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

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));

+	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));