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

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Message-ID: <20250911161850.00005667@huawei.com>
Date: Thu, 11 Sep 2025 16:18:50 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: James Morse <james.morse@....com>
CC: <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-acpi@...r.kernel.org>, D Scott Phillips OS
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Subject: Re: [PATCH v2 11/29] arm_mpam: Probe hardware to find the supported
 partid/pmg values

On Wed, 10 Sep 2025 20:42:51 +0000
James Morse <james.morse@....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@....com>
Trivial stuff inline.
Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.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.  

>  }