Re: [PATCH v11 11/13] arm_mpam: add MPAM-Fb MSC firmware access support

[Andre Przywara <andre.przywara@arm.com>]

Hi Ben,

On 9/30/26 16:07, Ben Horgan wrote:
> Hi Andre,
> 
> On 24/09/2026 16:27, Andre Przywara wrote:
>> The Arm MPAM Firmware-backed (Fb) Profile document[1] describes an
>> alternative way of accessing the "Memory System Components" (MSC) in an
>> MPAM enabled system.
>>
>> Normally the MSCs are MMIO mapped, but in some implementations this
>> might not be possible (MSC located outside of the local socket, MSC
>> mapped secure-only) or desirable (direct MMIO access too slow or needs
>> to be mediated through a control processor). MPAM-fb standardises a
>> protocol to abstract MSC accesses, building on the SCMI protocol.
>>
>> Add functions that do an MSC read or write access by redirecting the
>> request through a firmware interface. For now this done via an ACPI
>> PCC shared memory and mailbox combination.
>>
>> Since the protocol used is only a small subset of the full SCMI spec,
>> and the SCMI protocol has no full ACPI support anyway, open-code the
>> (simple) SCMI message generation, for just the fields we need.
>>
>> [1] https://developer.arm.com/documentation/den0144/latest
>>
>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>> Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
>> Tested-by: Ritwick Sharma <ritwick.sharma@arm.com> # on Arm AGI CPU
>> Reviewed-by: Gavin Shan <gshan@redhat.com>
>> Reviewed-by: Srivathsa L Rao <srivathsa.rao@oss.qualcomm.com>
>> ---
>>   drivers/acpi/arm64/mpam.c       |   6 +-
>>   drivers/resctrl/Kconfig         |   1 +
>>   drivers/resctrl/Makefile        |   2 +-
>>   drivers/resctrl/mpam_devices.c  |  65 ++++++++--
>>   drivers/resctrl/mpam_fb.c       | 212 ++++++++++++++++++++++++++++++++
>>   drivers/resctrl/mpam_internal.h |  19 +++
>>   include/linux/arm_mpam.h        |   2 +-
>>   7 files changed, 291 insertions(+), 16 deletions(-)
>>   create mode 100644 drivers/resctrl/mpam_fb.c
>>
>> diff --git a/drivers/acpi/arm64/mpam.c b/drivers/acpi/arm64/mpam.c
>> index 84963a20c3e78..c57bd367922b7 100644
>> --- a/drivers/acpi/arm64/mpam.c
>> +++ b/drivers/acpi/arm64/mpam.c
>> @@ -220,8 +220,8 @@ static struct platform_device * __init acpi_mpam_parse_msc(struct acpi_mpam_msc_
>>   	struct platform_device *pdev __free(platform_device_put) =
>>   		platform_device_alloc("mpam_msc", tbl_msc->identifier);
>>   	int next_res = 0, next_prop = 0, err;
>> -	/* pcc, nrdy, affinity and a sentinel */
>> -	struct property_entry props[4] = { 0 };
>> +	/* pcc, msc-id, nrdy, affinity and a sentinel */
>> +	struct property_entry props[5] = { 0 };
>>   	/* mmio, 2xirq, no sentinel. */
>>   	struct resource res[3] = { 0 };
>>   	struct acpi_device *companion;
>> @@ -256,6 +256,8 @@ static struct platform_device * __init acpi_mpam_parse_msc(struct acpi_mpam_msc_
>>   	} else if (iface == MPAM_IFACE_PCC) {
>>   		props[next_prop++] = PROPERTY_ENTRY_U32("pcc-channel",
>>   							tbl_msc->base_address);
>> +		props[next_prop++] = PROPERTY_ENTRY_U32("mpam-fb-msc-id",
>> +							tbl_msc->identifier);
>>   	}
>>   
>>   	acpi_mpam_parse_irqs(pdev, tbl_msc, res, &next_res);
>> diff --git a/drivers/resctrl/Kconfig b/drivers/resctrl/Kconfig
>> index 672abea3b03cc..9591d792736e5 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 MAILBOX
>>   	help
>>   	  Memory System Resource Partitioning and Monitoring (MPAM) driver for
>>   	  System IP, e.g. caches and memory controllers.
>> diff --git a/drivers/resctrl/Makefile b/drivers/resctrl/Makefile
>> index 4f6d0e81f9b8f..097c036724e97 100644
>> --- a/drivers/resctrl/Makefile
>> +++ b/drivers/resctrl/Makefile
>> @@ -1,5 +1,5 @@
>>   obj-$(CONFIG_ARM64_MPAM_DRIVER)			+= mpam.o
>> -mpam-y						+= mpam_devices.o
>> +mpam-y						+= mpam_devices.o mpam_fb.o
>>   mpam-$(CONFIG_ARM64_MPAM_RESCTRL_FS)		+= mpam_resctrl.o
>>   
>>   ccflags-$(CONFIG_ARM64_MPAM_DRIVER_DEBUG)	+= -DDEBUG
>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
>> index 4c548ec1bd7be..90161696aada0 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -12,6 +12,7 @@
>>   #include <linux/cpu.h>
>>   #include <linux/cpumask.h>
>>   #include <linux/device.h>
>> +#include <linux/errname.h>
>>   #include <linux/errno.h>
>>   #include <linux/gfp.h>
>>   #include <linux/interrupt.h>
>> @@ -82,6 +83,21 @@ static DECLARE_WORK(mpam_broken_work, &mpam_disable);
>>   
>>   /* When mpam is disabled, the printed reason to aid debugging */
>>   static char *mpam_disable_reason;
>> +static int mpam_disable_errno;
>> +static int mpam_disable_mpam_fb_err;
>> +
>> +void mpam_fb_disable_mpam(int err, int mpam_fb_err)
>> +{
>> +	/* Prevent repeated calls when mpam_disable() does MSC accesses. */
>> +	if (!mpam_is_enabled())
>> +		return;
>> +
>> +	mpam_disable_errno = err;
>> +	mpam_disable_mpam_fb_err = mpam_fb_err;
>> +
>> +	mpam_disable_reason = "MPAM-Fb error";
>> +	schedule_work(&mpam_broken_work);
>> +}
>>   
>>   /*
>>    * Whether resctrl has been setup. Used by cpuhp in preference to
>> @@ -179,8 +195,11 @@ static void mpam_assert_partid_sizes_fixed(void)
>>   
>>   static int __mpam_read_reg(struct mpam_msc *msc, u16 reg, u32 *res)
>>   {
>> -	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>>   
>> +	if (msc->iface == MPAM_IFACE_PCC)
>> +		return mpam_fb_send_read_request(msc, reg, res);
>> +
>> +	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>>   	*res = readl_relaxed(msc->mapped_hwpage + reg);
>>   
>>   	return 0;
>> @@ -197,9 +216,12 @@ static inline int _mpam_read_partsel_reg(struct mpam_msc *msc, u16 reg,
>>   
>>   static int __mpam_write_reg(struct mpam_msc *msc, u16 reg, u32 val)
>>   {
>> -	WARN_ON_ONCE(reg + sizeof(u32) > msc->mapped_hwpage_sz);
>> -	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>>   
>> +	if (msc->iface == MPAM_IFACE_PCC)
>> +		return mpam_fb_send_write_request(msc, reg, val);
>> +
>> +	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>> +	WARN_ON_ONCE(reg + sizeof(u32) > msc->mapped_hwpage_sz);
>>   	writel_relaxed(val, msc->mapped_hwpage + reg);
>>   
>>   	return 0;
>> @@ -1143,8 +1165,11 @@ static int mpam_msc_read_mbwu_l(struct mpam_msc *msc, u64 *res)
>>   
>>   	mpam_mon_sel_lock_held(msc);
>>   
>> -	WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
>> -	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>> +	if (msc->iface == MPAM_IFACE_MMIO) {
>> +		WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
>> +		WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(),
>> +					       &msc->accessibility));
>> +	}
>>   
>>   	ret = __mpam_read_reg(msc, MSMON_MBWU_L + 4, &mbwu_l_high2);
>>   	if (ret)
>> @@ -1178,8 +1203,11 @@ static int mpam_msc_zero_mbwu_l(struct mpam_msc *msc)
>>   
>>   	mpam_mon_sel_lock_held(msc);
>>   
>> -	WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
>> -	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>> +	if (msc->iface == MPAM_IFACE_MMIO) {
>> +		WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
>> +		WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(),
>> +					       &msc->accessibility));
>> +	}
>>   
>>   	ret = __mpam_write_reg(msc, MSMON_MBWU_L, 0);
>>   	if (ret)
>> @@ -1490,11 +1518,16 @@ static int _msmon_read(struct mpam_component *comp, struct mon_read *arg)
>>   					 srcu_read_lock_held(&mpam_srcu)) {
>>   			arg->ris = ris;
>>   
>> -			err = smp_call_function_any(&msc->accessibility,
>> -						    __ris_msmon_read, arg,
>> -						    true);
>> -			if (!err && arg->err)
>> +			if (msc->iface == MPAM_IFACE_MMIO) {
>> +				err = smp_call_function_any(&msc->accessibility,
>> +							    __ris_msmon_read,
>> +							    arg, true);
>> +				if (!err)
>> +					err = arg->err;
>> +			} else {
>> +				__ris_msmon_read(arg);
>>   				err = arg->err;
>> +			}
>>   
>>   			/*
>>   			 * Save one error to be returned to the caller, but
>> @@ -1977,6 +2010,9 @@ static int mpam_get_msc_preferred_cpu(struct mpam_msc *msc)
>>   
>>   static int mpam_touch_msc(struct mpam_msc *msc, int (*fn)(void *a), void *arg)
>>   {
>> +	if (msc->iface != MPAM_IFACE_MMIO)
>> +		return fn(arg);
>> +
>>   	lockdep_assert_irqs_enabled();
>>   	lockdep_assert_cpus_held();
>>   	WARN_ON_ONCE(!srcu_read_lock_held((&mpam_srcu)));
>> @@ -3102,7 +3138,12 @@ void mpam_disable(struct work_struct *ignored)
>>   	mutex_unlock(&mpam_list_lock);
>>   	mpam_free_garbage();
>>   
>> -	pr_err_once("MPAM disabled due to %s\n", mpam_disable_reason);
>> +	if (mpam_disable_errno || mpam_disable_mpam_fb_err)
>> +		pr_err_once("MPAM disabled due to %s: %s, MPAM-Fb error %d\n",
>> +			    mpam_disable_reason, errname(mpam_disable_errno),
>> +			    mpam_disable_mpam_fb_err);
>> +	else
>> +		pr_err_once("MPAM disabled due to %s\n", mpam_disable_reason);
>>   }
>>   
>>   /*
>> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
>> new file mode 100644
>> index 0000000000000..8298fcedd2be7
>> --- /dev/null
>> +++ b/drivers/resctrl/mpam_fb.c
>> @@ -0,0 +1,212 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +// Copyright (C) 2024-2026 Arm Ltd.
>> +
>> +#include <linux/arm_mpam.h>
>> +#include <linux/cleanup.h>
>> +#include <linux/errno.h>
>> +#include <linux/mailbox_client.h>
>> +#include <linux/mutex.h>
>> +#include <linux/platform_device.h>
>> +#include <linux/types.h>
>> +
>> +#include <acpi/pcc.h>
>> +#include <asm/mpam.h>
>> +
>> +#include "mpam_internal.h"
>> +
>> +#define MPAM_MSC_TOKEN_MASK		GENMASK_U32(27, 18)
>> +#define MPAM_MSC_PROT_ID_MASK		GENMASK_U32(17, 10)
>> +
>> +#define MPAM_FB_PROTOCOL_ID		0x1a
>> +
>> +#define MPAM_PROTOCOL_VERSION_CMD	0x0
>> +#define MPAM_MSC_READ_CMD		0x4
>> +#define MPAM_MSC_WRITE_CMD		0x5
>> +
>> +#define MPAM_FB_PROT_HEADER_LEN		sizeof(u32)
>> +
>> +#define MPAM_FB_SUCCESS		 0
>> +#define MPAM_FB_ERR_NOT_SUPPORTED	-1
>> +#define MPAM_FB_ERR_INVALID_PARAMETERS	-2
>> +#define MPAM_FB_ERR_DENIED		-3
>> +#define MPAM_FB_ERR_NOT_FOUND		-4
>> +#define MPAM_FB_ERR_OUT_OF_RANGE	-5
>> +#define MPAM_FB_ERR_BUSY		-6
>> +#define MPAM_FB_ERR_COMMS_ERROR		-7
>> +#define MPAM_FB_ERR_GENERIC_ERROR	-8
>> +#define MPAM_FB_ERR_HW_ERROR		-9
>> +#define MPAM_FB_ERR_PROTOCOL_ERROR	-10
>> +#define MPAM_FB_ERR_IN_USE		-11
>> +
>> +static atomic_t mpam_fb_token = ATOMIC_INIT(0);
>> +
>> +static int mpam_fb_to_linux_errno(int mpam_fb_code)
>> +{
>> +	switch (mpam_fb_code) {
>> +	case MPAM_FB_ERR_NOT_SUPPORTED:
>> +		return -EOPNOTSUPP;
>> +	case MPAM_FB_ERR_INVALID_PARAMETERS:
>> +		return -EINVAL;
>> +	case MPAM_FB_ERR_NOT_FOUND:
>> +		return -ENOENT;
>> +	case MPAM_FB_ERR_OUT_OF_RANGE:
>> +		return -ERANGE;
>> +	case MPAM_FB_ERR_BUSY:
>> +		return -EBUSY;
>> +	default:
>> +		return -EINVAL;
>> +	}
>> +}
>> +
>> +static void mpam_fb_build_version_message(unsigned int token,
>> +					  void __iomem *msg_buf)
>> +{
>> +	struct acpi_pcct_ext_pcc_shared_memory __iomem *pcc_shmem = msg_buf;
>> +
>> +	/* .signature is filled by the platform */
>> +	writel_relaxed(PCC_CMD_COMPLETION_NOTIFY, &pcc_shmem->flags);
>> +	writel_relaxed(MPAM_FB_PROT_HEADER_LEN, &pcc_shmem->length);
>> +	writel_relaxed(MPAM_PROTOCOL_VERSION_CMD |
>> +		       FIELD_PREP(MPAM_MSC_TOKEN_MASK, token) |
>> +		       FIELD_PREP(MPAM_MSC_PROT_ID_MASK, MPAM_FB_PROTOCOL_ID),
>> +		       &pcc_shmem->command);
>> +}
>> +
>> +static void mpam_fb_build_read_message(int msc_fb_id, int reg,
>> +				       unsigned int token,
>> +				       void __iomem *msg_buf)
>> +{
>> +	struct acpi_pcct_ext_pcc_shared_memory __iomem *pcc_shmem = msg_buf;
>> +	struct mpam_fb_read_payload {
>> +		u32 msc_fb_id;
>> +		u32 flags;
>> +		u32 reg_offset;
>> +	} __packed __iomem *payload = msg_buf + sizeof(*pcc_shmem);
>> +	int msg_size = MPAM_FB_PROT_HEADER_LEN + sizeof(*payload);
>> +
>> +	/* .signature is filled by the platform */
>> +	writel_relaxed(PCC_CMD_COMPLETION_NOTIFY, &pcc_shmem->flags);
>> +	writel_relaxed(msg_size, &pcc_shmem->length);
>> +	writel_relaxed(MPAM_MSC_READ_CMD |
>> +		       FIELD_PREP(MPAM_MSC_TOKEN_MASK, token) |
>> +		       FIELD_PREP(MPAM_MSC_PROT_ID_MASK, MPAM_FB_PROTOCOL_ID),
>> +		       &pcc_shmem->command);
>> +
>> +	writel_relaxed(msc_fb_id, &payload->msc_fb_id);
>> +	writel_relaxed(0, &payload->flags);
>> +	writel_relaxed(reg, &payload->reg_offset);
>> +}> +
>> +static void mpam_fb_build_write_message(int msc_fb_id, int reg, u32 val,
>> +					unsigned int token,
>> +					void __iomem *msg_buf)
>> +{
>> +	struct acpi_pcct_ext_pcc_shared_memory __iomem *pcc_shmem = msg_buf;
>> +	struct mpam_fb_write_payload {
>> +		u32 msc_fb_id;
>> +		u32 flags;
>> +		u32 reg_offset;
>> +		u32 value;
>> +	} __packed __iomem *payload = msg_buf + sizeof(*pcc_shmem);
>> +	int msg_size = MPAM_FB_PROT_HEADER_LEN + sizeof(*payload);
>> +
>> +	/* .signature is filled by the platform */
>> +	writel_relaxed(PCC_CMD_COMPLETION_NOTIFY, &pcc_shmem->flags);
>> +	writel_relaxed(msg_size, &pcc_shmem->length);
>> +	writel_relaxed(MPAM_MSC_WRITE_CMD |
>> +		       FIELD_PREP(MPAM_MSC_TOKEN_MASK, token) |
>> +		       FIELD_PREP(MPAM_MSC_PROT_ID_MASK, MPAM_FB_PROTOCOL_ID),
>> +		       &pcc_shmem->command);
>> +
>> +	writel_relaxed(msc_fb_id, &payload->msc_fb_id);
>> +	writel_relaxed(0, &payload->flags);
>> +	writel_relaxed(reg, &payload->reg_offset);
>> +	writel_relaxed(val, &payload->value);
>> +}
>> +
>> +static int mpam_fb_send_request(struct mpam_msc *msc, u32 msc_fb_id,
>> +				u16 reg, u32 *result, int mpam_fb_command)
>> +{
>> +	unsigned int token = atomic_inc_return(&mpam_fb_token);
>> +	struct acpi_pcct_ext_pcc_shared_memory __iomem *pcc_shmem;
>> +	struct mpam_pcc_chan *pcc_chan;
>> +	struct pcc_mbox_chan *chan;
>> +	void __iomem *payload_ofs;
>> +	int mpam_fb_err = 0;
>> +	u32 status;
>> +	int ret;
>> +
>> +	pcc_chan = msc->pcc_chan;
>> +	if (!pcc_chan)
>> +		return -ENODEV;
>> +
>> +	chan = pcc_chan->pcc_chan;
>> +
>> +	/* prune token to fit into the 10 bits inside the command register */
>> +	token = FIELD_GET(MPAM_MSC_TOKEN_MASK,
>> +			  FIELD_PREP(MPAM_MSC_TOKEN_MASK, token));
>> +
>> +	mutex_lock(&pcc_chan->pcc_chan_lock);
>> +
>> +	switch (mpam_fb_command) {
>> +	case MPAM_PROTOCOL_VERSION_CMD:
>> +		mpam_fb_build_version_message(token, chan->shmem);
>> +		break;
>> +	case MPAM_MSC_READ_CMD:
>> +		mpam_fb_build_read_message(msc_fb_id, reg, token, chan->shmem);
>> +		break;
>> +	case MPAM_MSC_WRITE_CMD:
>> +		mpam_fb_build_write_message(msc_fb_id, reg, *result,
>> +					    token, chan->shmem);
>> +		break;
>> +	default:
>> +		dev_err(&msc->pdev->dev, "unsupported MPAM-Fb command %d\n",
>> +			mpam_fb_command);
>> +		ret = -EINVAL;
>> +		goto out_err;
>> +	}
>> +
> 
> Forwarding another message from sashiko, [1]. Do we need a dmb_wb() here to make sure the buffer is
> fully written before we send it?

Ah yes. I originally dismissed this comment because I thought we already 
use MMIO accessors, so ordering is of no concern for us, but indeed I 
used the _relaxed variant.
I added a "dma_wmb();" here now, I think that should be the right 
barrier, because it's a DMA style operation, and we only care about the 
sync-after-write part here.

Cheers,
Andre

> 
> [1] https://sashiko.dev/#/patchset/20260924152739.2865510-1-andre.przywara%40arm.com
> 
>> +	ret = mbox_send_message(chan->mchan, NULL);
>> +	if (ret < 0)
>> +		goto out_err;
>> +
>> +	pcc_shmem = chan->shmem;
>> +	payload_ofs = chan->shmem + sizeof(*pcc_shmem);
>> +	status = readl(&pcc_shmem->command);
>> +	if (FIELD_GET(MPAM_MSC_TOKEN_MASK, status) != token) {
>> +		ret = -ETIMEDOUT;
>> +		goto out_err;
>> +	}
>> +
>> +	mpam_fb_err = readl(payload_ofs + 0x0);
>> +	if (mpam_fb_err < 0) {
>> +		ret = mpam_fb_to_linux_errno(mpam_fb_err);
>> +		goto out_err;
>> +	}
>> +
>> +	if (mpam_fb_command != MPAM_MSC_WRITE_CMD)
>> +		*result = readl(payload_ofs + 0x4);
>> +
>> +	mutex_unlock(&pcc_chan->pcc_chan_lock);
>> +
>> +	return 0;
>> +
>> +out_err:
>> +	mutex_unlock(&pcc_chan->pcc_chan_lock);
>> +
>> +	mpam_fb_disable_mpam(ret, mpam_fb_err);
>> +
>> +	return ret;
>> +}
>> +
>> +int mpam_fb_send_read_request(struct mpam_msc *msc, u16 reg, u32 *result)
>> +{
>> +	return mpam_fb_send_request(msc, msc->fb_id, reg, result,
>> +				    MPAM_MSC_READ_CMD);
>> +}
>> +
>> +int mpam_fb_send_write_request(struct mpam_msc *msc, u16 reg, u32 value)
>> +{
>> +	return mpam_fb_send_request(msc, msc->fb_id, reg, &value,
>> +				    MPAM_MSC_WRITE_CMD);
>> +}
>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> index 58f7523bc087b..99057d6abee1e 100644
>> --- a/drivers/resctrl/mpam_internal.h
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -11,6 +11,7 @@
>>   #include <linux/io.h>
>>   #include <linux/jump_label.h>
>>   #include <linux/llist.h>
>> +#include <linux/mailbox_client.h>
>>   #include <linux/mutex.h>
>>   #include <linux/resctrl.h>
>>   #include <linux/spinlock.h>
>> @@ -57,6 +58,15 @@ struct mpam_garbage {
>>   	struct platform_device	*pdev;
>>   };
>>   
>> +struct mpam_pcc_chan {
>> +	struct list_head	pcc_chans;
>> +	struct mbox_client	pcc_cl;
>> +	struct pcc_mbox_chan	*pcc_chan;
>> +	struct mutex		pcc_chan_lock; /* only one message at a time */
>> +	struct kref		refcount;
>> +	int			subspace_id;
>> +};
>> +
>>   struct mpam_msc {
>>   	/* member of mpam_all_msc */
>>   	struct list_head	all_msc_list;
>> @@ -66,6 +76,8 @@ struct mpam_msc {
>>   
>>   	/* Not modified after mpam_is_enabled() becomes true */
>>   	enum mpam_msc_iface	iface;
>> +	struct mpam_pcc_chan	*pcc_chan;
>> +	int			fb_id;
>>   	u32			nrdy_usec;
>>   	cpumask_t		accessibility;
>>   	bool			has_extd_esr;
>> @@ -493,6 +505,9 @@ extern u8 mpam_pmg_max;
>>   void mpam_enable(struct work_struct *work);
>>   void mpam_disable(struct work_struct *work);
>>   
>> +/* helper function to call from outside mpam_devices.c */
>> +void mpam_fb_disable_mpam(int err, int mpam_fb_err);
>> +
>>   /* Reset all the RIS in a class under cpus_read_lock() */
>>   int mpam_reset_class_locked(struct mpam_class *class);
>>   
>> @@ -520,6 +535,10 @@ static inline void mpam_resctrl_offline_cpu(unsigned int cpu) { }
>>   static inline void mpam_resctrl_teardown_class(struct mpam_class *class) { }
>>   #endif /* CONFIG_RESCTRL_FS */
>>   
>> +/* 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);
>> +
>>   /*
>>    * MPAM MSCs have the following register layout. See:
>>    * Arm Memory System Resource Partitioning and Monitoring (MPAM) System
>> diff --git a/include/linux/arm_mpam.h b/include/linux/arm_mpam.h
>> index f92a36187a527..002f56e153626 100644
>> --- a/include/linux/arm_mpam.h
>> +++ b/include/linux/arm_mpam.h
>> @@ -12,7 +12,7 @@ struct mpam_msc;
>>   
>>   enum mpam_msc_iface {
>>   	MPAM_IFACE_MMIO,	/* a real MPAM MSC */
>> -	MPAM_IFACE_PCC,		/* a fake MPAM MSC */
>> +	MPAM_IFACE_PCC,		/* using the MPAM-Fb firmware redirection */
>>   };
>>   
>>   enum mpam_class_types {
>