Re: [PATCH v10 10/14] arm_mpam: add MPAM-Fb MSC firmware access support

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

Hi Ben,

thanks for having a look ;-)

On 9/11/26 17:13, Ben Horgan wrote:
> Hi Andre,
> 
> On 11/09/2026 12:28, 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       |   2 +
>>   drivers/resctrl/Kconfig         |   1 +
>>   drivers/resctrl/Makefile        |   2 +-
>>   drivers/resctrl/mpam_devices.c  |  65 ++++++++--
>>   drivers/resctrl/mpam_fb.c       | 211 ++++++++++++++++++++++++++++++++
>>   drivers/resctrl/mpam_internal.h |  19 +++
>>   include/linux/arm_mpam.h        |   2 +-
>>   7 files changed, 288 insertions(+), 14 deletions(-)
>>   create mode 100644 drivers/resctrl/mpam_fb.c
>>

[ ... ]
>> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
>> new file mode 100644
>> index 0000000000000..98212f761de38
>> --- /dev/null
>> +++ b/drivers/resctrl/mpam_fb.c
>> @@ -0,0 +1,211 @@
>> +// 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_id, int reg, unsigned int token,
>> +				       void __iomem *msg_buf)
> 
> As we're anticipating that msc_id may be different from msc->id can be use msc_fb_id instead of
> msc_id in this file.

Yes, that's a good idea, changed that.

Cheers,
Andre

>> +{
>> +	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_msc *msc, 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 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_id, reg, token, chan->shmem);
>> +		break;
>> +	case MPAM_MSC_WRITE_CMD:
>> +		mpam_fb_build_write_message(msc_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;
>> +	}
>> +
>> +	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->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, msc->mpam_fb_msc_id, reg, &value,
>> +				    MPAM_MSC_WRITE_CMD);
>> +}
>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> index 2414598100140..e5a2804facd2d 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;
>>   	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() */
>>   void 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 {
>