Re: [PATCH v2 18/29] arm_mpam: Register and enable IRQs

James Morse
2025-10-02T18:03:06Z

Hi Jonathan,

On 12/09/2025 13:12, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:42:58 +0000
> James Morse <james.morse@arm.com> wrote:
>
>> Register and enable error IRQs. All the MPAM error interrupts indicate a
>> software bug, e.g. out of range partid. If the error interrupt is ever
>> signalled, attempt to disable MPAM.
>>
>> Only the irq handler accesses the ESR register, so no locking is needed.
>> The work to disable MPAM after an error needs to happen at process
>> context as it takes mutex. It also unregisters the interrupts, meaning
>> it can't be done from the threaded part of a threaded interrupt.
>> Instead, mpam_disable() gets scheduled.
>>
>> Enabling the IRQs in the MSC may involve cross calling to a CPU that
>> can access the MSC.
>>
>> Once the IRQ is requested, the mpam_disable() path can be called
>> asynchronously, which will walk structures sized by max_partid. Ensure
>> this size is fixed before the interrupt is requested.

>> @@ -1318,11 +1405,172 @@ static void mpam_enable_merge_features(struct list_head *all_classes_list)
>> }
>> }
>>
>> +static char *mpam_errcode_names[16] = {
>> + [0] = "No error",
>
> I think you had a bunch of defines for these in an earlier patch. Can we use
> that to index here instead of [0] etc.

Sure,

>> + [1] = "PARTID_SEL_Range",
>> + [2] = "Req_PARTID_Range",
>> + [3] = "MSMONCFG_ID_RANGE",
>> + [4] = "Req_PMG_Range",
>> + [5] = "Monitor_Range",
>> + [6] = "intPARTID_Range",
>> + [7] = "Unexpected_INTERNAL",
>> + [8] = "Undefined_RIS_PART_SEL",
>> + [9] = "RIS_No_Control",
>> + [10] = "Undefined_RIS_MON_SEL",
>> + [11] = "RIS_No_Monitor",
>> + [12 ... 15] = "Reserved"
>> +};
>
>
>> +static void mpam_unregister_irqs(void)
>> +{
>> + int irq, idx;
>> + struct mpam_msc *msc;
>> +
>> + cpus_read_lock();
>
> guard(cpus_read_lock)();
> guard(srcu)(&mpam_srcu);

Sure, looks like I didn't realise there was a cpus_read_lock version of this when I went
looking for places to add this.

>> + /* take the lock as free_irq() can sleep */

The comment gets dropped as this mattered for an earlier locking scheme. (but free_irq()
can still sleep)

>> + idx = srcu_read_lock(&mpam_srcu);
>> + list_for_each_entry_srcu(msc, &mpam_all_msc, all_msc_list,
>> + srcu_read_lock_held(&mpam_srcu)) {
>> + irq = platform_get_irq_byname_optional(msc->pdev, "error");
>> + if (irq <= 0)
>> + continue;
>> +
>> + if (test_and_clear_bit(MPAM_ERROR_IRQ_HW_ENABLED, &msc->error_irq_flags))
>> + mpam_touch_msc(msc, mpam_disable_msc_ecr, msc);
>> +
>> + if (test_and_clear_bit(MPAM_ERROR_IRQ_REQUESTED, &msc->error_irq_flags)) {
>> + if (irq_is_percpu(irq)) {
>> + msc->reenable_error_ppi = 0;
>> + free_percpu_irq(irq, msc->error_dev_id);
>> + } else {
>> + devm_free_irq(&msc->pdev->dev, irq, msc);
>> + }
>> + }
>> + }
>> + srcu_read_unlock(&mpam_srcu, idx);
>> + cpus_read_unlock();
>> +}

>> @@ -1332,6 +1580,27 @@ static void mpam_enable_once(void)
>> partid_max_published = true;
>> spin_unlock(&partid_max_lock);
>>
>> + /*
>> + * If all the MSC have been probed, enabling the IRQs happens next.
>> + * That involves cross-calling to a CPU that can reach the MSC, and
>> + * the locks must be taken in this order:
>> + */
>> + cpus_read_lock();
>> + mutex_lock(&mpam_list_lock);
>> + mpam_enable_merge_features(&mpam_classes);
>> +
>> + err = mpam_register_irqs();
>> + if (err)
>> + pr_warn("Failed to register irqs: %d\n", err);
>
> Perhaps move the print into the if (err) below?

More types of error get later, and its maybe useful to know which of these failed.

>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> index 6e047fbd3512..f04a9ef189cf 100644
>> --- a/drivers/resctrl/mpam_internal.h
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -32,6 +32,10 @@ struct mpam_garbage {
>> struct platform_device *pdev;
>> };
>>
>> +/* Bit positions for error_irq_flags */
>> +#define MPAM_ERROR_IRQ_REQUESTED 0
>> +#define MPAM_ERROR_IRQ_HW_ENABLED 1
>
> If there aren't going to be load more of these (I've not really thought
> about whether there might) then using a bitmap for these seems to add complexity
> that we wouldn't see with
> bool error_irq_req;
> bool error_irq_hw_enabled;

It's a bitmap so that mpam_unregister_irqs() can use test_and_clear_bit() on them,
because with a real interrupt mpam_unregister_irqs() can run multiple times in parallel
with itself.
Doing this as bools would mean having a mutex to prevent that from happening.

I'll do that as its a slightly simpler.

Thanks,

James