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Simone 4697ce3e3b 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>

2026-06-27 15:01:50 +02:00

4.7 KiB

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Intel Skylake-X iMC SMBus I2C Driver

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:

The X299 System Management Mode (SMM) firmware traps standard port-based PCI configuration writes, breaking standard driver access.
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 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:

# Debian / Ubuntu
sudo apt-get install build-essential linux-headers-$(uname -r)

# Fedora / RHEL
sudo dnf install gcc make kernel-devel

Manual Build & Load

Compile the driver:
```
make
```
Test style compliance:
```
make checkpatch
```
Load immediately (testing):
```
make reload
```
Install permanently into the kernel tree:
```
sudo make install
```

DKMS Support (Dynamic Kernel Module Support)

To build and load the driver automatically on every kernel upgrade:

Install DKMS:
```
sudo apt-get install dkms
```
Register the module:
```
sudo dkms add .
```

Build and install:

sudo dkms install -m i2c-imc-skylake -v 1.0

Verification & Testing

Once loaded, verify that the two channels are registered successfully:

i2cdetect -l | grep iMC

Expected output:

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:

sudo i2cdetect -y 6

License

This driver is licensed under the GPL v2 only (GPL-2.0-only).

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