i2c: imc-skylake: add driver for Intel Skylake-X iMC SMBus engine

Add a driver for the integrated memory controller (iMC) SMBus engine on
Intel Skylake-X / Cascade Lake-X processors (socket LGA 2066, platform
X299, PCU function 8086:2085).

The engine provides two SMBus channels — one per pair of DIMM slots —
over which SPD EEPROMs, DDR4 thermal sensors and third-party LED
controllers are accessible. Exposing it as a pair of standard Linux I2C
adapters lets existing tools (i2c-tools, lm-sensors) use it without
bespoke sysfs hacks.

Key design decisions:
- ECAM MMIO access instead of CF8/CFC port I/O (SMM traps port writes)
- Dynamic MCFG table parsing for mmcfg_base (no hardcoding)
- Support for SMBus BYTE_DATA and WORD_DATA transfers
- devm-managed resources with automatic cleanup
- Global mutex to serialize transactions across both channels sharing
  the same ECAM mapping

Signed-off-by: Simone Chifari <simone.chifari@gmail.com>

Makefile

@@ -0,0 +1,63 @@
+obj-m := i2c-imc-skylake.o
+
+KVER   ?= $(shell uname -r)
+KDIR   ?= /lib/modules/$(KVER)/build
+MODDIR  = /lib/modules/$(KVER)/kernel/drivers/i2c/busses
+PWD    := $(shell pwd)
+
+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: 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
+
+# Remove module and .ko/.ko.zst from kernel tree
+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)
+
+# Quick rmmod + insmod cycle for leak/oops testing
+reload: all
+	sudo rmmod i2c-imc-skylake 2>/dev/null || true
+	sudo insmod i2c-imc-skylake.ko
+
+# Load without build (use after install)
+load:
+	sudo modprobe i2c-dev
+	sudo modprobe i2c-imc-skylake
+
+unload:
+	sudo rmmod i2c-imc-skylake 2>/dev/null || true
+
+# Checkpatch (must be clean before submission)
+checkpatch:
+	perl $(KDIR)/scripts/checkpatch.pl --strict --no-tree -f i2c-imc-skylake.c
+
+# Sparse static analysis (requires sparse >= kernel version; may show header errors on old sparse)
+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
+
+log:
+	sudo dmesg | grep i2c-imc-skylake | tail -80
+
+.PHONY: all clean install uninstall reload load unload checkpatch sparse test log