Re: [PATCH v7 11/11] arm_mpam: detect and enable MPAM-Fb PCC support

Hi Andre,

On 7/31/2026 10:33 PM, Andre Przywara 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 <[email protected]>
> Tested-by: Ritwick Sharma <[email protected]>
> Reviewed-by: Jonathan Cameron <[email protected]>
> ---
>   drivers/resctrl/mpam_devices.c  | 113 +++++++++++++++++++++++++++++++-
>   drivers/resctrl/mpam_fb.c       |  41 ++++++++++++
>   drivers/resctrl/mpam_internal.h |   2 +
>   3 files changed, 154 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index b238feb559d4..40b247a89f8e 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -19,14 +19,19 @@
>   #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>
> +#include <linux/property.h>
>   #include <linux/srcu.h>
>   #include <linux/spinlock.h>
>   #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 +54,88 @@ 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_chan_release(struct kref *ref)
> +{
> +	struct mpam_pcc_chan *cur = container_of(ref, struct mpam_pcc_chan,
> +						 refcount);
> +
> +	pcc_mbox_free_channel(cur->pcc_chan);
> +	list_del(&cur->pcc_chans);
> +	mutex_destroy(&cur->pcc_chan_lock);
> +	kfree(cur);
> +}
> +
> +static struct mpam_pcc_chan *mpam_pcc_chan_get(struct device *dev,
> +					       int subspace_id)
> +{
> +	struct mpam_pcc_chan *cur;
> +
> +	guard(mutex)(&pcc_chan_list_lock);
> +
> +	list_for_each_entry(cur, &pcc_chan_list, pcc_chans) {
> +		if (cur->subspace_id == subspace_id) {
> +			kref_get(&cur->refcount);
> +
> +			return cur;
> +		}
> +	}
> +
> +	cur = kzalloc_obj(*cur);
> +	if (!cur)
> +		return ERR_PTR(-ENOMEM);
> +
> +	cur->pcc_cl.dev = dev;

While testing v7 with multiple PCC MSCs sharing a single PCC channel, I
noticed that pcc_cl.dev is set once at channel creation time (from the
first MSC's device) and never updated when subsequent MSCs reuse the
channel.In mpam_pcc_chan_get(), the first call sets:
  cur->pcc_cl.dev = dev;

Subsequent calls return the existing channel without updating 
pcc_cl.dev. Here dev_err() calls through the channel report errors 
against the first MSC's device, even when a different MSC triggered them.

I reproduced this with four PCC MSCs sharing subspace 0. After unbinding
the first MSC (kref 4→3), triggering dev_err() via a test hook showed 
the device prefix change from "mpam_msc mpam_msc.1:" to "platform 
mpam_msc.1:"

A simple fix maybe to use the MSC's own device for error messages in
mpam_fb_send_request() instead of pcc_chan->pcc_cl.dev

> +	cur->pcc_cl.tx_block = true;
> +
> +	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);
> +		return ERR_PTR(err);
> +	}
> +
> +	/*
> +	 * Timeout based on the "nominal latency" in us, from the
> +	 * PCC ACPI table. tx_tout is in ms.
> +	 * Add some margin here to be on the safe side.
> +	 */
> +	cur->pcc_cl.tx_tout = DIV_ROUND_UP(cur->pcc_chan->latency * 5, 1000);
> +
> +	mutex_init(&cur->pcc_chan_lock);
> +
> +	cur->subspace_id = subspace_id;
> +	kref_init(&cur->refcount);
> +
> +	list_add_tail(&cur->pcc_chans, &pcc_chan_list);
> +
> +	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;
> +
> +	guard(mutex)(&pcc_chan_list_lock);
> +
> +	list_for_each_entry_safe(cur, tmp, &pcc_chan_list, pcc_chans) {
> +		if (cur == pcc_chan) {
> +			kref_put(&cur->refcount, mpam_pcc_chan_release);
> +
> +			return 0;
> +		}
> +	}
> +
> +	return -ENOENT;
> +}
> +
>   /*
>    * Number of MSCs that have been probed. Once all MSCs have been probed MPAM
>    * can be enabled.
> @@ -2274,6 +2361,8 @@ 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);
> @@ -2284,7 +2373,7 @@ 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;
> @@ -2328,7 +2417,7 @@ 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(dev, "pcc-channel", &pcc_subspace_id))
>   		msc->iface = MPAM_IFACE_MMIO;
>   	else
>   		msc->iface = MPAM_IFACE_PCC;
> @@ -2349,6 +2438,26 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>   		}
>   		msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
>   		msc->mapped_hwpage = io;
> +	} else if (msc->iface == MPAM_IFACE_PCC) {
> +		msc->pcc_chan = mpam_pcc_chan_get(dev, pcc_subspace_id);
> +		if (IS_ERR(msc->pcc_chan)) {
> +			pr_err("Failed to request MSC PCC channel\n");
> +			return ERR_CAST(msc->pcc_chan);
> +		}
> +
> +		err = mpam_fb_check_shared_buffer_size(msc);
> +		if (err) {
> +			mpam_pcc_chan_put(msc->pcc_chan);
> +
> +			return ERR_PTR(err);
> +		}
> +
> +		err = mpam_fb_check_protocol_version(msc);
> +		if (err) {
> +			mpam_pcc_chan_put(msc->pcc_chan);
> +
> +			return ERR_PTR(err);
> +		}
>   	} else {
>   		return ERR_PTR(-EINVAL);
>   	}
> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
> index 61a91c6cec0a..7a7fb6d067ba 100644
> --- a/drivers/resctrl/mpam_fb.c
> +++ b/drivers/resctrl/mpam_fb.c
> @@ -37,6 +37,11 @@
>   #define MPAM_MSC_TOKEN_MASK		GENMASK(27, 18)
>   
>   #define MPAM_FB_PROT_HEADER_LEN		sizeof(u32)
> +/* The longest message is MPAM_MSC_WRITE, with 4 parameters. */
> +#define MPAM_FB_MAX_MSG_SIZE		(4 * sizeof(u32))
> +
> +#define MPAM_FB_VERSION_MAJOR_MASK	GENMASK(31, 16)
> +#define MPAM_FB_VERSION_MINOR_MASK	GENMASK(15, 0)
>   
>   static atomic_t mpam_fb_token = ATOMIC_INIT(0);
>   
> @@ -207,3 +212,39 @@ int mpam_fb_send_write_request(struct mpam_msc *msc, u16 reg, u32 value)
>   	return mpam_fb_send_request(msc->pcc_chan, msc->id, reg, &value,
>   				    MPAM_MSC_WRITE_CMD);
>   }
> +
> +/* We only support MPAM-Fb protocol version 1.x */
> +int mpam_fb_check_protocol_version(struct mpam_msc *msc)
> +{
> +	u32 version;
> +	int ret;
> +
> +	ret = mpam_fb_send_request(msc->pcc_chan, 0,
> +				   0, &version, MPAM_PROTOCOL_VERSION_CMD);
> +	if (ret)
> +		return ret;
> +
> +	if (FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, version) != 1) {
> +		pr_err("Incompatible MPAM-Fb protocol version %ld.%ld\n",
> +		       FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, version),
> +		       FIELD_GET(MPAM_FB_VERSION_MINOR_MASK, version));
> +
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +int mpam_fb_check_shared_buffer_size(struct mpam_msc *msc)
> +{
> +	int min_buffer_size = MPAM_FB_MAX_MSG_SIZE +
> +			      sizeof(struct acpi_pcct_ext_pcc_shared_memory);
> +
> +	if (msc->pcc_chan->pcc_chan->shmem_size < min_buffer_size) {
> +		pr_err("MPAM-Fb PCC channel size too small.\n");
> +
> +		return -ENOMEM;
> +	}
> +
> +	return 0;
> +}
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index f13a79a84c58..8859189088f3 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -528,6 +528,8 @@ static inline void mpam_resctrl_teardown_class(struct mpam_class *class) { }
>   /* MPAM-Fb Firmware-backed protocol wrappers */
>   int mpam_fb_send_read_request(struct mpam_msc *msc, u16 reg, u32 *result);
>   int mpam_fb_send_write_request(struct mpam_msc *msc, u16 reg, u32 value);
> +int mpam_fb_check_protocol_version(struct mpam_msc *msc);
> +int mpam_fb_check_shared_buffer_size(struct mpam_msc *msc);
>   
>   /*
>    * MPAM MSCs have the following register layout. See:

--
Best Regards,
Srivathsa