Simone
6bf5879a9f
Add support for I2C_SMBUS_QUICK and I2C_SMBUS_BYTE transfers to maintain compatibility with older userspace tools (e.g., legacy i2cdetect versions). - I2C_SMBUS_QUICK: Emulated as read-byte from offset 0, allowing device probing without sending data. - I2C_SMBUS_BYTE: For reads, the command byte becomes the register offset; for writes, the command byte is the value written to offset 0. This preserves the LKML v3 corrections (channel index, FRAME decomposition, timeout documentation, ECAM overflow checks) while adding backward compatibility for tools that expect QUICK/BYTE support. Note: This functionality is only in the compat branch and will not be submitted upstream, as the hardware requires a register offset for every transaction.
Intel Skylake-X iMC SMBus I2C Driver
-orange.svg){kind=icon}
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 ACPIMCFGtable and accessing the registers directly via ECAM MMIO memory-mapped pages. - Dual Channel Support: Registers two independent I2C buses (
iMC SMBus Skylake-X channel 0andchannel 1), mapping physically to DIMM slots 1-2 and 3-4 respectively. - Clean Lifecycle: Fully compliant with the
devmkernel 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
The engine has no SMBus QUICK/BYTE primitive, so i2cdetect cannot scan the bus. Read a device directly with a byte-data access (a register offset is always required), e.g. SPD byte 2 (DDR4 type code 0x0c) from the EEPROM at 0x50 on channel 0:
sudo i2cget -y 6 0x50 0x02
License
This driver is licensed under the GPL v2 only (GPL-2.0-only).