Re: [PATCH v7 10/11] arm_mpam: change MPAM-Fb error IRQ to use a threaded IRQ handler

[Jonathan Cameron]

On Fri, 31 Jul 2026 19:03:23 +0200
Andre Przywara <andre.przywara@xxxxxxx> 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
> an MSC access as well.
> 
> When an MSC is using MPAM-Fb, change the error IRQ to use a threaded IRQ
> handler, with an empty hard IRQ routine, and doing all the MSC accesses
> (to access the status and disable the IRQ line) in the threaded part.
> Also forbid per-CPU interrupts (PPIs) for MPAM-Fb, as we cannot use a
> threaded IRQ here.
> 
> The change in the actual IRQ handler is minimal, we just check for the
> first MSC access error and bail out early. MMIO based MSCs keep using a
> hard-IRQ handler, since they must be at least non-migrate-able when doing
> MSC accesses, for the CPU affinity check to work.
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>

A small thing inline about a refactor that I can't spot a reason for.
May it's one of those cases where the code evolved into something that
would benefit from another look.

Either way I'm fine with this
Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>

> ---
>  drivers/resctrl/mpam_devices.c | 52 +++++++++++++++++++++++++---------
>  1 file changed, 39 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index ea08c12f717a..b238feb559d4 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -2741,19 +2755,19 @@ static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
>  			   msc->id, mpam_errcode_names[errcode], partid, pmg,
>  			   ris);
>  
> -	/* Disable this interrupt. */
> +out_disable:
> +	/* Disable this interrupt. Ignore errors, we need to proceed anyway. */
>  	mpam_disable_msc_ecr(msc);
>  
> -	/* Are we 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.
> +	 * Check whether we are racing with the thread disabling MPAM.
>  	 */
> -	mpam_disable_reason = "hardware error interrupt";
> -	schedule_work(&mpam_broken_work);
> +	if (mpam_is_enabled()) {
> +		mpam_disable_reason = "hardware error interrupt";
> +		schedule_work(&mpam_broken_work);
> +	}

Why the logic flip here?  (!mpam_is_enabled() and early return vs
if (mpam_is_enabled())

It seems to be doing exactly the same.
Is it to couple the comment more tightly with the code?

I don't mind it done this way but it is a reasonable amount of churn
for no strong benefit that I can immediately spot.

>  
>  	return IRQ_HANDLED;
>  }