Re: [PATCH v10 12/14] arm_mpam: detect and enable MPAM-Fb PCC support

[Ben Horgan <ben.horgan@arm.com>]

Hi Andre,

On 11/09/2026 12:28, 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 <andre.przywara@arm.com>
> Tested-by: Ritwick Sharma <ritwick.sharma@arm.com> # on Arm AGI CPU
> Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
> Tested-by: Srivathsa L Rao <srivathsa.rao@oss.qualcomm.com>
> Reviewed-by: Srivathsa L Rao <srivathsa.rao@oss.qualcomm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> ---
>  drivers/resctrl/Kconfig         |   1 +
>  drivers/resctrl/mpam_devices.c  | 119 +++++++++++++++++++++++++++++++-
>  drivers/resctrl/mpam_fb.c       |  41 +++++++++++
>  drivers/resctrl/mpam_internal.h |   2 +
>  4 files changed, 161 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/resctrl/Kconfig b/drivers/resctrl/Kconfig
> index 9591d792736e5..3f81638f57be3 100644
> --- a/drivers/resctrl/Kconfig
> +++ b/drivers/resctrl/Kconfig
> @@ -2,6 +2,7 @@ menuconfig ARM64_MPAM_DRIVER
>  	bool "MPAM driver"
>  	depends on ARM64 && ARM64_MPAM
>  	select ACPI_MPAM if ACPI
> +	select PCC if ACPI
>  	select MAILBOX
>  	help
>  	  Memory System Resource Partitioning and Monitoring (MPAM) driver for
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 14dc8461e4b8f..150a529458388 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -20,14 +20,19 @@
>  #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 */
> @@ -50,6 +55,87 @@ 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.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.
> @@ -2285,6 +2371,8 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>  	if (!msc)
>  		return;
>  
> +	mpam_pcc_chan_put(msc->pcc_chan);
> +
>  	mutex_lock(&mpam_list_lock);
>  	mpam_msc_destroy(msc);
>  	mutex_unlock(&mpam_list_lock);
> @@ -2295,7 +2383,7 @@ 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;
> @@ -2339,7 +2427,7 @@ 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;
> @@ -2360,6 +2448,33 @@ 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) {
> +		u32 mpam_fb_msc_id;
> +
> +		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);
> +		}
> +
> +		if (device_property_read_u32(&pdev->dev, "mpam-fb-msc-id",
> +					     &mpam_fb_msc_id))
> +			mpam_fb_msc_id = msc->id;
> +		msc->mpam_fb_msc_id = mpam_fb_msc_id;

This looks like the change earlier in the series to drivers/acpi/arm64/mpam.c makes no functional
effect unless the proposed spec changes are accepted; the default is msc->id. I'd just move the
earlier drivers/acpi/arm64/mpam.c to your last patch and introduce this device_property_read_u32()
their too (or a separate patch if it's easier for anyone). That way all but the last two patches can
be considered for merging without knowing what's going to happen with the spec.

The struct msc property mpam_fb_msc_id is a bit long. Better as msc->fb_id?

Thanks,

Ben

>  	} else {
>  		return ERR_PTR(-EINVAL);
>  	}
> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
> index 98212f761de38..8fc1d78a7b5ab 100644
> --- a/drivers/resctrl/mpam_fb.c
> +++ b/drivers/resctrl/mpam_fb.c
> @@ -20,10 +20,15 @@
>  #define MPAM_FB_PROTOCOL_ID		0x1a
>  
>  #define MPAM_PROTOCOL_VERSION_CMD	0x0
> +#define MPAM_FB_VERSION_MAJOR_MASK		GENMASK_U32(31, 16)
> +#define MPAM_FB_VERSION_MINOR_MASK		GENMASK_U32(15, 0)
> +
>  #define MPAM_MSC_READ_CMD		0x4
>  #define MPAM_MSC_WRITE_CMD		0x5
>  
>  #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_SUCCESS		 0
>  #define MPAM_FB_ERR_NOT_SUPPORTED	-1
> @@ -209,3 +214,39 @@ int mpam_fb_send_write_request(struct mpam_msc *msc, u16 reg, u32 value)
>  	return mpam_fb_send_request(msc, msc->mpam_fb_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, 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 %d.%d\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 e5a2804facd2d..314d3b48ca9fd 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -538,6 +538,8 @@ 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: