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>

test-smoke.sh

@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+# Smoke test for i2c-imc-skylake (pci_driver, no bus/dev/fn params).
+# Run after: make install  OR  make reload
+# Requires: root, i2c-tools (i2cdetect), module already loaded.
+set -euo pipefail
+
+PASS=0
+FAIL=0
+
+_ok()   { echo "PASS: $*"; ((PASS++)) || true; }
+_fail() { echo "FAIL: $*"; ((FAIL++)) || true; }
+
+echo "=== adapter list ==="
+i2cdetect -l | grep -i imc || _fail "no imc adapters visible"
+
+_bus() {
+	i2cdetect -l 2>/dev/null | grep -i "imc.*channel $1" | awk '{print $1}' | tr -d 'i2c-'
+}
+
+CH0=$(_bus 0)
+CH1=$(_bus 1)
+
+if [ -n "$CH0" ]; then
+	_ok "ch0 adapter at i2c-$CH0"
+else
+	_fail "ch0 adapter not found"
+fi
+
+if [ -n "$CH1" ]; then
+	_ok "ch1 adapter at i2c-$CH1"
+else
+	_fail "ch1 adapter not found"
+fi
+
+if [ -n "$CH0" ]; then
+	echo ""
+	echo "=== SPD scan ch0 (expect 0x50-0x57) ==="
+	i2cdetect -r -y "$CH0" 2>/dev/null
+	# at least one SPD slot must respond
+	if i2cdetect -r -y "$CH0" 2>/dev/null | grep '50' >/dev/null; then
+		_ok "SPD device at 0x50 on ch0"
+	else
+		_fail "no SPD device on ch0"
+	fi
+fi
+
+if [ -n "$CH1" ]; then
+	echo ""
+	echo "=== SPD scan ch1 ==="
+	i2cdetect -r -y "$CH1" 2>/dev/null
+fi
+
+echo ""
+echo "=== udev autoload ==="
+sudo rmmod i2c-imc-skylake
+sleep 1
+sudo udevadm trigger --subsystem-match=pci --action=add
+sleep 2
+if lsmod | grep i2c_imc_skylake >/dev/null; then
+	_ok "udev autoload"
+else
+	_fail "udev did not reload module — check /etc/modules-load.d/ or udev rules"
+fi
+
+echo ""
+echo "=== load/unload x5 (leak/oops) ==="
+for i in $(seq 1 5); do
+	sudo rmmod i2c-imc-skylake
+	sudo modprobe i2c-imc-skylake
+	echo "  cycle $i OK"
+done
+_ok "5 load/unload cycles completed"
+
+echo ""
+echo "=== dmesg tail ==="
+dmesg | tail -20
+
+echo ""
+echo "=== result: $PASS passed, $FAIL failed ==="
+[ "$FAIL" -eq 0 ]