Re: [PATCH v12 12/13] arm_mpam: change MPAM-Fb error IRQ to use a threaded IRQ handler

[Ben Horgan]

Hi Andre,

On 01/10/2026 16:34, Andre Przywara wrote:
> When an MPAM MSC gets into an error condition, it can trigger an error
> IRQ. We cannot really do much about those errors, but we at least query
> and log the error, then disable MPAM functionality.
> 
> This error report relies on reading the MSC's error status register
> (ESR) in the current hard-IRQ handler, which is not possible for MPAM-Fb
> based MSC accesses, since they involve mailbox routines that might sleep.
> The same is true for clearing the interrupt at the source, which requires
> a (potentially sleeping) MSC access as well.
> 
> Change the error IRQ handler to be a threaded interrupt, but keep the
> handling in the hard-IRQ part for MMIO MSCs. This is needed since the
> CPU affinity check in the MSC accessors requires a non-preemptible
> context.
> When the MSC is using an MPAM-Fb based access, we push the work into the
> threaded part of the handler, where the accessors are allowed to sleep.
> Also forbid per-CPU interrupts (PPIs) for MPAM-Fb, as we cannot use a
> threaded IRQ here.
> 
> The actual IRQ handler learns how to deal with errors. We cannot really
> handle them, but we can try our best to disable the IRQ anyway.
> Should the level IRQ line deactivation fail on the device side, we mask
> the IRQ on the irqchip level, to prevent an interrupt storm.
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
> Reviewed-by: Srivathsa L Rao <srivathsa.rao@xxxxxxxxxxxxxxxx>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>

Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>

Thanks,

Ben

> ---
>  drivers/resctrl/mpam_devices.c | 77 ++++++++++++++++++++++++++++------
>  1 file changed, 65 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 63469f3b0a90a..08b1e84e3f794 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -2734,24 +2734,42 @@ static int mpam_disable_msc_ecr(void *_msc)
>  	return __mpam_write_reg(msc, MPAMF_ECR, 0);
>  }
>  
> +/*
> + * This will run as the threaded IRQ handler part when using MPAM-Fb, but
> + * as the sole hard-IRQ handler for MMIO based accesses.
> + */
>  static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  {
>  	u64 reg;
> +	int ret;
>  	u16 partid;
>  	u8 errcode, pmg, ris;
>  
> -	if (WARN_ON_ONCE(!msc) ||
> +	if (WARN_ON_ONCE(!msc))
> +		return IRQ_NONE;
> +
> +	if (msc->iface == MPAM_IFACE_MMIO &&
>  	    WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(),
>  					   &msc->accessibility)))
>  		return IRQ_NONE;
>  
> -	mpam_msc_read_esr(msc, &reg);
> +	ret = mpam_msc_read_esr(msc, &reg);
> +	if (ret) {
> +		pr_err_ratelimited("unknown error irq from msc:%u\n", msc->id);
> +
> +		/* Try out best here ... */
> +		goto out_disable;
> +	}
>  
>  	errcode = FIELD_GET(MPAMF_ESR_ERRCODE, reg);
>  	if (!errcode)
>  		return IRQ_NONE;
>  
> -	/* Clear level triggered irq */
> +	/*
> +	 * Clear the level triggered IRQ. If that fails, we cannot do anything
> +	 * about it, so ignore any errors. We will disable the IRQ either on
> +	 * the device side or on the irqchip level next anyway.
> +	 */
>  	mpam_msc_clear_esr(msc);
>  
>  	partid = FIELD_GET(MPAMF_ESR_PARTID_MON, reg);
> @@ -2762,16 +2780,24 @@ static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  			   msc->id, mpam_errcode_names[errcode], partid, pmg,
>  			   ris);
>  
> -	/* Disable this interrupt. */
> -	mpam_disable_msc_ecr(msc);
> +out_disable:
> +	/*
> +	 * Disable this interrupt on the device side. If that fails, disable
> +	 * the IRQ on the irqchip level, as we must prevent further handler
> +	 * invocations. We only take the interrupt once anyway, as we
> +	 * are going to free the IRQ next, in mpam_disable().
> +	 */
> +	ret = mpam_disable_msc_ecr(msc);
> +	if (ret)
> +		disable_irq_nosync(irq);
>  
> -	/* Are we racing with the thread disabling MPAM? */
> +	/* Check whether we are racing with the thread disabling MPAM. */
>  	if (!mpam_is_enabled())
>  		return IRQ_HANDLED;
>  
>  	/*
> -	 * Schedule the teardown work. Don't use a threaded IRQ as we can't
> -	 * unregister the interrupt from the threaded part of the handler.
> +	 * Schedule the teardown work. We have to defer it as we can't
> +	 * unregister the interrupt from the threaded part of a handler.
>  	 */
>  	mpam_disable_reason = "hardware error interrupt";
>  	schedule_work(&mpam_broken_work);
> @@ -2786,10 +2812,30 @@ static irqreturn_t mpam_ppi_handler(int irq, void *dev_id)
>  	return __mpam_irq_handler(irq, msc);
>  }
>  
> -static irqreturn_t mpam_spi_handler(int irq, void *dev_id)
> +/*
> + * MMIO based MSC accesses must run in non-preemptible context, as they
> + * might have affinity requirements to check.
> + * MPAM-Fb based MSC accesses must NOT run in hard-IRQ context, as they
> + * can sleep.
> + * So split the IRQ handling up, depending on the MSC access type.
> + */
> +static irqreturn_t mpam_shared_hard_irq(int irq, void *dev_id)
>  {
>  	struct mpam_msc *msc = dev_id;
>  
> +	if (msc->iface == MPAM_IFACE_MMIO)
> +		return __mpam_irq_handler(irq, msc);
> +
> +	return IRQ_WAKE_THREAD;
> +}
> +
> +static irqreturn_t mpam_shared_thread_irq(int irq, void *dev_id)
> +{
> +	struct mpam_msc *msc = dev_id;
> +
> +	if (msc->iface == MPAM_IFACE_MMIO)
> +		return IRQ_HANDLED;
> +
>  	return __mpam_irq_handler(irq, msc);
>  }
>  
> @@ -2810,6 +2856,11 @@ static int mpam_register_irqs(void)
>  		/* The MPAM spec says the interrupt can be SPI, PPI or LPI */
>  		/* We anticipate sharing the interrupt with other MSCs */
>  		if (irq_is_percpu(irq)) {
> +			if (msc->iface != MPAM_IFACE_MMIO) {
> +				dev_err(&msc->pdev->dev,
> +					"Only MMIO MSCs can use per-CPU interrupts\n");
> +				return -EINVAL;
> +			}
>  			err = request_percpu_irq(irq, &mpam_ppi_handler,
>  						 "mpam:msc:error",
>  						 msc->error_dev_id);
> @@ -2821,9 +2872,11 @@ static int mpam_register_irqs(void)
>  					       &_enable_percpu_irq, &irq,
>  					       true);
>  		} else {
> -			err = devm_request_irq(&msc->pdev->dev, irq,
> -					       &mpam_spi_handler, IRQF_SHARED,
> -					       "mpam:msc:error", msc);
> +			err = devm_request_threaded_irq(&msc->pdev->dev, irq,
> +							&mpam_shared_hard_irq,
> +							&mpam_shared_thread_irq,
> +							IRQF_SHARED | IRQF_ONESHOT,
> +							"mpam:msc:error", msc);
>  			if (err)
>  				return err;
>  		}