Re: [PATCH v5 08/10] arm_mpam: add MPAM-Fb MSC firmware access support

[Srivathsa L Rao]

Hi Andre,

On 7/29/2026 7:11 PM, 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@xxxxxxx>
> ---
>   drivers/resctrl/Makefile        |   2 +-
>   drivers/resctrl/mpam_devices.c  |  52 ++++++--
>   drivers/resctrl/mpam_fb.c       | 214 ++++++++++++++++++++++++++++++++
>   drivers/resctrl/mpam_internal.h |  22 ++++
>   include/linux/arm_mpam.h        |   2 +-
>   5 files changed, 279 insertions(+), 13 deletions(-)
>   create mode 100644 drivers/resctrl/mpam_fb.c
>
> diff --git a/drivers/resctrl/Makefile b/drivers/resctrl/Makefile
> index 4f6d0e81f9b8..097c036724e9 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 f6910ab3bbc2..abe1e628928f 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -83,6 +83,16 @@ static DECLARE_WORK(mpam_broken_work, &mpam_disable);
>   /* When mpam is disabled, the printed reason to aid debugging */
>   static char *mpam_disable_reason;
>   
> +void mpam_fb_disable_mpam(int err)
> +{
> +	static char mpam_fb_reason[32];
> +
> +	snprintf(mpam_fb_reason, sizeof(mpam_fb_reason), "MPAM-Fb error %d",
> +		 err);
> +	mpam_disable_reason = mpam_fb_reason;
> +	schedule_work(&mpam_broken_work);
> +}
> +
>   /*
>    * Whether resctrl has been setup. Used by cpuhp in preference to
>    * mpam_is_enabled(). The disable call after an error interrupt makes
> @@ -179,8 +189,11 @@ 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 +210,12 @@ 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;
> @@ -1141,8 +1157,11 @@ 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)
> @@ -1176,8 +1195,11 @@ 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 +1512,16 @@ 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
> @@ -1921,6 +1948,9 @@ 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)));
> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
> new file mode 100644
> index 000000000000..dbe984241b0d
> --- /dev/null
> +++ b/drivers/resctrl/mpam_fb.c
> @@ -0,0 +1,214 @@
> +// 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/types.h>
> +
> +#include <acpi/pcc.h>
> +#include <asm/mpam.h>
> +
> +#include "mpam_internal.h"
> +
> +#define MPAM_FB_PROTOCOL_ID		0x1a
> +
> +#define MPAM_PROTOCOL_VERSION_CMD	0x0
> +#define MPAM_MSC_ATTRIBUTES_CMD		0x3
> +#define MPAM_MSC_READ_CMD		0x4
> +#define MPAM_MSC_WRITE_CMD		0x5
> +
> +#define MPAM_FB_ERR_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
> +
> +#define MPAM_MSC_PROT_ID_MASK	GENMASK(17, 10)
> +#define MPAM_MSC_TOKEN_MASK	GENMASK(27, 18)
> +
> +static atomic_t mpam_fb_token = ATOMIC_INIT(0);
> +
> +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_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_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_id, &payload->msc_id);
> +	writel_relaxed(0, &payload->flags);
> +	writel_relaxed(reg, &payload->reg_offset);
> +}
> +
> +static void mpam_fb_build_write_message(int msc_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_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_id, &payload->msc_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_pcc_chan *pcc_chan, u32 msc_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 pcc_mbox_chan *chan;
> +	void __iomem *payload_ofs;
> +	u32 status;
> +	int ret;
> +
> +	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));
> +
> +	guard(mutex)(&pcc_chan->pcc_chan_lock);
> +
> +	switch (mpam_fb_command) {
> +	case MPAM_MSC_WRITE_CMD:
> +		mpam_fb_build_write_message(msc_id, reg, *result,
> +					    token, chan->shmem);
> +		break;
> +	case MPAM_MSC_READ_CMD:
> +		mpam_fb_build_read_message(msc_id, reg, token, chan->shmem);
> +		break;
> +	case MPAM_PROTOCOL_VERSION_CMD:
> +		mpam_fb_build_version_message(token, chan->shmem);
> +		break;
> +	default:
> +		dev_err(pcc_chan->pcc_cl.dev, "unsupported MPAM-Fb command %d\n",
> +			mpam_fb_command);
> +		ret = -EINVAL;
> +		goto out_err;
> +	}
> +
> +	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;
> +	}
> +
> +	ret = readl(payload_ofs + 0x0);
> +	if (ret < 0) {
> +		switch (ret) {
> +		case MPAM_FB_ERR_NOT_SUPPORTED:
> +			ret = -EOPNOTSUPP;
> +			break;
> +		case MPAM_FB_ERR_INVALID_PARAMETERS:
> +			ret = -EINVAL;
> +			break;
> +		case MPAM_FB_ERR_NOT_FOUND:
> +			ret = -ENOENT;
> +			break;
> +		case MPAM_FB_ERR_OUT_OF_RANGE:
> +			ret = -ERANGE;
> +			break;

While testing v4 on a QEMU setup with a fake PCC-backed MSC, I added a
small error injection mechanism to verify the firmware response status
code translations in mpam_fb_send_request(). I injected each defined
MPAM_FB_ERR_* code and observed the following.

> +		default:
> +			ret = -EINVAL;
> +		}
> +
> +		goto out_err;
> +	}

MPAM_FB_ERR_BUSY (-6) falls through to this default and gets -EINVAL.
Would -EAGAIN be more appropriate here?  Callers could then maybe add a 
short retry loop inside mpam_fb_send_request() itself, or in the probe 
path, convert -EAGAIN to -EPROBE_DEFER so the driver core retries probe 
automatically.

The other unhandled codes also collapse to -EINVAL, like EPROTO, EBUSY, 
I guess that can come later.

> +
> +	if (mpam_fb_command != MPAM_MSC_WRITE_CMD)
> +		*result = readl(payload_ofs + 0x4);
> +
> +	return 0;
> +
> +out_err:
> +	mpam_fb_disable_mpam(ret);
> +
> +	return ret;
> +}
> +
> +int mpam_fb_send_read_request(struct mpam_msc *msc, u16 reg, u32 *result)
> +{
> +	return mpam_fb_send_request(msc->pcc_chan, msc->mpam_fb_msc_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->pcc_chan, msc->mpam_fb_msc_id,
> +				    reg, &value, MPAM_MSC_WRITE_CMD);
> +}
> +
> +int mpam_fb_get_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;
> +
> +	return version;
> +}
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 2b81b6b0bf4e..a2193e7df57c 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			mpam_fb_msc_id;	/* in its own name space */
>   	u32			nrdy_usec;
>   	cpumask_t		accessibility;
>   	bool			has_extd_esr;
> @@ -484,6 +496,9 @@ 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);
> +
>   /* Reset all the RIS in a class under cpus_read_lock() */
>   void mpam_reset_class_locked(struct mpam_class *class);
>   
> @@ -511,6 +526,13 @@ 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);
> +int mpam_fb_get_protocol_version(struct mpam_msc *msc);
> +
> +#define MPAM_FB_PROT_HEADER_LEN		sizeof(u32)
> +
>   /*
>    * 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 f92a36187a52..002f56e15362 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 {

Best Regards,
Srivathsa