Re: [PATCH v10 RESEND 05/15] arm_mpam: Don't stall MPAM enable when an MSC fails to probe

[Ben Horgan]

Hi Jinjie,



On 20/10/2026 14:42, Jinjie Ruan wrote:
> mpam_msc_drv_probe() registers the discovery cpuhp callbacks only when
> the number of successfully probed MSCs reaches fw_num_msc. Every failure
> path returns before the counter is incremented, and a failed probe is
> final - the driver core does not retry. One failing MSC therefore leaves
> the counter below fw_num_msc forever: the discovery callbacks are never
> registered, mpam_enable() is never scheduled, and MPAM stays dead
> system-wide - the resctrl filesystem is never registered, and no error
> message hints at the cause.
> 
> An MSC can legitimately fail to probe on an otherwise working system:
> firmware may describe MSCs that are not accessible from any possible
> CPU, e.g. when the possible CPUs are restricted with nr_cpus=. The MSCs
> of the clipped CPUs then fail with "MSC is not accessible from any
> CPU!", or their cache resources become unresolvable from PPTT.

As the MPAM resources are system wide we need to query the support from each of the MSC to know what
resources will end up. I do appreciate that if you have full control of your system then you can
know this in advance but then you can also adjust your acpi tables to mark the non-accessible MSC as
disabled. As such, I don't think changes such as this should be in the driver. Here is a previous
explanation for a similar patch [1]

Thanks,

Ben

[1] https://lore.kernel.org/lkml/59c21340-4e3d-4c48-9d9f-c0f78a023bac@xxxxxxx/

> 
> Count every completed probe, successful or not. Failed MSCs are not on
> mpam_all_msc (either never added, or removed by mpam_msc_drv_remove()),
> so the discovery path skips them naturally, and MPAM is enabled for the
> surviving devices.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Ben Horgan <ben.horgan@xxxxxxx>
> Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Cc: Fenghua Yu <fenghuay@xxxxxxxxxx>
> Cc: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Fixes: f04046f2577a5 ("arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate")
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> ---
>  drivers/resctrl/mpam_devices.c | 26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 61cbc75625fa..a44d66a5733d 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -2120,21 +2120,31 @@ static int mpam_msc_drv_probe(struct platform_device *pdev)
>  	msc = do_mpam_msc_drv_probe(pdev);
>  	mutex_unlock(&mpam_list_lock);
>  
> -	if (IS_ERR(msc))
> -		return PTR_ERR(msc);
> +	if (IS_ERR(msc)) {
> +		err = PTR_ERR(msc);
> +		goto out;
> +	}
>  
>  	/* Create RIS entries described by firmware */
>  	err = acpi_mpam_parse_resources(msc, plat_data);
> -	if (err) {
> +	if (err)
>  		mpam_msc_drv_remove(pdev);
> -		return err;
> -	}
> -
> -	if (atomic_add_return(1, &mpam_num_msc) == fw_num_msc)
> +out:
> +	/*
> +	 * A failed MSC must not prevent MPAM being enabled for the
> +	 * surviving devices. Firmware may describe MSCs that are not
> +	 * accessible from any possible CPU, e.g. when the possible CPUs
> +	 * were restricted by nr_cpus=. The failed device is not
> +	 * re-probed, so count it here too: otherwise the counter never
> +	 * reaches fw_num_msc, the discovery cpuhp callbacks are never
> +	 * registered and mpam_enable() is never scheduled - MPAM stays
> +	 * dead system-wide, without any error message.
> +	 */
> +	if (atomic_inc_return(&mpam_num_msc) == fw_num_msc)
>  		mpam_register_cpuhp_callbacks(mpam_discovery_cpu_online, NULL,
>  					      "mpam:drv_probe");
>  
> -	return 0;
> +	return err;
>  }
>  
>  static struct platform_driver mpam_msc_driver = {