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

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Message-ID: <f43bbd94-f611-4817-b5b2-35c5c1d236e8@nvidia.com>
Date: Sat, 4 Oct 2025 17:09:38 -0700
From: Fenghua Yu <fenghuay@...dia.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>,
 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 13:42, 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;
>   
>   	void __iomem		*mapped_hwpage;
>   	size_t			mapped_hwpage_sz;
> @@ -83,6 +88,30 @@ struct mpam_msc {
>   	struct mpam_garbage	garbage;
>   };
>   
> +/* Returning false here means accesses to mon_sel must fail and report an error. */
> +static inline bool __must_check mpam_mon_sel_lock(struct mpam_msc *msc)
> +{
> +	WARN_ON_ONCE(msc->iface != MPAM_IFACE_MMIO);
> +
> +	raw_spin_lock_irqsave(&msc->_mon_sel_lock, msc->_mon_sel_flags);
> +	return true;
> +}
> +
> +static inline void mpam_mon_sel_unlock(struct mpam_msc *msc)
> +{
> +	raw_spin_unlock_irqrestore(&msc->_mon_sel_lock, msc->_mon_sel_flags);
> +}
> +
> +static inline void mpam_mon_sel_lock_held(struct mpam_msc *msc)
> +{
> +	lockdep_assert_held_once(&msc->_mon_sel_lock);
> +}
> +
> +static inline void mpam_mon_sel_lock_init(struct mpam_msc *msc)
> +{
> +	raw_spin_lock_init(&msc->_mon_sel_lock);
> +}
> +
>   struct mpam_class {
>   	/* mpam_components in this class */
>   	struct list_head	components;

The inner and outer locks were defined and used in patch #7; but they 
are replaced by _mon_sel_lock in this patch.

I'm wondering if this patch should be merged into patch #7. This patch 
seems is redundant.

Thanks.

-Fenghua