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 3C0EA371EA6; Tue, 9 Sep 2025 16:57:33 +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=1757437055; cv=none; b=XLMVw+1XmXkOvS58lzCr+jxsXDJeb7BDt5PwljHVuM7lqMHIQylrG/zCz66OjKPwwm9HDAgzJ00VnwebFVcOJawFkhUJZT3fW1xAdQkYV0hiqqHAnR3AS+fEGb2ANSV/FsBfAbqnxZr5DABwiw/9fKKVNDlkdTqR9EX/IvlJ0cw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757437055; c=relaxed/simple; bh=ln9VFRt7yyw3cu3AlJWX2zgOXdCM8GySL4/KhrfAA9o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ocE4LURd0rDKTumWMFNa1z8bhsPut5Q1wPR1htWJsk+pi5OfKMWwxhZYc8OUAVja0qkh2hJZj0aAglRHVcvSV/ugWjdYaG9YiZLLsL35rqqEEAxyiKzy5mshcyIwsZsHpwB3GKMO0Ye/DnyRxv5Mn8zluVqFOD3U9dzfhYP+wu4= 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; 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 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 1201815A1; Tue, 9 Sep 2025 09:57:25 -0700 (PDT) Received: from [10.1.197.69] (eglon.cambridge.arm.com [10.1.197.69]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9EF853F694; Tue, 9 Sep 2025 09:57:27 -0700 (PDT) Message-ID: Date: Tue, 9 Sep 2025 17:57:06 +0100 Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 16/33] arm_mpam: Add helpers for managing the locking around the mon_sel registers To: Fenghua Yu , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-acpi@vger.kernel.org, devicetree@vger.kernel.org Cc: shameerali.kolothum.thodi@huawei.com, D Scott Phillips OS , carl@os.amperecomputing.com, lcherian@marvell.com, bobo.shaobowang@huawei.com, tan.shaopeng@fujitsu.com, baolin.wang@linux.alibaba.com, Jamie Iles , Xin Hao , peternewman@google.com, dfustini@baylibre.com, amitsinght@marvell.com, David Hildenbrand , Rex Nie , Dave Martin , Koba Ko , Shanker Donthineni , baisheng.gao@unisoc.com, Jonathan Cameron , Rob Herring , Rohit Mathew , Rafael Wysocki , Len Brown , Lorenzo Pieralisi , Hanjun Guo , Sudeep Holla , Krzysztof Kozlowski , Conor Dooley , Catalin Marinas , Will Deacon , Greg Kroah-Hartman , Danilo Krummrich References: <20250822153048.2287-1-james.morse@arm.com> <20250822153048.2287-17-james.morse@arm.com> <4c8da7b3-6235-4d0a-aeb1-81c8bdfd051a@nvidia.com> Content-Language: en-GB From: James Morse In-Reply-To: <4c8da7b3-6235-4d0a-aeb1-81c8bdfd051a@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Fenghua, On 28/08/2025 18:07, Fenghua Yu wrote: > On 8/22/25 08:29, James Morse wrote: >> The MSC MON_SEL register needs to be accessed from hardirq context by the >> PMU drivers, making an irqsave spinlock the obvious lock to protect these >> registers. On systems with SCMI mailboxes it must be able to sleep, meaning >> a mutex must be used. >> >> Clearly these two can't exist at the same time. >> >> Add helpers for the MON_SEL locking. The outer lock must be taken in a >> pre-emptible context before the inner lock can be taken. On systems with >> SCMI mailboxes where the MON_SEL accesses must sleep - the inner lock >> will fail to be 'taken' if the caller is unable to sleep. This will allow >> the PMU driver to fail without having to check the interface type of >> each MSC. >> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h >> index a623f405ddd8..c6f087f9fa7d 100644 >> --- a/drivers/resctrl/mpam_internal.h >> +++ b/drivers/resctrl/mpam_internal.h >> @@ -68,10 +68,19 @@ struct mpam_msc { >>         /* >>        * mon_sel_lock protects access to the MSC hardware registers that are >> -     * affeted by MPAMCFG_MON_SEL. >> +     * affected by MPAMCFG_MON_SEL, and the mbwu_state. >> +     * Both the 'inner' and 'outer' must be taken. >> +     * For real MMIO MSC, the outer lock is unnecessary - but keeps the >> +     * code common with: >> +     * Firmware backed MSC need to sleep when accessing the MSC, which >> +     * means some code-paths will always fail. For these MSC the outer >> +     * lock is providing the protection, and the inner lock fails to >> +     * be taken if the task is unable to sleep. >> +     * >>        * If needed, take msc->probe_lock first. >>        */ >>       struct mutex        outer_mon_sel_lock; >> +    bool            outer_lock_held; > Is it better to define outer_lock_held at atomic_t? Writes a protected by the outer lock, its just something to generate a warning for debug. I can make it a READ_ONCE() if you're worried about torn values in the failure case. (as its just for debug, I'm not worried about false-negatives) >>       raw_spinlock_t        inner_mon_sel_lock; >>       unsigned long        inner_mon_sel_flags; >>   @@ -81,6 +90,52 @@ struct mpam_msc { >>       struct mpam_garbage    garbage; >>   }; >>   +static inline bool __must_check mpam_mon_sel_inner_lock(struct mpam_msc *msc) >> +{ >> +    /* >> +     * The outer lock may be taken by a CPU that then issues an IPI to run >> +     * a helper that takes the inner lock. lockdep can't help us here. >> +     */ >> +    WARN_ON_ONCE(!msc->outer_lock_held); > > At this point, msc->outer_lock_held might not be true yet due to no memory barrier on it > on this CPU. The IPI machinery has this covered: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/irqchip/irq-gic.c#n838 > If it's atomic_t and it's set as true on another CPU by smp_store_release(), > it's guaranteed to be visible as true on this CPU. Without atomic setting, we may see a > false warning here and cause debug difficult. Thanks, James