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

[Vladimir Murzin <vladimir.murzin@arm.com>]

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