Re: [PATCH v2 12/29] arm_mpam: Add helpers for managing the locking around the mon_sel registers

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Wed, 10 Sep 2025 20:42:52 +0000
James Morse <james.morse@arm.com> 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.

Comments talk about outer locks, but not actually seeing that in the current code.

> 
> Signed-off-by: James Morse <james.morse@arm.com>

 ---
> Change since v1:
>  * Made accesses to outer_lock_held READ_ONCE() for torn values in the failure
>    case.
Comment on wrong patch?  No READ_ONCE() in here.

> ---
>  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;