Re: [PATCH v7 11/11] arm_mpam: detect and enable MPAM-Fb PCC support

Hi,

On 8/3/26 15:37, Andre Przywara wrote:
> Hi Srivathsa,
> 
> On 8/3/26 12:49, Srivathsa L Rao wrote:
>> Hi Andre,
>>
>> On 7/31/2026 10:33 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 <[email protected]>
>>> Tested-by: Ritwick Sharma <[email protected]>
>>> Reviewed-by: Jonathan Cameron <[email protected]>
>>> ---
>>>   drivers/resctrl/mpam_devices.c  | 113 +++++++++++++++++++++++++++++++-
>>>   drivers/resctrl/mpam_fb.c       |  41 ++++++++++++
>>>   drivers/resctrl/mpam_internal.h |   2 +
>>>   3 files changed, 154 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/ 
>>> mpam_devices.c
>>> index b238feb559d4..40b247a89f8e 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>
>>> +#include <acpi/pcc.h>
>>> +#include <acpi/acpi_io.h>
>>> +
>>>   #include "mpam_internal.h"
>>>   /* Values for the T241 errata workaround */
>>> @@ -49,6 +54,88 @@ 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.dev = dev;
>>
>> While testing v7 with multiple PCC MSCs sharing a single PCC channel, I
>> noticed that pcc_cl.dev is set once at channel creation time (from the
>> first MSC's device) and never updated when subsequent MSCs reuse the
>> channel.In mpam_pcc_chan_get(), the first call sets:
>>   cur->pcc_cl.dev = dev;
>>
>> Subsequent calls return the existing channel without updating 
>> pcc_cl.dev. Here dev_err() calls through the channel report errors 
>> against the first MSC's device, even when a different MSC triggered them.
> 
> Mmh, that's true, but I don't think there is much we can do about it? 
> Ultimately it's a channel shared between multiple devices.
> And I am less concerned about our own dev_err() (we can easily - and 
> actually should - replace this with pr_err()), but more about the mbox 
> uses of the dev pointer. Did you by any chance find more critical users 
> of the dev pointer other than dev_err() prints, in the mailbox code?

To answer my own question: I could only find code using the dev pointer 
during initialisation, to attribute the error messages to the right 
device. Which means when we drop the single usage in mpam_fb.c, there 
should be no more problems.
Fixed that accordingly.

Cheers,
Andre

> I don't know if the correct MSC attribution really matters, or if we can 
> live with just a valid reference to some "related" device, but ...
> 
>> I reproduced this with four PCC MSCs sharing subspace 0. After unbinding
>> the first MSC (kref 4→3), triggering dev_err() via a test hook showed 
>> the device prefix change from "mpam_msc mpam_msc.1:" to "platform 
>> mpam_msc.1:"
> 
> Mmmh, interesting, where does the change come from? I would think that 
> would result in a use after free?
> Which means we should make sure that some other MSC donates their dev 
> pointer if the first MSC goes away? I will have a look into that 
> direction, though it still feels a bit odd.
> 
>> A simple fix maybe to use the MSC's own device for error messages in
>> mpam_fb_send_request() instead of pcc_chan->pcc_cl.dev
> 
> But we don't have that pointer at this time, do we?  But as mentioned, 
> we could just revert to a simple pr_err() instead.
> 
> Thanks for the heads up anyway!
> Cheers,
> Andre
> 
> 
>>
>>> +    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.
>>> @@ -2274,6 +2361,8 @@ 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);
>>> @@ -2284,7 +2373,7 @@ 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;
>>> @@ -2328,7 +2417,7 @@ 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;
>>> @@ -2349,6 +2438,26 @@ 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) {
>>> +        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);
>>> +        }
>>>       } else {
>>>           return ERR_PTR(-EINVAL);
>>>       }
>>> diff --git a/drivers/resctrl/mpam_fb.c b/drivers/resctrl/mpam_fb.c
>>> index 61a91c6cec0a..7a7fb6d067ba 100644
>>> --- a/drivers/resctrl/mpam_fb.c
>>> +++ b/drivers/resctrl/mpam_fb.c
>>> @@ -37,6 +37,11 @@
>>>   #define MPAM_MSC_TOKEN_MASK        GENMASK(27, 18)
>>>   #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_VERSION_MAJOR_MASK    GENMASK(31, 16)
>>> +#define MPAM_FB_VERSION_MINOR_MASK    GENMASK(15, 0)
>>>   static atomic_t mpam_fb_token = ATOMIC_INIT(0);
>>> @@ -207,3 +212,39 @@ int mpam_fb_send_write_request(struct mpam_msc 
>>> *msc, u16 reg, u32 value)
>>>       return mpam_fb_send_request(msc->pcc_chan, 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->pcc_chan, 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 %ld.%ld\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 f13a79a84c58..8859189088f3 100644
>>> --- a/drivers/resctrl/mpam_internal.h
>>> +++ b/drivers/resctrl/mpam_internal.h
>>> @@ -528,6 +528,8 @@ 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:
>>
>> -- 
>> Best Regards,
>> Srivathsa
>>
>