From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id C59A7486E6A for ; Thu, 1 Oct 2026 12:12:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790856775; cv=none; b=ofYldji7zeUyjcqyCL5UMUMKkViJEgXEbIdL7WtmlDntyk7j4B1RVc4J2pPz71ZJiQJT/m02BisDUB0PUDA1JIe/vWIa782z59HKfm2JmvFYrk70A793gMIYasWt47NckBJpprszD1KEP9Gc5PL17d+9D1DdZTfX7B/Ws5cv4vA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790856775; c=relaxed/simple; bh=lIRzzkK1k4nk3ikZ7PiqNu7IU4+fhqAmi6tUa1OnN9k=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uaSZbbkkSqkeMRD/aib82+Oi0ftpjrAFl3VAkszZRzyTyqkdcDg1Hr1jMjJO4RbpnQQO+wSX7DCcKC+Ag83u7ITjrBCsPyAd5wfyuOYy1/gopTHfjEWev8KKaGNvF3IcYggLxE5W1cbr4COA0AF25YYiy7eiTV5nuxdR6sbMgb0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=bjKNRXn7; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="bjKNRXn7" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A8084497; Thu, 1 Oct 2026 05:12:44 -0700 (PDT) Received: from [10.0.153.43] (e121487-lin.cambridge.arm.com [10.0.153.43]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EBCB73F85F; Thu, 1 Oct 2026 05:12:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1790856768; bh=lIRzzkK1k4nk3ikZ7PiqNu7IU4+fhqAmi6tUa1OnN9k=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=bjKNRXn7yo/UqNgFkcF3IY3sku/lgQnv7xJJVzYk+46XnA4wAlk5OdTu9PnKXdeYN vXViWtLWHi30C7LI2m58Qt5PdpIFSYBfaLSQI6o7rzhaHMCZdjFvwPaNturdSmpsO2 Uevn/9QA66598RIWWXygq335bWJ2XEVjQu+dqSTE= Message-ID: <1575b9c1-872f-4c09-89d7-8b2bd284e770@arm.com> Date: Thu, 1 Oct 2026 13:12:44 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/2] arm64: errata: Add Cortex-A725 erratum 3821522 workaround To: Beata Michalska , linux-arm-kernel@lists.infradead.org Cc: catalin.marinas@arm.com, will@kernel.org, suzuki.poulose@arm.com, James.Morse@arm.com, xuewen.yan94@gmail.com, seanwang1@lenovo.com, linux-kernel@vger.kernel.org References: <20261001090050.3849551-1-beata.michalska@arm.com> <20261001090050.3849551-3-beata.michalska@arm.com> Content-Language: en-GB From: Vladimir Murzin In-Reply-To: <20261001090050.3849551-3-beata.michalska@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/1/26 10:00, 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: > Signed-off-by: Beata Michalska > --- > 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 > Reviewed-by: Vladimir Murzin Thanks Vladimir