Re: [PATCH v3 16/16] arm_mpam: detect and enable MPAM-Fb PCC support

[Jonathan Cameron]

On Fri, 10 Jul 2026 16:45:20 +0200
Andre Przywara <andre.przywara@xxxxxxx> wrote:

> The Arm MPAM-Fb specification [1] describes a protocol to access MSC
> registers through a firmware interface. This requires a shared memory
> region to hold the message, and a mailbox to trigger the access.
> For ACPI this is wrapped as a PCC channel, described using existing
> ACPI abstractions.
> 
> Add code to parse those PCC table descriptions associated with an MSC,
> and store the parsed information in the MSC struct.
> There can be multiple PCC channels, and each channel can serve multiple
> MSCs, so we need to keep track of the channel usage, using a list and
> a refcount.
> This will be used by the MPAM-Fb access wrapper code.
> 
> [1] https://developer.arm.com/documentation/den0144/latest
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
Hi Andre,

A few things inline.

Thanks,

Jonathan

> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 220b4a06e739..7be98f13dd63 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -19,6 +19,7 @@
>  #include <linux/irqdesc.h>
>  #include <linux/list.h>
>  #include <linux/lockdep.h>
> +#include <linux/mailbox_client.h>
>  #include <linux/mutex.h>
>  #include <linux/platform_device.h>
>  #include <linux/printk.h>
> @@ -27,6 +28,9 @@
>  #include <linux/types.h>
>  #include <linux/workqueue.h>
>  
> +#include <acpi/pcc.h>
> +#include <acpi/acpi_io.h>
> +
>  #include "mpam_internal.h"
>  
>  /* Values for the T241 errata workaround */
> @@ -49,6 +53,88 @@ static LIST_HEAD(mpam_all_msc);
>  
>  struct srcu_struct mpam_srcu;
>  
> +/* PCC channels might be serving multiple MSCs, so keep a refcounted list. */
> +static DEFINE_MUTEX(pcc_chan_list_lock);
> +static LIST_HEAD(pcc_chan_list);
> +
> +static void mpam_pcc_rx_callback(struct mbox_client *cl, void *msg)
> +{
> +	/* TODO: wake up tasks blocked on this MSC's PCC channel */

That sounds like as significant todo. Does it matter for now?

> +}
> +
> +static struct mpam_pcc_chan *mpam_pcc_chan_get(struct device *dev,
> +					       int subspace_id)
> +{
> +	struct mpam_pcc_chan *cur;
> +
> +	mutex_lock(&pcc_chan_list_lock);

guard()

> +
> +	list_for_each_entry(cur, &pcc_chan_list, pcc_chans) {
> +		if (cur->subspace_id == subspace_id) {
> +			cur->refcount++;
> +			mutex_unlock(&pcc_chan_list_lock);
> +
> +			return cur;
> +		}
> +	}
> +
> +	cur = kzalloc_obj(*cur);
> +	if (!cur) {
> +		mutex_unlock(&pcc_chan_list_lock);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
> +	cur->pcc_cl.dev = dev;
> +	cur->pcc_cl.rx_callback = mpam_pcc_rx_callback;
> +	cur->pcc_cl.tx_block = true;
> +	cur->pcc_cl.tx_tout = 1000; /* 1s */
> +
> +	cur->pcc_chan = pcc_mbox_request_channel(&cur->pcc_cl, subspace_id);
> +	if (IS_ERR(cur->pcc_chan)) {
> +		long err = PTR_ERR(cur->pcc_chan);
> +
> +		kfree(cur);
> +		mutex_unlock(&pcc_chan_list_lock);
> +		return ERR_PTR(err);
> +	}
> +
> +	mutex_init(&cur->pcc_chan_lock);
> +	cur->subspace_id = subspace_id;
> +	cur->refcount = 1;
> +
> +	list_add_tail(&cur->pcc_chans, &pcc_chan_list);
> +
> +	mutex_unlock(&pcc_chan_list_lock);
> +
> +	return cur;
> +}
> +
> +static int mpam_pcc_chan_put(struct mpam_pcc_chan *pcc_chan)
> +{
> +	struct mpam_pcc_chan *cur, *tmp;
> +
> +	if (!pcc_chan)
> +		return 0;
> +
> +	mutex_lock(&pcc_chan_list_lock);

guard()

> +
> +	list_for_each_entry_safe(cur, tmp, &pcc_chan_list, pcc_chans) {
> +		if (cur == pcc_chan) {
> +			if (!--cur->refcount) {

Maybe embed a kref_t? Then release. As much as anything acts as documentation
that this is reference counting.

> +				pcc_mbox_free_channel(cur->pcc_chan);
> +				list_del(&pcc_chan->pcc_chans);
> +				kfree(cur);
> +			}
> +			mutex_unlock(&pcc_chan_list_lock);
> +			return 0;
> +		}
> +	}
> +
> +	mutex_unlock(&pcc_chan_list_lock);
> +
> +	return -ENOENT;
> +}
> +
>  /*
>   * Number of MSCs that have been probed. Once all MSCs have been probed MPAM
>   * can be enabled.
> @@ -2208,6 +2294,8 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>  {
>  	struct mpam_msc *msc = platform_get_drvdata(pdev);
>  
> +	mpam_pcc_chan_put(msc->pcc_chan);
> +
>  	mutex_lock(&mpam_list_lock);
>  	mpam_msc_destroy(msc);
>  	mutex_unlock(&mpam_list_lock);
> @@ -2218,7 +2306,7 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>  static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>  {
>  	int err;
> -	u32 tmp;
> +	u32 pcc_subspace_id;
>  	struct mpam_msc *msc;
>  	struct resource *msc_res;
>  	struct device *dev = &pdev->dev;
> @@ -2263,7 +2351,8 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>  	if (err)
>  		return ERR_PTR(err);
>  
> -	if (device_property_read_u32(&pdev->dev, "pcc-channel", &tmp))
> +	if (device_property_read_u32(&pdev->dev, "pcc-channel",
> +				     &pcc_subspace_id))
>  		msc->iface = MPAM_IFACE_MMIO;
>  	else
>  		msc->iface = MPAM_IFACE_PCC;
> @@ -2279,6 +2368,39 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>  		}
>  		msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
>  		mcc->mapped_hwpage = io;
> +	} else if (msc->iface == MPAM_IFACE_PCC) {
> +		u32 msc_id;
> +		int ret;
> +
> +		if (device_property_read_u32(&pdev->dev, "msc-id", &msc_id)) {

earlier patch included of.h, should have been property.h given quite correctly
you are using the generic firmware accessors.

> +			pr_err("missing MPAM-Fb MSC identifier\n");
> +			return ERR_PTR(-EINVAL);
> +		}
> +		msc->mpam_fb_msc_id = msc_id;
> +
> +		msc->pcc_chan = mpam_pcc_chan_get(&pdev->dev, pcc_subspace_id);
> +		if (IS_ERR(msc->pcc_chan)) {
> +			pr_err("Failed to request MSC PCC channel\n");
> +			return (void *)msc->pcc_chan;

ERR_CAST()

> +		}
> +
> +		if (msc->pcc_chan->pcc_chan->shmem_size < MPAM_FB_MAX_MSG_SIZE) {
> +			pr_err("MPAM-Fb PCC channel size too small.\n");
> +			mpam_pcc_chan_put(msc->pcc_chan);
> +			return ERR_PTR(-ENOMEM);
> +		}

Blank line here as next bit is unrelated to the bit above.

> +		ret = mpam_fb_get_protocol_version(msc);
> +		if (ret < 0) {
> +			pr_err("Cannot query MPAM-Fb protocol version.\n");
> +			mpam_pcc_chan_put(msc->pcc_chan);
> +			return ERR_PTR(-EIO);
If there is a good reason to eat the error code, then add a comment. If not.
			return ERR_PTR(ret);

> +		}
> +		if ((ret >> 16) != 1) {
> +			pr_err("Incompatible MPAM-Fb protocol version %d.%d\n",
> +			       ret >> 16, ret & 0xffff);
> +			mpam_pcc_chan_put(msc->pcc_chan);
> +			return ERR_PTR(-EINVAL);
> +		}
>  	} else {
>  		return ERR_PTR(-EINVAL);
>  	}