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Date: Fri, 2 Oct 2026 13:44:44 +0100
From: Will Deacon <will@kernel.org>
To: Tomohiro Misono <misono.tomohiro@fujitsu.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Randy Dunlap <rdunlap@infradead.org>, Marc Zyngier <maz@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Radu Rendec <radu@rendec.net>,
	Kohei Enju <enju.kohei@fujitsu.com>,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-perf-users@vger.kernel.org
Subject: Re: [PATCH 3/4] perf: arm_pmu: Add workaround for FUJITSU-MONAKA
 Erratum E#030002
Message-ID: <ar-nPJyVzPcvbICa@willie-the-truck>
References: <20261002-monaka-fix-for-upstream-v1-0-4aec0b0cbe34@fujitsu.com>
 <20261002-monaka-fix-for-upstream-v1-3-4aec0b0cbe34@fujitsu.com>
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In-Reply-To: <20261002-monaka-fix-for-upstream-v1-3-4aec0b0cbe34@fujitsu.com>

On Fri, Oct 02, 2026 at 07:26:51PM +0900, Tomohiro Misono wrote:
> FUJITSU-MONAKA Erratum E#030002 affects FUJITSU-MONAKA CPU.
> 
> On affected FUJITSU-MONAKA CPUs, after a PMU event counter is
> stopped at a value just before overflow, an overflow might still
> be incorrectly detected, resulting interrupt storm in some case.
> 
> Work around the erratum by avoiding programming PMU event counters
> with values within five counts of overflow.

What's the user-visible behaviour of this erratum? It's not entirely clear
to me why we need to do anything at all.

Will