Re: [PATCH v4 08/10] arm_mpam: add MPAM-Fb MSC firmware access support

[Sudeep Holla <sudeep.holla@kernel.org>]

On Thu, Jul 23, 2026 at 05:54:52PM +0200, Andre Przywara wrote:
> The Arm MPAM Firmware-backed (Fb) Profile document[1] describes an
> alternative way of accessing the "Memory System Components" (MSC) in an
> MPAM enabled system.
> 
> Normally the MSCs are MMIO mapped, but in some implementations this
> might not be possible (MSC located outside of the local socket, MSC
> mapped secure-only) or desirable (direct MMIO access too slow or needs
> to be mediated through a control processor). MPAM-fb standardises a
> protocol to abstract MSC accesses, building on the SCMI protocol.
> 
> Add functions that do an MSC read or write access by redirecting the
> request through a firmware interface. For now this done via an ACPI
> PCC shared memory and mailbox combination.
> 
> Since the protocol used is only a small subset of the full SCMI spec,
> and the SCMI protocol has no full ACPI support anyway, open-code the
> (simple) SCMI message generation, for just the fields we need.
> 
> [1] https://developer.arm.com/documentation/den0144/latest
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  drivers/resctrl/Makefile        |   2 +-
>  drivers/resctrl/mpam_devices.c  |  27 ++++-
>  drivers/resctrl/mpam_fb.c       | 197 ++++++++++++++++++++++++++++++++
>  drivers/resctrl/mpam_internal.h |  20 ++++
>  include/linux/arm_mpam.h        |   2 +-
>  5 files changed, 240 insertions(+), 8 deletions(-)
>  create mode 100644 drivers/resctrl/mpam_fb.c
> 
> diff --git a/drivers/resctrl/Makefile b/drivers/resctrl/Makefile
> index 4f6d0e81f9b8..097c036724e9 100644
> --- a/drivers/resctrl/Makefile
> +++ b/drivers/resctrl/Makefile
> @@ -1,5 +1,5 @@
>  obj-$(CONFIG_ARM64_MPAM_DRIVER)			+= mpam.o
> -mpam-y						+= mpam_devices.o
> +mpam-y						+= mpam_devices.o mpam_fb.o
>  mpam-$(CONFIG_ARM64_MPAM_RESCTRL_FS)		+= mpam_resctrl.o
>  
>  ccflags-$(CONFIG_ARM64_MPAM_DRIVER_DEBUG)	+= -DDEBUG
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index e2cb884eacf4..b8108ed4b32c 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -181,6 +181,9 @@ static int __mpam_read_reg(struct mpam_msc *msc, u16 reg, u32 *res)
>  {
>  	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>

The above doesn't make any sense for PCC accesses, perhaps need to be pushed
down ?

> +	if (msc->iface == MPAM_IFACE_PCC)
> +		return mpam_fb_send_read_request(msc, reg, res);
> +
>  	*res = readl_relaxed(msc->mapped_hwpage + reg);
>  
>  	return 0;
> @@ -197,9 +200,12 @@ static inline int _mpam_read_partsel_reg(struct mpam_msc *msc, u16 reg,
>  
>  static int __mpam_write_reg(struct mpam_msc *msc, u16 reg, u32 val)
>  {
> -	WARN_ON_ONCE(reg + sizeof(u32) > msc->mapped_hwpage_sz);
>  	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>  

Ditto

-- 
Regards,
Sudeep