[PATCH v10 12/14] arm_mpam: detect and enable MPAM-Fb PCC support
Andre Przywara
andre.przywara at arm.com
Mon Sep 21 09:09:27 PDT 2026
Hi Ben,
On 9/11/26 17:09, Ben Horgan wrote:
> Hi Andre,
>
> On 11/09/2026 12:28, Andre Przywara wrote:
>> The Arm MPAM-Fb specification [1] describes a protocol to access MSC
>> registers through a firmware interface. This requires a shared memory
>> region to hold the message, and a mailbox to trigger the access.
>> For ACPI this is wrapped as a PCC channel, described using existing
>> ACPI abstractions.
>>
>> Add code to parse those PCC table descriptions associated with an MSC,
>> and store the parsed information in the MSC struct.
>> There can be multiple PCC channels, and each channel can serve multiple
>> MSCs, so we need to keep track of the channel usage, using a list and
>> a refcount.
>>
>> This will be used by the MPAM-Fb access wrapper code.
>>
>> [1] https://developer.arm.com/documentation/den0144/latest
>>
>> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
>> Tested-by: Ritwick Sharma <ritwick.sharma at arm.com> # on Arm AGI CPU
>> Reviewed-by: Jonathan Cameron <jonathan.cameron at oss.qualcomm.com>
>> Tested-by: Srivathsa L Rao <srivathsa.rao at oss.qualcomm.com>
>> Reviewed-by: Srivathsa L Rao <srivathsa.rao at oss.qualcomm.com>
>> Reviewed-by: Gavin Shan <gshan at redhat.com>
>> ---
>> drivers/resctrl/Kconfig | 1 +
>> drivers/resctrl/mpam_devices.c | 119 +++++++++++++++++++++++++++++++-
>> drivers/resctrl/mpam_fb.c | 41 +++++++++++
>> drivers/resctrl/mpam_internal.h | 2 +
>> 4 files changed, 161 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/resctrl/Kconfig b/drivers/resctrl/Kconfig
>> index 9591d792736e5..3f81638f57be3 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 PCC if ACPI
>> select MAILBOX
>> help
>> Memory System Resource Partitioning and Monitoring (MPAM) driver for
>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
>> index 14dc8461e4b8f..150a529458388 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -20,14 +20,19 @@
>> #include <linux/irqdesc.h>
>> #include <linux/list.h>
>> #include <linux/lockdep.h>
>> +#include <linux/mailbox_client.h>
>> #include <linux/mutex.h>
>> #include <linux/platform_device.h>
>> #include <linux/printk.h>
>> +#include <linux/property.h>
>> #include <linux/srcu.h>
>> #include <linux/spinlock.h>
>> #include <linux/types.h>
>> #include <linux/workqueue.h>
>>
>> +#include <acpi/pcc.h>
>> +#include <acpi/acpi_io.h>
>> +
>> #include "mpam_internal.h"
>>
>> /* Values for the T241 errata workaround */
>> @@ -50,6 +55,87 @@ static LIST_HEAD(mpam_all_msc);
>>
>> struct srcu_struct mpam_srcu;
>>
>> +/* PCC channels might be serving multiple MSCs, so keep a refcounted list. */
>> +static DEFINE_MUTEX(pcc_chan_list_lock);
>> +static LIST_HEAD(pcc_chan_list);
>> +
>> +static void mpam_pcc_chan_release(struct kref *ref)
>> +{
>> + struct mpam_pcc_chan *cur = container_of(ref, struct mpam_pcc_chan,
>> + refcount);
>> +
>> + pcc_mbox_free_channel(cur->pcc_chan);
>> + list_del(&cur->pcc_chans);
>> + mutex_destroy(&cur->pcc_chan_lock);
>> + kfree(cur);
>> +}
>> +
>> +static struct mpam_pcc_chan *mpam_pcc_chan_get(struct device *dev,
>> + int subspace_id)
>> +{
>> + struct mpam_pcc_chan *cur;
>> +
>> + guard(mutex)(&pcc_chan_list_lock);
>> +
>> + list_for_each_entry(cur, &pcc_chan_list, pcc_chans) {
>> + if (cur->subspace_id == subspace_id) {
>> + kref_get(&cur->refcount);
>> +
>> + return cur;
>> + }
>> + }
>> +
>> + cur = kzalloc_obj(*cur);
>> + if (!cur)
>> + return ERR_PTR(-ENOMEM);
>> +
>> + cur->pcc_cl.tx_block = true;
>> +
>> + cur->pcc_chan = pcc_mbox_request_channel(&cur->pcc_cl, subspace_id);
>> + if (IS_ERR(cur->pcc_chan)) {
>> + long err = PTR_ERR(cur->pcc_chan);
>> +
>> + kfree(cur);
>> + return ERR_PTR(err);
>> + }
>> +
>> + /*
>> + * Timeout based on the "nominal latency" in us, from the
>> + * PCC ACPI table. tx_tout is in ms.
>> + * Add some margin here to be on the safe side.
>> + */
>> + cur->pcc_cl.tx_tout = DIV_ROUND_UP(cur->pcc_chan->latency * 5, 1000);
>> +
>> + mutex_init(&cur->pcc_chan_lock);
>> +
>> + cur->subspace_id = subspace_id;
>> + kref_init(&cur->refcount);
>> +
>> + list_add_tail(&cur->pcc_chans, &pcc_chan_list);
>> +
>> + return cur;
>> +}
>> +
>> +static int mpam_pcc_chan_put(struct mpam_pcc_chan *pcc_chan)
>> +{
>> + struct mpam_pcc_chan *cur, *tmp;
>> +
>> + if (!pcc_chan)
>> + return 0;
>> +
>> + guard(mutex)(&pcc_chan_list_lock);
>> +
>> + list_for_each_entry_safe(cur, tmp, &pcc_chan_list, pcc_chans) {
>> + if (cur == pcc_chan) {
>> + kref_put(&cur->refcount, mpam_pcc_chan_release);
>> +
>> + return 0;
>> + }
>> + }
>> +
>> + return -ENOENT;
>> +}
>> +
>> /*
>> * Number of MSCs that have been probed. Once all MSCs have been probed MPAM
>> * can be enabled.
>> @@ -2285,6 +2371,8 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>> if (!msc)
>> return;
>>
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> +
>> mutex_lock(&mpam_list_lock);
>> mpam_msc_destroy(msc);
>> mutex_unlock(&mpam_list_lock);
>> @@ -2295,7 +2383,7 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>> static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>> {
>> int err;
>> - u32 tmp;
>> + u32 pcc_subspace_id;
>> struct mpam_msc *msc;
>> struct resource *msc_res;
>> struct device *dev = &pdev->dev;
>> @@ -2339,7 +2427,7 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>> if (err)
>> return ERR_PTR(err);
>>
>> - if (device_property_read_u32(&pdev->dev, "pcc-channel", &tmp))
>> + if (device_property_read_u32(dev, "pcc-channel", &pcc_subspace_id))
>> msc->iface = MPAM_IFACE_MMIO;
>> else
>> msc->iface = MPAM_IFACE_PCC;
>> @@ -2360,6 +2448,33 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
>> }
>> msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
>> msc->mapped_hwpage = io;
>> + } else if (msc->iface == MPAM_IFACE_PCC) {
>> + u32 mpam_fb_msc_id;
>> +
>> + msc->pcc_chan = mpam_pcc_chan_get(dev, pcc_subspace_id);
>> + if (IS_ERR(msc->pcc_chan)) {
>> + pr_err("Failed to request MSC PCC channel\n");
>> + return ERR_CAST(msc->pcc_chan);
>> + }
>> +
>> + err = mpam_fb_check_shared_buffer_size(msc);
>> + if (err) {
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> +
>> + return ERR_PTR(err);
>> + }
>> +
>> + err = mpam_fb_check_protocol_version(msc);
>> + if (err) {
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> +
>> + return ERR_PTR(err);
>> + }
>> +
>> + if (device_property_read_u32(&pdev->dev, "mpam-fb-msc-id",
>> + &mpam_fb_msc_id))
>> + mpam_fb_msc_id = msc->id;
>> + msc->mpam_fb_msc_id = mpam_fb_msc_id;
>
> This looks like the change earlier in the series to drivers/acpi/arm64/mpam.c makes no functional
> effect unless the proposed spec changes are accepted; the default is msc->id. I'd just move the
Yes, but it reduces churn, and more importantly, is more true to the
MPAM-Fb spec, which says:
"The MSC identifiers are sequential and start from 0 for each agent
which uses this protocol."
So this requires that we have per-channel IDs for the MSCs, that's why I
think it's justified to introduce this already now, even when it's
somewhat redundant as the code stands now, as we always assign the
system-wide ID to it.
I can of course move that out (and have that already on top of v9,
internally), but wanted to check this with you.
> earlier drivers/acpi/arm64/mpam.c to your last patch and introduce this device_property_read_u32()
> their too (or a separate patch if it's easier for anyone). That way all but the last two patches can
> be considered for merging without knowing what's going to happen with the spec.
Well, sure, though I think a separation of MPAM-FB MSC IDs and
system-wide MSC IDs is inevitable, regardless of which spec is changed
and how the numbers are assigned, exactly.
> The struct msc property mpam_fb_msc_id is a bit long. Better as msc->fb_id?
Sure, can shorten that.
Cheers,
Andre.
>> } else {
>> return ERR_PTR(-EINVAL);
>> }
>> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
>> index 98212f761de38..8fc1d78a7b5ab 100644
>> --- a/drivers/resctrl/mpam_fb.c
>> +++ b/drivers/resctrl/mpam_fb.c
>> @@ -20,10 +20,15 @@
>> #define MPAM_FB_PROTOCOL_ID 0x1a
>>
>> #define MPAM_PROTOCOL_VERSION_CMD 0x0
>> +#define MPAM_FB_VERSION_MAJOR_MASK GENMASK_U32(31, 16)
>> +#define MPAM_FB_VERSION_MINOR_MASK GENMASK_U32(15, 0)
>> +
>> #define MPAM_MSC_READ_CMD 0x4
>> #define MPAM_MSC_WRITE_CMD 0x5
>>
>> #define MPAM_FB_PROT_HEADER_LEN sizeof(u32)
>> +/* The longest message is MPAM_MSC_WRITE, with 4 parameters. */
>> +#define MPAM_FB_MAX_MSG_SIZE (4 * sizeof(u32))
>>
>> #define MPAM_FB_SUCCESS 0
>> #define MPAM_FB_ERR_NOT_SUPPORTED -1
>> @@ -209,3 +214,39 @@ 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);
>> }
>> +
>> +/* We only support MPAM-Fb protocol version 1.x */
>> +int mpam_fb_check_protocol_version(struct mpam_msc *msc)
>> +{
>> + u32 version;
>> + int ret;
>> +
>> + ret = mpam_fb_send_request(msc, 0, 0, &version,
>> + MPAM_PROTOCOL_VERSION_CMD);
>> + if (ret)
>> + return ret;
>> +
>> + if (FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, version) != 1) {
>> + pr_err("Incompatible MPAM-Fb protocol version %d.%d\n",
>> + FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, version),
>> + FIELD_GET(MPAM_FB_VERSION_MINOR_MASK, version));
>> +
>> + return -EINVAL;
>> + }
>> +
>> + return 0;
>> +}
>> +
>> +int mpam_fb_check_shared_buffer_size(struct mpam_msc *msc)
>> +{
>> + int min_buffer_size = MPAM_FB_MAX_MSG_SIZE +
>> + sizeof(struct acpi_pcct_ext_pcc_shared_memory);
>> +
>> + if (msc->pcc_chan->pcc_chan->shmem_size < min_buffer_size) {
>> + pr_err("MPAM-Fb PCC channel size too small.\n");
>> +
>> + return -ENOMEM;
>> + }
>> +
>> + return 0;
>> +}
>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
>> index e5a2804facd2d..314d3b48ca9fd 100644
>> --- a/drivers/resctrl/mpam_internal.h
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -538,6 +538,8 @@ static inline void mpam_resctrl_teardown_class(struct mpam_class *class) { }
>> /* 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_check_protocol_version(struct mpam_msc *msc);
>> +int mpam_fb_check_shared_buffer_size(struct mpam_msc *msc);
>>
>> /*
>> * MPAM MSCs have the following register layout. See:
>
More information about the linux-arm-kernel
mailing list