linux-kernel - Re: [PATCH v2 12/29] arm_mpam: Add helpers for managing the locking around the mon

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Message-ID: <aaefa134-b520-42d2-a700-4f7382b0318f@arm.com>
Date: Thu, 11 Sep 2025 16:31:43 +0100
From: Ben Horgan <ben.horgan@....com>
To: James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org
Cc: D Scott Phillips OS <scott@...amperecomputing.com>,
 carl@...amperecomputing.com, lcherian@...vell.com,
 bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
 baolin.wang@...ux.alibaba.com, Jamie Iles <quic_jiles@...cinc.com>,
 Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
 dfustini@...libre.com, amitsinght@...vell.com,
 David Hildenbrand <david@...hat.com>, Dave Martin <dave.martin@....com>,
 Koba Ko <kobak@...dia.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 fenghuay@...dia.com, baisheng.gao@...soc.com,
 Jonathan Cameron <jonathan.cameron@...wei.com>, Rob Herring
 <robh@...nel.org>, Rohit Mathew <rohit.mathew@....com>,
 Rafael Wysocki <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>, Hanjun Guo
 <guohanjun@...wei.com>, Sudeep Holla <sudeep.holla@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 12/29] arm_mpam: Add helpers for managing the locking
 around the mon_sel registers

Hi James,

On 9/10/25 21:42, James Morse wrote:
> The MSC MON_SEL register needs to be accessed from hardirq for the overflow
> interrupt, and when taking an IPI to access these registers on platforms
> where MSC are not accesible from every CPU. This makes 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. The
> SCMI platforms can't support an overflow interrupt.
> 
> Clearly these two can't exist for one MSC 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
> callers to fail without having to explicitly check the interface type of
> each MSC.
> 
> Signed-off-by: James Morse <james.morse@....com>
> ---
> Change since v1:
>  * Made accesses to outer_lock_held READ_ONCE() for torn values in the failure
>    case.
> ---
>  drivers/resctrl/mpam_devices.c  |  3 +--
>  drivers/resctrl/mpam_internal.h | 37 +++++++++++++++++++++++++++++----
>  2 files changed, 34 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 24dc81c15ec8..a26b012452e2 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -748,8 +748,7 @@ static int mpam_msc_drv_probe(struct platform_device *pdev)
>  
>  		mutex_init(&msc->probe_lock);
>  		mutex_init(&msc->part_sel_lock);
> -		mutex_init(&msc->outer_mon_sel_lock);
> -		raw_spin_lock_init(&msc->inner_mon_sel_lock);
> +		mpam_mon_sel_lock_init(msc);
>  		msc->id = pdev->id;
>  		msc->pdev = pdev;
>  		INIT_LIST_HEAD_RCU(&msc->all_msc_list);
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 828ce93c95d5..4cc44d4e21c4 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -70,12 +70,17 @@ struct mpam_msc {
>  
>  	/*
>  	 * mon_sel_lock protects access to the MSC hardware registers that are
> -	 * affected by MPAMCFG_MON_SEL.
> +	 * affected by MPAMCFG_MON_SEL, and the mbwu_state.
> +	 * Access to mon_sel is needed from both process and interrupt contexts,
> +	 * but is complicated by firmware-backed platforms that can't make any
> +	 * access unless they can sleep.
> +	 * Always use the mpam_mon_sel_lock() helpers.
> +	 * Accessed to mon_sel need to be able to fail if they occur in the wrong
> +	 * context.
>  	 * If needed, take msc->probe_lock first.
>  	 */
> -	struct mutex		outer_mon_sel_lock;
> -	raw_spinlock_t		inner_mon_sel_lock;
> -	unsigned long		inner_mon_sel_flags;
> +	raw_spinlock_t		_mon_sel_lock;
> +	unsigned long		_mon_sel_flags;
>  

These stale variables can be removed in the patch that introduced them,
outer_mon_sel_lock, inner_mon_sel_lock, inner_mon_sel_flags. Jonathan
has already pointed out the stale comment and paragraph in the commit
message.

Thanks,

Ben