Re: [PATCH v3 13/16] arm_mpam: prepare mon_sel locking for MPAM-Fb

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Fri, 10 Jul 2026 16:45:17 +0200
Andre Przywara <andre.przywara@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.
> 
> Change the mon_sel locking wrapper function to only use a spinlock when
> the MSC is accessed directly via MMIO. In case of MPAM-Fb, we use a
> mutex, but only if we are in a sleepable context. If that's not the
> case, we return an error. This should not happen, as MPAM-Fb by design
> does not require an MSC access to happen from a specific CPU, so there
> is no need for any IPIs or preemption disabling to satisfy CPU
> constraints. And since overflow interrupts are not supported at the moment
> anyway, we also wouldn't meet the other case.
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>

I've not been paying attention to this topic, so I might come back
with some more feedback after I've had time to think about it a bit!
So trivial stuff only for now.

>  
>  static inline void mpam_mon_sel_lock_init(struct mpam_msc *msc)
>  {
>  	raw_spin_lock_init(&msc->_mon_sel_lock);
> +	mutex_init(&msc->mon_sel_mutex);

I'm not that fussed, but maybe add a call to mutex_destroy().
Most likely it'll never help catch anything in this code though..

>  }
>  
>  /* Bits for mpam features bitmaps */