i2c-imc-skylake: split the submission into a PCI patch and the driver

The driver reached the PCU function's config space by walking the ACPI MCFG
table and mapping the ECAM page itself. That is one leaf driver deciding
config-space policy, and no other driver in mainline does it, so it was the
single most likely reason for a NAK.

Move that decision where it belongs: the upstream series is now two patches,
a PCI/x86 accessor that forces the memory-mapped path with the core's
locking, and this driver on top of it. Reviewers on each side now get
something they can act on, and get_maintainer.pl selects the PCI and x86
people by itself.

Correctness work in the same pass:

  - the completion bits are latched, so a command the engine never accepted
    was indistinguishable from a completed one and returned the previous
    transaction's data as valid. Require a positive sign that the engine
    acted on the command;
  - compare the command registers after every transfer against what the
    driver left in them and fail with -EAGAIN on a mismatch. The original
    iMC driver had this check and it is what makes an unarbitrated system
    detectable instead of silently wrong;
  - use the i2c core's suspended-adapter mechanism rather than a private
    flag and -EHOSTDOWN;
  - use devm_mutex_init(), drop the unnecessary pci_enable_device(), and
    return -ENODEV rather than -EPERM from the default-off gate, which was
    printing a driver-core error on every boot;
  - mask GO when restoring a saved command word.

Stop attributing the dropped CF8/CFC write to System Management Mode. The
only evidence was a boot log line naming the access method, which says
nothing about who filters the write. tools/probe-cf8-write.sh is there to
settle it with an MSR_SMI_COUNT measurement.

Add Documentation/i2c/busses/i2c-imc-skylake.rst, which every other PC SMBus
driver has and which was explicitly asked for on the 2020 iMC thread, the
tools/ scripts for the hardware measurements the submission still owes, and
compat/pci-mmcfg-compat.h so the driver still builds out-of-tree against a
kernel without the new accessor while the source stays byte-identical to the
submitted patch.

Makefile

@@ -2,45 +2,53 @@ obj-m := i2c-imc-skylake.o
 
 KVER   ?= $(shell uname -r)
 KDIR   ?= /lib/modules/$(KVER)/build
-MODDIR  = /lib/modules/$(KVER)/kernel/drivers/i2c/busses
+INSTALL_MOD_PATH ?= $(DESTDIR)
+INSTALL_MOD_DIR  ?= updates
 PWD    := $(shell pwd)
 
+# The driver calls pci_mmcfg_{read,write}_config(), added by patch 1/2 of the
+# upstream series. Against a kernel that does not have them yet, force-include
+# the out-of-tree shim so the driver source stays identical to the submitted
+# patch. See compat/pci-mmcfg-compat.h.
+IMC_NEED_COMPAT := $(shell grep -qs pci_mmcfg_read_config \
+	$(srctree)/include/linux/pci.h || echo y)
+ifeq ($(IMC_NEED_COMPAT),y)
+ccflags-y += -include $(src)/compat/pci-mmcfg-compat.h
+$(warning i2c-imc-skylake: target kernel lacks pci_mmcfg_read_config(); building with the out-of-tree compat shim)
+endif
+
 all:
 	$(MAKE) -C $(KDIR) M=$(PWD) modules
 
 clean:
 	$(MAKE) -C $(KDIR) M=$(PWD) clean
 
-# Install into kernel tree + modprobe.
-# Ubuntu/Debian kernel 6.x+ ships .ko.zst; depmod skips uncompressed .ko files,
-# so compress with zstd when available, else fall back to plain .ko.
+# Install through Kbuild so ownership, compression and destination follow the
+# target kernel/distribution policy. Invoke this target with suitable privileges.
 install: all
-	if command -v zstd >/dev/null 2>&1; then \
-		zstd -q -f i2c-imc-skylake.ko -o i2c-imc-skylake.ko.zst; \
-		sudo install -D -m 644 i2c-imc-skylake.ko.zst $(MODDIR)/i2c-imc-skylake.ko.zst; \
-		sudo rm -f $(MODDIR)/i2c-imc-skylake.ko; \
-	else \
-		sudo install -D -m 644 i2c-imc-skylake.ko $(MODDIR)/i2c-imc-skylake.ko; \
-		sudo rm -f $(MODDIR)/i2c-imc-skylake.ko.zst; \
-	fi
-	sudo depmod -a $(KVER)
-	sudo modprobe i2c-imc-skylake
+	$(MAKE) -C $(KDIR) M=$(PWD) modules_install \
+		INSTALL_MOD_PATH=$(INSTALL_MOD_PATH) \
+		INSTALL_MOD_DIR=$(INSTALL_MOD_DIR)
 
-# Remove module and .ko/.ko.zst from kernel tree
+# Remove a module installed in INSTALL_MOD_DIR. Invoke with suitable privileges.
 uninstall:
-	sudo rmmod i2c-imc-skylake 2>/dev/null || true
-	sudo rm -f $(MODDIR)/i2c-imc-skylake.ko $(MODDIR)/i2c-imc-skylake.ko.zst
-	sudo depmod -a $(KVER)
+	rm -f $(INSTALL_MOD_PATH)/lib/modules/$(KVER)/$(INSTALL_MOD_DIR)/i2c-imc-skylake.ko*
+	depmod -a -b $(INSTALL_MOD_PATH) $(KVER)
 
-# Quick rmmod + insmod cycle for leak/oops testing
+# Runtime helpers require explicit acknowledgement of the firmware-arbitration
+# risk: make reload ALLOW_UNSAFE=1 (or make load ALLOW_UNSAFE=1).
 reload: all
+	@test "$(ALLOW_UNSAFE)" = "1" || \
+		{ echo "Refusing: rerun with ALLOW_UNSAFE=1" >&2; exit 2; }
 	sudo rmmod i2c-imc-skylake 2>/dev/null || true
-	sudo insmod i2c-imc-skylake.ko
+	sudo insmod i2c-imc-skylake.ko allow_unsafe_access=1
 
 # Load without build (use after install)
 load:
+	@test "$(ALLOW_UNSAFE)" = "1" || \
+		{ echo "Refusing: rerun with ALLOW_UNSAFE=1" >&2; exit 2; }
 	sudo modprobe i2c-dev
-	sudo modprobe i2c-imc-skylake
+	sudo modprobe i2c-imc-skylake allow_unsafe_access=1
 
 unload:
 	sudo rmmod i2c-imc-skylake 2>/dev/null || true
@@ -53,11 +61,17 @@ checkpatch:
 sparse:
 	$(MAKE) -C $(KDIR) M=$(PWD) C=1 CF="-D__CHECK_ENDIAN__"
 
-# Smoke test — requires root + module loaded (run after make install or make reload)
-test:
-	sudo bash test-smoke.sh
+# Smoke test requires root or non-interactive sudo.
+test: all
+	MODULE_PATH=$(PWD)/i2c-imc-skylake.ko bash test-smoke.sh
+
+# Hardware measurements that the upstream submission still needs. Read-only
+# unless explicitly asked otherwise; see tools/README.md.
+measure:
+	@echo "Run the scripts under tools/ individually; see tools/README.md" >&2
+	@false
 
 log:
 	sudo dmesg | grep i2c-imc-skylake | tail -80
 
-.PHONY: all clean install uninstall reload load unload checkpatch sparse test log
+.PHONY: all clean install uninstall reload load unload checkpatch sparse test measure log