Re: [PATCH v3 14/16] arm_mpam: add MPAM-Fb MSC firmware access support
From: Andre Przywara
Date: Wed Jul 22 2026 - 13:04:04 EST
Hi Ben,
On 7/15/26 18:17, Ben Horgan wrote:
Hi Andre,
On 7/10/26 15:45, 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
SCMI message generation and handshake, 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 | 26 +++-
drivers/resctrl/mpam_fb.c | 208 ++++++++++++++++++++++++++++++++
drivers/resctrl/mpam_internal.h | 20 +++
include/linux/arm_mpam.h | 2 +-
5 files changed, 251 insertions(+), 7 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 000000000000..7d7409910f28
--- /dev/null
+++ b/drivers/resctrl/mpam_fb.c
@@ -0,0 +1,208 @@
[ ... ]
+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 *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;
+
+ guard(mutex)(&pcc_chan->pcc_chan_lock);
+
+ switch (mpam_fb_command) {
+ case MPAM_MSC_WRITE_CMD:
+ ret = mpam_fb_build_write_message(msc_id, reg, *result,
+ token, chan->shmem);
+ break;
+ case MPAM_MSC_READ_CMD:
+ ret = mpam_fb_build_read_message(msc_id, reg,
+ token, chan->shmem);
+ break;
+ case MPAM_PROTOCOL_VERSION:
+ ret = mpam_fb_build_version_message(token, chan->shmem);
+ break;
+ }
+ if (ret < 0)
+ return ret;
+
+ ret = mbox_send_message(chan->mchan, NULL);
+ if (ret < 0)
+ return ret;
+
+ 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)
+ return -ETIMEDOUT;
+
+ ret = readl(payload_ofs + 0x0);
+ if (ret < 0) {
+ switch (ret) {
+ 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;
Does it make sense to have individual translations for any of the other errors? Which one do you
think is most likely?
I don't think it's really that useful. For most errors there is no good mapping between MPAM-Fb and the generic Linux error codes, and when they are propagated up it's unclear what they mean, really.
I wonder if we should print or log the original error code, but otherwise not sweat it to find a perfect match for every MPAM-Fb code.
Cheers,
Andre
Thanks,
Ben
+ default:
+ return -EINVAL;
+ }
+ }
+
+ if (mpam_fb_command != MPAM_MSC_WRITE_CMD)
+ *result = readl(payload_ofs + 0x4);
+
+ return 0;
+}
+
+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);
+ if (ret)
+ return ret;
+
+ return version;
+}
diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 7b6e0df904f8..c3c4ddb4a561 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 */
+ int subspace_id;
+ int refcount;
+};
+
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;
@@ -501,6 +513,14 @@ 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 PCC_TYPE3_MSG_PAYLOAD_OFS 0x10
+#define MPAM_FB_MAX_MSG_SIZE (PCC_TYPE3_MSG_PAYLOAD_OFS + 4 * 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 {