[PATCH v4 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@xxxxxxxxxxxxxxx>
Signed-off-by: Beata Michalska <beata.michalska@xxxxxxx>
---
 Documentation/arch/arm64/silicon-errata.rst |  2 ++
 arch/arm64/Kconfig                          | 22 +++++++++++++++++++++
 arch/arm64/kernel/cpu_errata.c              |  4 ++++
 arch/arm64/kernel/topology.c                |  9 +++++----
 4 files changed, 33 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 8ce559ac073c7..40c14e8600ae0 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -386,6 +386,10 @@ static const struct midr_range workaround_amu_constcnt_list[] = {
 #ifdef CONFIG_ARM64_ERRATUM_2457168
 	/* Cortex-A510 r0p0-r1p1 */
 	MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1),
+#endif
+#ifdef CONFIG_ARM64_ERRATUM_3821522
+	/* Cortex-A725 r0p0 - r0p2 */
+	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..f1bdb49d5d92e 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 a HW erratum.
+	 * 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