-A725 erratum 3821522 into the same handling.

---
v3:
- splitted the commit and droped changes for cpucap_is_possible
---

Beata Michalska (2):
  arm64: errata: Factor out broken AMU const counter cap
  arm64: errata: Add Cortex-A725 erratum 3821522 workaround

 Documentation/arch/arm64/silicon-errata.rst |  2 ++
 arch/arm64/Kconfig                          | 30 +++++++++++++++++++++
 arch/arm64/kernel/cpu_errata.c              | 29 +++++++++++++++-----
 arch/arm64/kernel/cpufeature.c              |  2 +-
 arch/arm64/kernel/topology.c                | 16 ++++++-----
 arch/arm64/tools/cpucaps                    |  2 +-
 6 files changed, 66 insertions(+), 15 deletions(-)

-- 
2.43.0

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

[Beata Michalska]

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[] = {
+#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)
+		.matches = cpucap_multi_entry_cap_matches,
+		.match_list = arm64_repeat_amu_constcnt_list
 	},
 #endif
 #ifdef CONFIG_ARM64_ERRATUM_2038923
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 17b83a2518a8f..1569f84214763 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2064,7 +2064,7 @@ static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
 		cpumask_set_cpu(smp_processor_id(), &amu_cpus);
 
 		/* 0 reference values signal broken/disabled counters */
-		if (!this_cpu_has_cap(ARM64_WORKAROUND_2457168))
+		if (!this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT))
 			update_freq_counters_refs();
 	}
 }
diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index d28438f8b83f1..64588554e7f76 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -402,7 +402,7 @@ static void cpu_read_constcnt(void *val)
 	 * the counter is disabled. A return value of 0 at counter read is
 	 * properly handled as an error case by the users of the counter.
 	 */
-	*(u64 *)val = this_cpu_has_cap(ARM64_WORKAROUND_2457168) ?
+	*(u64 *)val = this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT) ?
 		      0UL : read_constcnt();
 }
 
@@ -463,8 +463,9 @@ static void amu_read_core_const_ctrs(void *val)
 
 	/*
 	 * cpu_read_constcnt() incurs slight latency due to the
-	 * ARM64_WORKAROUND_2457168 check. Read it first to minimize
-	 * the sampling skew between the const and core counters.
+	 * ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT check.
+	 * Read it first to minimize the sampling skew between the const
+	 * and core counters.
 	 */
 	cpu_read_constcnt(&ctrs->constcnt);
 	cpu_read_corecnt(&ctrs->corecnt);
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 2775ba3359cfe..58b6c084f9f25 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -105,7 +105,6 @@ WORKAROUND_1902691
 WORKAROUND_2038923
 WORKAROUND_2064142
 WORKAROUND_2077057
-WORKAROUND_2457168
 WORKAROUND_2645198
 WORKAROUND_2658417
 WORKAROUND_4193714
@@ -132,3 +131,4 @@ WORKAROUND_REPEAT_TLBI_SYNC
 WORKAROUND_SPECULATIVE_AT
 WORKAROUND_SPECULATIVE_SSBS
 WORKAROUND_SPECULATIVE_UNPRIV_LOAD
+WORKAROUND_BROKEN_AMU_CONSTCNT
-- 
2.43.0

[PATCH v3 2/2] arm64: errata: Add Cortex-A725 erratum 3821522 workaround

[Beata Michalska]

Cortex-A725 erratum 3821522 affects the CNT_CYCLES event, which can
incur a significant increment error when a CPU enters and subsequently
exits WFE or WFI, and may no longer track the system counter
frequency.

The AMEVCNTR01_EL0 counter is used as the AMU constant counter for
frequency invariance and CPPC FFH feedback counters. Wire the affected
Cortex-A725 range into the shared broken AMU constant-counter capability
so the affected counter is treated as unavailable by returning zero in
the AMU counter paths. This prevents the broken counter from being used
as a reference source.

The erratum can also affect PMUv3 users of the CNT_CYCLES event,
but this workaround intentionally does not change PMU event handling.
Hiding or rejecting the PMU event from the erratum code would change
the perf-visible PMU event interface, including raw event selection,
and would need a separate PMU-specific approach rather than being
folded into the AMU reference-counter workaround.

Cc: <stable@vger.kernel.org>
Signed-off-by: Beata Michalska <beata.michalska@arm.com>
---
 Documentation/arch/arm64/silicon-errata.rst |  2 ++
 arch/arm64/Kconfig                          | 22 +++++++++++++++++++++
 arch/arm64/kernel/cpu_errata.c              |  6 ++++++
 arch/arm64/kernel/topology.c                |  9 +++++----
 4 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/Documentation/arch/arm64/silicon-errata.rst b/Documentation/arch/arm64/silicon-errata.rst
index ac3248b9f2f3b..99a1eb4b833dd 100644
--- a/Documentation/arch/arm64/silicon-errata.rst
+++ b/Documentation/arch/arm64/silicon-errata.rst
@@ -174,6 +174,8 @@ stable kernels.
 +----------------+-----------------+-----------------+-----------------------------+
 | ARM            | Cortex-A725     | #3456106        | ARM64_ERRATUM_3194386       |
 +----------------+-----------------+-----------------+-----------------------------+
+| ARM            | Cortex-A725     | #3821522        | ARM64_ERRATUM_3821522       |
++----------------+-----------------+-----------------+-----------------------------+
 | ARM            | Cortex-X1       | #1502854        | N/A                         |
 +----------------+-----------------+-----------------+-----------------------------+
 | ARM            | Cortex-X1       | #3324344        | ARM64_ERRATUM_3194386       |
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 3b6c052d46a18..cab741a695f52 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1081,6 +1081,28 @@ config ARM64_ERRATUM_2645198
 
 	  If unsure, say Y.
 
+config ARM64_ERRATUM_3821522
+	bool "Cortex-A725: 3821522: workaround for possible CNT_CYCLES increment error due to WFE/WFI"
+	depends on ARM64_AMU_EXTN
+	select ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
+	default y
+	help
+	  This option adds the workaround for ARM Cortex-A725 erratum 3821522.
+
+	  On affected A725 cores, the CNT_CYCLES event may incur a significant
+	  increment error when entering and subsequently exiting WFx.
+	  As a result, the CNT_CYCLES may diverge from the system counter
+	  frequency at which it is expected to increment.
+	  This renders the AMU counter AMEVCNTR01, that implements CNT_CYCLES,
+	  being unreliable and unsuitable for use.
+
+	  Since there is no hardware workaround, reads of the affected CNT_CYCLES
+	  counter return 0 in the relevant paths. This causes users of the counter
+	  to treat it as unavailable and is functionally equivalent to firmware
+	  disabling the affected counter.
+
+	  If unsure, say Y.
+
 config ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD
 	bool
 
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index f18683e56184b..42a6fff8c3f44 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -388,6 +388,12 @@ static const struct arm64_cpu_capabilities arm64_repeat_amu_constcnt_list[] = {
 		/* Cortex-A510 r0p0-r1p1 */
 		CAP_MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1)
 	},
+#endif
+#ifdef CONFIG_ARM64_ERRATUM_3821522
+	{
+		/* Cortex-A725 r0p0 - r0p2 */
+		CAP_MIDR_RANGE(MIDR_CORTEX_A725, 0, 0, 0, 2)
+	},
 #endif
 	{}
 };
diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 64588554e7f76..fc5d3abd12905 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -397,10 +397,11 @@ static void cpu_read_corecnt(void *val)
 static void cpu_read_constcnt(void *val)
 {
 	/*
-	 * Return 0 if the current CPU is affected by erratum 2457168. A value
-	 * of 0 is also returned if the current CPU does not support AMUs or if
-	 * the counter is disabled. A return value of 0 at counter read is
-	 * properly handled as an error case by the users of the counter.
+	 * Return 0 if the current CPU is affected by either erratum 2457168
+	 * or erratum 3821522. A value of 0 is also returned if the current
+	 * CPU does not support AMUs or if the counter is disabled. A return
+	 * value of 0 at counter read is properly handled as an error case
+	 * by the users of the counter.
 	 */
 	*(u64 *)val = this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT) ?
 		      0UL : read_constcnt();
-- 
2.43.0

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

[Vladimir Murzin]

Hi Beata,

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)

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

Re: [PATCH v3 2/2] arm64: errata: Add Cortex-A725 erratum 3821522 workaround

[Vladimir Murzin]

Hi Beata,

On 9/29/26 16:42, Beata Michalska wrote:
> Cortex-A725 erratum 3821522 affects the CNT_CYCLES event, which can
> incur a significant increment error when a CPU enters and subsequently
> exits WFE or WFI, and may no longer track the system counter
> frequency.
> 
> The AMEVCNTR01_EL0 counter is used as the AMU constant counter for
> frequency invariance and CPPC FFH feedback counters. Wire the affected
> Cortex-A725 range into the shared broken AMU constant-counter capability
> so the affected counter is treated as unavailable by returning zero in
> the AMU counter paths. This prevents the broken counter from being used
> as a reference source.
> 
> The erratum can also affect PMUv3 users of the CNT_CYCLES event,
> but this workaround intentionally does not change PMU event handling.
> Hiding or rejecting the PMU event from the erratum code would change
> the perf-visible PMU event interface, including raw event selection,
> and would need a separate PMU-specific approach rather than being
> folded into the AMU reference-counter workaround.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> ---
>  Documentation/arch/arm64/silicon-errata.rst |  2 ++
>  arch/arm64/Kconfig                          | 22 +++++++++++++++++++++
>  arch/arm64/kernel/cpu_errata.c              |  6 ++++++
>  arch/arm64/kernel/topology.c                |  9 +++++----
>  4 files changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/arch/arm64/silicon-errata.rst b/Documentation/arch/arm64/silicon-errata.rst
> index ac3248b9f2f3b..99a1eb4b833dd 100644
> --- a/Documentation/arch/arm64/silicon-errata.rst
> +++ b/Documentation/arch/arm64/silicon-errata.rst
> @@ -174,6 +174,8 @@ stable kernels.
>  +----------------+-----------------+-----------------+-----------------------------+
>  | ARM            | Cortex-A725     | #3456106        | ARM64_ERRATUM_3194386       |
>  +----------------+-----------------+-----------------+-----------------------------+
> +| ARM            | Cortex-A725     | #3821522        | ARM64_ERRATUM_3821522       |
> ++----------------+-----------------+-----------------+-----------------------------+
>  | ARM            | Cortex-X1       | #1502854        | N/A                         |
>  +----------------+-----------------+-----------------+-----------------------------+
>  | ARM            | Cortex-X1       | #3324344        | ARM64_ERRATUM_3194386       |
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 3b6c052d46a18..cab741a695f52 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1081,6 +1081,28 @@ config ARM64_ERRATUM_2645198
>  
>  	  If unsure, say Y.
>  
> +config ARM64_ERRATUM_3821522
> +	bool "Cortex-A725: 3821522: workaround for possible CNT_CYCLES increment error due to WFE/WFI"
> +	depends on ARM64_AMU_EXTN
> +	select ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
> +	default y
> +	help
> +	  This option adds the workaround for ARM Cortex-A725 erratum 3821522.
> +
> +	  On affected A725 cores, the CNT_CYCLES event may incur a significant
> +	  increment error when entering and subsequently exiting WFx.
> +	  As a result, the CNT_CYCLES may diverge from the system counter
> +	  frequency at which it is expected to increment.
> +	  This renders the AMU counter AMEVCNTR01, that implements CNT_CYCLES,
> +	  being unreliable and unsuitable for use.
> +
> +	  Since there is no hardware workaround, reads of the affected CNT_CYCLES
> +	  counter return 0 in the relevant paths. This causes users of the counter
> +	  to treat it as unavailable and is functionally equivalent to firmware
> +	  disabling the affected counter.
> +
> +	  If unsure, say Y.
> +
>  config ARM64_WORKAROUND_SPECULATIVE_UNPRIV_LOAD
>  	bool
>  
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index f18683e56184b..42a6fff8c3f44 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -388,6 +388,12 @@ static const struct arm64_cpu_capabilities arm64_repeat_amu_constcnt_list[] = {
>  		/* Cortex-A510 r0p0-r1p1 */
>  		CAP_MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1)
>  	},
> +#endif
> +#ifdef CONFIG_ARM64_ERRATUM_3821522
> +	{
> +		/* Cortex-A725 r0p0 - r0p2 */
> +		CAP_MIDR_RANGE(MIDR_CORTEX_A725, 0, 0, 0, 2)
> +	},
>  #endif
>  	{}
>  };
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 64588554e7f76..fc5d3abd12905 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -397,10 +397,11 @@ static void cpu_read_corecnt(void *val)
>  static void cpu_read_constcnt(void *val)
>  {
>  	/*
> -	 * Return 0 if the current CPU is affected by erratum 2457168. A value
> -	 * of 0 is also returned if the current CPU does not support AMUs or if
> -	 * the counter is disabled. A return value of 0 at counter read is
> -	 * properly handled as an error case by the users of the counter.
> +	 * Return 0 if the current CPU is affected by either erratum 2457168
> +	 * or erratum 3821522. A value of 0 is also returned if the current

Perhaps wording it generically like:

  Return 0 if the current CPU is affected by a HW erratum.

would save us from having to change the comment next time we encounter a broken AMU :)

Thanks
Vladimir