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

James Morse
2025-09-29T17:45:00Z

Hi Ben,

On 11/09/2025 16:31, Ben Horgan wrote:
> 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.

>> 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.

Yeah - I forgot to rewrite the commit message when I split this patch.
I'll pull those earlier bits into the now:later patch that splits the locking up.

Thanks,

James