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

[Andre Przywara]

Hi,

On 7/24/26 20:03, Sudeep Holla wrote:
> On Fri, Jul 24, 2026 at 06:08:52PM +0100, Ben Horgan wrote:
> > Hi Andre,
> >
> > On 7/23/26 16:54, 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@xxxxxxx>
> > > ---
> > >   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
>
> [...]

[ ... ]

> > > +
> > > +	writel_relaxed(PCC_CHAN_FLAGS_IRQ, &pcc_shmem->flags);
> >
> > Are you sure it's ok to unconditionally set this irq bit? I know we found out that we do need it set
> > sometimes. What does setting it actually based on what the platform does look like?
>
> You need to set it if you are not polling the PCC channels for completions.

So I was wondering about that: why does the PCC *user* need to set it 
here? That's probably more of a protocol issue, but to me it breaks the 
abstraction: MPAM-Fb just uses the mailbox provided via PCC, and doesn't 
care about whether it's polling or using an interrupt, because that is a 
PCC detail.
So is there a way to figure this out cleanly? I only found that one 
could possibly chase down the "struct mbox_chan" controller struct, then 
look at txdone_method, or something similar, but that sounds very dodgy?

Do you have any ideas on this?

Cheers,
Andre