Re: [PATCH v11 11/13] arm_mpam: add MPAM-Fb MSC firmware access support
From: Andre Przywara
Date: Thu Oct 01 2026 - 11:10:53 EST
Hi Ben,
On 9/30/26 18:14, 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@xxxxxxx>
Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>
Tested-by: Ritwick Sharma <ritwick.sharma@xxxxxxx> # on Arm AGI CPU
Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
Reviewed-by: Srivathsa L Rao <srivathsa.rao@xxxxxxxxxxxxxxxx>
---
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;
+ }
+
+ 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;
Any error code not listed in rdtgroup_mondata_show() can be swallowed by resctrl when reading
monitors and it will end up showing an invalid value to the user. I saw 0.
""
if (rr.err == -EIO)
seq_puts(m, "Error\n");
else if (rr.err == -EINVAL)
seq_puts(m, "Unavailable\n");
else if (rr.err == -ENOENT)
seq_puts(m, "Unassigned\n");
else if (evt->is_floating_point)
print_event_value(m, evt->binary_bits, rr.val);
else
seq_printf(m, "%llu\n", rr.val);"
""
Triggering an -ETIMEDOUT response from mpam_fb_send_request() after 5000 requests I see:
/sys/fs/resctrl$ while cat mon_data/mon_L3_01/mbm_total_bytes; do :; done
45273792
[...]
252762048
256527040
0
Unavailable
Unavailable
Unavailable
cat: mon_data/mon_L3_01/mbm_total_bytes: No such file or directory
If, instead of -ETIMEDOUT, the error code return is changed to -EIO I see:
4950622336
[...]
4954631104
4958864064
Error
Unavailable
Unavailable
Unavailable
cat: mon_data/mon_L3_01/mbm_total_bytes: No such file or directory
For now, we probably want to just return -EIO on any error in mpam_fb_send_request() and then we can
update the resctrl code to accept more error values.
I see what you mean. It's a shame, but I indeed just return -EIO in the mpam_fb_send_request() error path now. Added a TODO comment so that we don't forget coming back to this.
Cheers,
Andre
On another fs/resctrl note, if monitor values are read for a control group then mon_event_count()
reads for the control group and any child monitor groups. If any of the reads succeed then it
purposely discards any other errors. This means that incorrect and non-increasing monitor reads are
possible. This isn't desirable but is existing behaviour and can be fixed later.
Thanks,
Ben
+ 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 {