Re: [PATCH v2 11/29] arm_mpam: Probe hardware to find the supported partid/pmg values

Jonathan Cameron
2025-09-11T15:18:55Z

On Wed, 10 Sep 2025 20:42:51 +0000
James Morse <james.morse@arm.com> wrote:

> CPUs can generate traffic with a range of PARTID and PMG values,
> but each MSC may also have its own maximum size for these fields.
> Before MPAM can be used, the driver needs to probe each RIS on
> each MSC, to find the system-wide smallest value that can be used.
> The limits from requestors (e.g. CPUs) also need taking into account.
>
> While doing this, RIS entries that firmware didn't describe are created
> under MPAM_CLASS_UNKNOWN.
>
> While we're here, implement the mpam_register_requestor() call
> for the arch code to register the CPU limits. Future callers of this
> will tell us about the SMMU and ITS.
>
> Signed-off-by: James Morse <james.morse@arm.com>
Trivial stuff inline.
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

> ---
> Changes since v1:
> * Change to lock ordering now that the list-lock mutex isn't held from
> the cpuhp call.
> * Removed irq-unmaksed assert in requestor register.
> * Changed captialisation in print message.
> ---
> drivers/resctrl/mpam_devices.c | 150 +++++++++++++++++++++++++++++++-
> drivers/resctrl/mpam_internal.h | 6 ++
> include/linux/arm_mpam.h | 14 +++
> 3 files changed, 169 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index c265376d936b..24dc81c15ec8 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c

> +int mpam_register_requestor(u16 partid_max, u8 pmg_max)
> +{
> + int err = 0;
> +
> + spin_lock(&partid_max_lock);
guard() perhaps so you can return early in the error pat and avoid
need for local variable err.

> + if (!partid_max_init) {
> + mpam_partid_max = partid_max;
> + mpam_pmg_max = pmg_max;
> + partid_max_init = true;
> + } else if (!partid_max_published) {
> + mpam_partid_max = min(mpam_partid_max, partid_max);
> + mpam_pmg_max = min(mpam_pmg_max, pmg_max);
> + } else {
> + /* New requestors can't lower the values */
> + if (partid_max < mpam_partid_max || pmg_max < mpam_pmg_max)
> + err = -EBUSY;
> + }
> + spin_unlock(&partid_max_lock);
> +
> + return err;
> +}
> +EXPORT_SYMBOL(mpam_register_requestor);

> @@ -470,9 +547,37 @@ int mpam_ris_create(struct mpam_msc *msc, u8 ris_idx,
> return err;
> }
>
> +static struct mpam_msc_ris *mpam_get_or_create_ris(struct mpam_msc *msc,
> + u8 ris_idx)
> +{
> + int err;
> + struct mpam_msc_ris *ris, *found = ERR_PTR(-ENOENT);
> +
> + lockdep_assert_held(&mpam_list_lock);
> +
> + if (!test_bit(ris_idx, &msc->ris_idxs)) {
> + err = mpam_ris_create_locked(msc, ris_idx, MPAM_CLASS_UNKNOWN,
> + 0, 0);
> + if (err)
> + return ERR_PTR(err);
> + }
> +
> + list_for_each_entry(ris, &msc->ris, msc_list) {
> + if (ris->ris_idx == ris_idx) {
> + found = ris;
I'd return ris;

Then can do return ERR_PTR(-ENOENT) below and not bother with found.

Ignore if this gets more complex later.

> + break;
> + }
> + }
> +
> + return found;
> +}

> @@ -675,9 +813,18 @@ static struct platform_driver mpam_msc_driver = {
>
> static void mpam_enable_once(void)
> {
> + /*
> + * Once the cpuhp callbacks have been changed, mpam_partid_max can no
> + * longer change.
> + */
> + spin_lock(&partid_max_lock);
> + partid_max_published = true;
> + spin_unlock(&partid_max_lock);
> +
> mpam_register_cpuhp_callbacks(mpam_cpu_online, mpam_cpu_offline);
>
> - pr_info("MPAM enabled\n");
> + printk(KERN_INFO "MPAM enabled with %u PARTIDs and %u PMGs\n",
> + mpam_partid_max + 1, mpam_pmg_max + 1);

Not sure why pr_info before and printk now.

> }