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>

README.md

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+# Intel Skylake-X iMC SMBus I2C Driver
+
+[![License: GPL v2](https://img.shields.io/badge/License-GPL%20v2-blue.svg)](https://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
+[![Kernel Compatibility](https://img.shields.io/badge/Kernel-6.x%20%7C%207.x-green.svg)](#)
+[![Code Style: checkpatch](https://img.shields.io/badge/checkpatch-clean-brightgreen.svg)](#)
+[![Upstream Status](https://img.shields.io/badge/Upstream-Submitted%20(v2)-orange.svg)](https://lore.kernel.org/linux-i2c/)
+
+A modern Linux PCI bus driver for the integrated Memory Controller (iMC) SMBus controller found in Intel Skylake-X and Cascade Lake-X (HEDT X299, function ID `8086:2085`) processors. 
+
+This driver maps the iMC SMBus engine and registers it as two standard Linux I2C adapters (one per physical channel), allowing native userspace tools like `i2c-tools` and `lm-sensors` to interact with target devices (such as DDR4 DIMM SPD EEPROMs and thermal sensors) without custom raw PCI port-IO writes.
+
+---
+
+## Origin & Use Case
+
+This driver was born out of the need to control the RGB lighting effects on **Kingston FURY DDR4 RGB** memory modules under Linux on the Intel X299 (HEDT) platform.
+
+These memory modules carry an onboard **ENE KB9012** LED controller accessible via the SMBus channels of the CPU's integrated memory controller (iMC). However, standard Linux tools and OpenRGB could not communicate with them because:
+1. The X299 System Management Mode (SMM) firmware traps standard port-based PCI configuration writes, breaking standard driver access.
+2. The iMC SMBus channels were not exposed as standard Linux I2C buses.
+
+By resolving the MMCONFIG base and using ECAM MMIO, this driver bypasses SMM and registers the physical memory channels as standard `/dev/i2c-*` adapters. 
+
+While the kernel driver itself is kept 100% generic and brand-agnostic (to comply with Linux upstream requirements), its primary practical use cases are:
+* Driving userspace Linux daemons for hardware/software-rendered RGB lighting animations without screen or GUI lag.
+* Enabling compatibility with OpenRGB for memory modules on Skylake-X / Cascade Lake-X systems.
+* Reading standard DDR4 SPD data and DIMM thermal sensor metrics using standard utilities like `decode-dimms`.
+
+---
+
+## Upstream Status
+
+This driver is currently submitted for review to the Linux kernel `linux-i2c` subsystem mailing list for mainlining (patch series v2). See the [lore.kernel.org linux-i2c archive](https://lore.kernel.org/linux-i2c/) for the discussion thread.
+
+---
+
+## Technical Features & SMM Bypass
+
+* **ECAM MMIO Access**: On the X299 platform, System Management Mode (SMM) firmware traps and drops legacy port-based (`CF8/CFC`) configuration space writes to the PCU function. This driver bypasses the trap by resolving the MMCONFIG base from the ACPI `MCFG` table and accessing the registers directly via ECAM MMIO memory-mapped pages.
+* **Dual Channel Support**: Registers two independent I2C buses (`iMC SMBus Skylake-X channel 0` and `channel 1`), mapping physically to DIMM slots 1-2 and 3-4 respectively.
+* **Clean Lifecycle**: Fully compliant with the `devm` kernel framework. All mapped registers, adapters, and mutexes are cleaned up automatically upon driver unbinding.
+* **Zero Out-of-Tree Brandings**: Code is completely generic and compliant with standard Linux kernel styling guidelines.
+
+---
+
+## Installation
+
+### Prerequisites
+
+Ensure you have kernel headers and compiler utilities installed:
+
+```bash
+# Debian / Ubuntu
+sudo apt-get install build-essential linux-headers-$(uname -r)
+
+# Fedora / RHEL
+sudo dnf install gcc make kernel-devel
+```
+
+### Manual Build & Load
+
+1. **Compile the driver**:
+   ```bash
+   make
+   ```
+2. **Test style compliance**:
+   ```bash
+   make checkpatch
+   ```
+3. **Load immediately (testing)**:
+   ```bash
+   make reload
+   ```
+4. **Install permanently into the kernel tree**:
+   ```bash
+   sudo make install
+   ```
+
+---
+
+## DKMS Support (Dynamic Kernel Module Support)
+
+To build and load the driver automatically on every kernel upgrade:
+
+1. **Install DKMS**:
+   ```bash
+   sudo apt-get install dkms
+   ```
+2. **Register the module**:
+   ```bash
+   sudo dkms add .
+   ```
+3. **Build and install**:
+   ```bash
+   sudo dkms install -m i2c-imc-skylake -v 1.0
+   ```
+
+---
+
+## Verification & Testing
+
+Once loaded, verify that the two channels are registered successfully:
+
+```bash
+i2cdetect -l | grep iMC
+```
+
+Expected output:
+```text
+i2c-6   smbus           iMC SMBus Skylake-X channel 0                SMBus adapter
+i2c-7   smbus           iMC SMBus Skylake-X channel 1                SMBus adapter
+```
+
+You can scan for SPD EEPROMs (usually located at addresses `0x50-0x57`) on channel 0:
+```bash
+sudo i2cdetect -y 6
+```
+
+---
+
+## License
+
+This driver is licensed under the **GPL v2 only** (`GPL-2.0-only`).