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

[Ben Horgan <ben.horgan@arm.com>]

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 <andre.przywara@arm.com>
> ---
>  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.

>  	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?

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);
>  	}