[PATCH v4 08/10] arm_mpam: add MPAM-Fb MSC firmware access support
Andre Przywara
andre.przywara at arm.com
Tue Jul 28 09:19:25 PDT 2026
Hi Srivathsa,
thanks for having a look!
On 7/28/26 08:27, Srivathsa L Rao wrote:
> Hi Andre,
>
> On 7/23/2026 9:24 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 at arm.com>
>> ---
>> drivers/resctrl/Makefile | 2 +-
>> drivers/resctrl/mpam_devices.c | 27 ++++-
>> drivers/resctrl/mpam_fb.c | 197 ++++++++++++++++++++++++++++++++
>> drivers/resctrl/mpam_internal.h | 20 ++++
>> include/linux/arm_mpam.h | 2 +-
>> 5 files changed, 240 insertions(+), 8 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..ec4bf0dd9182
>> --- /dev/null
>> +++ b/drivers/resctrl/mpam_fb.c
>> @@ -0,0 +1,197 @@
[ ... ]
>> +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:
>> + mpam_fb_build_version_message(token, chan->shmem);
>> + break;
>> + }
>
> The switch here has no default case. If an unrecognised command value is
> passed (in future), it maybe safer and future proof to have a default case?
Well, mpam_fb_send_request is a static function, and all callers pass in
constant values, so this wouldn't easily happen and be more a compile
time error.
But I can of course add a defensive default check.
Cheers,
Andre
> default:
> return -EINVAL; ?> +
>> + 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;
>> + 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 b3a6ed9ed175..e4686ff0ba2e 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;
>> @@ -511,6 +523,14 @@ 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 MPAM_FB_PROT_HEADER_LEN sizeof(u32)
>> +#define MPAM_FB_MAX_MSG_SIZE (MPAM_FB_PROT_HEADER_LEN + 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 {
>
> Best Regards,
> Srivathsa
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