Subject: Re: [PATCH 5/5] arm_mpam: detect and enable MPAM-Fb PCC support

From: Andre Przywara <[email-masked]>
Date: Wed, 13 May 2026 16:51:36 +0200

Hi Ben,

On 5/8/26 12:48, Ben Horgan wrote:
> Hi Andre,
> 
> On 4/29/26 15:13, 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.
>> 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-masked]>
>> ---
>>   drivers/acpi/arm64/mpam.c      |  2 ++
>>   drivers/resctrl/mpam_devices.c | 46 +++++++++++++++++++++++++++++++---
>>   2 files changed, 45 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/acpi/arm64/mpam.c b/drivers/acpi/arm64/mpam.c
>> index 99c2bdbb3314..edb4d10e8dc3 100644
>> --- a/drivers/acpi/arm64/mpam.c
>> +++ b/drivers/acpi/arm64/mpam.c
>> @@ -341,6 +341,8 @@ static struct platform_device * __init acpi_mpam_parse_msc(struct acpi_mpam_msc_
>>   	} else if (iface == MPAM_IFACE_PCC) {
>>   		props[next_prop++] = PROPERTY_ENTRY_U32("pcc-channel",
>>   							tbl_msc->base_address);
>> +		props[next_prop++] = PROPERTY_ENTRY_U32("msc-id",
>> +							tbl_msc->identifier);
>>   	}
>>   
>>   	acpi_mpam_parse_irqs(pdev, tbl_msc, res, &next_res);
>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
>> index 62aa04cb6905..6f0d0959d3a4 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -19,6 +19,7 @@
>>   #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>
>> @@ -27,6 +28,9 @@
>>   #include <linux/types.h>
>>   #include <linux/workqueue.h>
>>   
>> +#include <acpi/pcc.h>
>> +#include <acpi/acpi_io.h>
>> +
>>   #include "mpam_internal.h"
>>   #include "mpam_fb.h"
>>   
>> @@ -1042,7 +1046,8 @@ static u64 mpam_msc_read_mbwu_l(struct mpam_msc *msc)
>>   
>>   	mpam_mon_sel_lock_held(msc);
>>   
>> -	WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
>> +	if (msc->iface == MPAM_IFACE_MMIO)
>> +		WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
> 
> This should be in a different patch.

Which one, preferably? A separate one? Or patch 3/5, which adds the PCC 
access code?
This one here is the patch that enables non-MMIO accesses, so I figured 
that's the place we should relax this check.

> 
>>   	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>>   
>>   	mbwu_l_high2 = __mpam_read_reg(msc, MSMON_MBWU_L + 4);
>> @@ -2042,10 +2047,15 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
>>   	mpam_free_garbage();
>>   }
>>   
>> +static void mpam_pcc_rx_callback(struct mbox_client *cl, void *msg)
>> +{
>> +	/* TODO: wake up tasks blocked on this MSC's PCC channel */
>> +}
>> +
>>   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;
>> @@ -2090,7 +2100,8 @@ 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(&pdev->dev, "pcc-channel",
>> +				     &pcc_subspace_id))
>>   		msc->iface = MPAM_IFACE_MMIO;
>>   	else
>>   		msc->iface = MPAM_IFACE_PCC;
>> @@ -2106,6 +2117,35 @@ 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 msc_id;
>> +
>> +		msc->pcc_cl.dev = &pdev->dev;
>> +		msc->pcc_cl.rx_callback = mpam_pcc_rx_callback;
>> +		msc->pcc_cl.tx_block = false;
> 
> How do we make sure that, for instance, a MON_SEL write has completed before we start reading
> the associated counters? Is there an ordering guarantee?

What is your concern here, exactly? To group all the accesses selected 
by one MON_SEL write, we have the mon_sel lock (now mutex) already. And 
on the mbox side I think we are safe because the PCC channel is 
exclusive, we have the per-channel mutex (pcc_chan_lock, in patch 3/5), 
and we wait for confirmation from the other end, even for writes. So as 
long as the MON_SEL write appears in program order before the counter 
read, all should be good, no? Am I missing something?

Cheers,
Andre

> 
> Thanks,
> 
> Ben
> 
>> +		msc->pcc_cl.tx_tout = 1000; /* 1s */
>> +		msc->pcc_cl.knows_txdone = false;
>> +
>> +		if (device_property_read_u32(&pdev->dev, "msc-id", &msc_id)) {
>> +			pr_err("missing MPAM-Fb MSC identifier\n");
>> +			return ERR_PTR(-EINVAL);
>> +		}
>> +		msc->mpam_fb_msc_id = msc_id;
>> +
>> +		msc->pcc_chan = pcc_mbox_request_channel(&msc->pcc_cl,
>> +							 pcc_subspace_id);
>> +		if (IS_ERR(msc->pcc_chan)) {
>> +			pr_err("Failed to request MSC PCC channel\n");
>> +			return (void *)msc->pcc_chan;
>> +		}
>> +
>> +		if (msc->pcc_chan->shmem_size < MPAM_FB_MAX_MSG_SIZE) {
>> +			pr_err("MPAM-Fb PCC channel size too small.\n");
>> +			pcc_mbox_free_channel(msc->pcc_chan);
>> +			return ERR_PTR(-ENOMEM);
>> +		}
>> +
>> +		mutex_init(&msc->pcc_chan_lock);
>>   	} else {
>>   		return ERR_PTR(-EINVAL);
>>   	}
>