[PATCH v5 10/10] arm_mpam: detect and enable MPAM-Fb PCC support
Andre Przywara
andre.przywara at arm.com
Thu Jul 30 03:05:56 PDT 2026
Hi Srivathsa,
many thanks for having a thorough look!
On 7/30/26 11:58, Srivathsa L Rao wrote:
> Hi Andre ,
>
> On 7/29/2026 7:11 PM, 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>
>> ---
>> drivers/resctrl/mpam_devices.c | 119 +++++++++++++++++++++++++++++++-
>> drivers/resctrl/mpam_fb.c | 4 +-
>> drivers/resctrl/mpam_internal.h | 6 +-
>> 3 files changed, 124 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/
>> mpam_devices.c
>> index e535603d2c7d..e360051f4aa7 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -19,14 +19,19 @@
>> #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>
>>
> [...]
>
>> /*
>> * Number of MSCs that have been probed. Once all MSCs have been
>> probed MPAM
>> * can be enabled.
>> @@ -2221,6 +2304,8 @@ static void mpam_msc_drv_remove(struct
>> platform_device *pdev)
>> {
>> struct mpam_msc *msc = platform_get_drvdata(pdev);
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> +
>> mutex_lock(&mpam_list_lock);
>> mpam_msc_destroy(msc);
>> mutex_unlock(&mpam_list_lock);
>> @@ -2231,7 +2316,7 @@ 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;
>> @@ -2279,7 +2364,7 @@ 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;
>> @@ -2295,6 +2380,36 @@ static struct mpam_msc
>> *do_mpam_msc_drv_probe(struct platform_device *pdev)
>> }
>> msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
>
> Ritwick pointed out in the v4 thread that mapped_hwpage_sz should use
> resource_size() rather than open-coding (end - start). That fix doesn't
> appear to have made it into v5.
Yes, that's because that's an existing bug, so it deserves a separate
fix (from him), with a Fixes: tag. I didn't want to complicate things by
making it part of this series unnecessarily. He will send something
ASAP. That being a fix it's less timing critical (as in: could be taken
any time).
Cheers,
Andre
>
> Also I did verify the resource_size() in my setup by adding prints, it
> did work well.> msc->mapped_hwpage = io;
>> + } else if (msc->iface == MPAM_IFACE_PCC) {
>> + int ret;
>> +
>> + 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);
>> + }
>> +
>> + if (msc->pcc_chan->pcc_chan->shmem_size <
>> MPAM_FB_MAX_MSG_SIZE) {
>> + pr_err("MPAM-Fb PCC channel size too small.\n");
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> + return ERR_PTR(-ENOMEM);
>> + }
>> + ret = mpam_fb_get_protocol_version(msc);
>> + if (ret < 0) {
>> + pr_err("Cannot query MPAM-Fb protocol version.\n");
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> +
>> + /* ret is an MPAM-Fb error code, return a Linux one. */
>> + return ERR_PTR(-EIO);
>> + }
>> + if (FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, ret) != 1) {
>> + pr_err("Incompatible MPAM-Fb protocol version %ld.%ld\n",
>> + FIELD_GET(MPAM_FB_VERSION_MAJOR_MASK, ret),
>> + FIELD_GET(MPAM_FB_VERSION_MINOR_MASK, ret));
>> + mpam_pcc_chan_put(msc->pcc_chan);
>> +
>> + return ERR_PTR(-EINVAL);
>> + }
>> } else {
>> return ERR_PTR(-EINVAL);
>> }
>> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
>> index dbe984241b0d..55c4778a8379 100644
>> --- a/drivers/resctrl/mpam_fb.c
>> +++ b/drivers/resctrl/mpam_fb.c
>> @@ -190,13 +190,13 @@ static int mpam_fb_send_request(struct
>> mpam_pcc_chan *pcc_chan, u32 msc_id,
>> 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,
>> + return mpam_fb_send_request(msc->pcc_chan, 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,
>> + return mpam_fb_send_request(msc->pcc_chan, msc->id,
>> reg, &value, MPAM_MSC_WRITE_CMD);
>> }
>> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/
>> mpam_internal.h
>> index a2193e7df57c..e926b7feb72a 100644
>> --- a/drivers/resctrl/mpam_internal.h
>> +++ b/drivers/resctrl/mpam_internal.h
>> @@ -77,7 +77,6 @@ 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;
>> @@ -532,6 +531,11 @@ 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)
>> +/* The longest message is MPAM_MSC_WRITE, with 4 parameters. */
>> +#define MPAM_FB_MAX_MSG_SIZE (MPAM_FB_PROT_HEADER_LEN + 4 *
>> sizeof(u32))
>> +
>> +#define MPAM_FB_VERSION_MAJOR_MASK GENMASK(31, 16)
>> +#define MPAM_FB_VERSION_MINOR_MASK GENMASK(15, 0)
>> /*
>> * MPAM MSCs have the following register layout. See:
>
>
> --
> Best Regards,
> Srivathsa
>
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