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

[James Morse]

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