Intel Skylake-X iMC SMBus I2C Driver

An experimental Linux PCI driver for the SMBus engine in the integrated memory controller (iMC) of Intel Skylake-X and Cascade Lake-X processors (PCU function 8086:2085).

The engine reaches the DDR4 DIMMs: SPD EEPROMs at 0x50-0x57, thermal sensors (TSOD) at 0x18-0x1f, and whatever else a given module carries on those lines. The driver registers it as two standard Linux I2C adapters, one per hardware channel, so i2c-tools, decode-dimms and lm-sensors work without custom raw PCI writes.

Warning

The engine has no arbitration mechanism. SMM, a BMC and the memory controller's own closed-loop thermal throttling (CLTT) can drive the same registers, and the PCI ID does not tell the driver whether any of them is active. The driver is disabled by default and binds only with allow_unsafe_access=1. Use it only where concurrent firmware access has been excluded — and check the log for the interference warning described below.

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Upstream status

Patch series v2 is archived on lore.kernel.org and received no replies.

The next posting is [PATCH v3], a single patch. See docs/submission/cover-letter.txt.

No iMC I2C driver has ever been merged into mainline. Andy Lutomirski's i2c_imc (2013-2016) and Stefan Schaeckeler's 2020 Broadwell rewrite both stalled on the same firmware-arbitration problem. This driver does not solve it either; it detects it. See the cover letter for what that means in practice.

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Retracted: the CF8/CFC claim

The v2 posting claimed that firmware filters CF8/CFC config writes to the PCU function, and that the registers were reachable only through the memory-mapped window. That was wrong. It was never tested directly; the only evidence was a boot log line naming the access method.

Measured on this board, driver unloaded:

# setpci -s 16:1e.5 9c.L=20085002    # read SPD 0x50, register 0x02
# setpci -s 16:1e.5 9c.L
20005002                             # GO consumed
# setpci -s 16:1e.5 a8.L
0000500c                             # READ_DONE, no error
# setpci -s 16:1e.5 b4.L
0000000c                             # SPD byte 2: DDR4 type code

The command executes and returns correct data through the ordinary accessors, and MSR_SMI_COUNT does not move across it. The driver now uses pci_read_config_dword() and pci_write_config_dword() like any other driver. Reproduce with tools/probe-cf8-transaction.sh.

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What the driver does about concurrency

• serializes both channels under one mutex and waits for the whole engine to be idle before touching any register;
• saves the command state of both channels, clears TSOD_ACTIVE for the duration of the transfer, restores both afterwards;
• compares the command registers after every transfer against what it left in them, and fails with -EAGAIN if anything else changed;
• reads the command word back and requires it to be the one just written, because the done bits are latched and a stale DONE would otherwise let a command that never reached the register look like a completed transfer.

If your log shows

i2c-imc-skylake 0000:16:1e.5: ch0 command changed under us (...): another master is using the engine

then firmware is using the engine on your system. Unload the driver.

Not implemented: the Broadwell-E iMC driver additionally stops the PCU's TSOD polling for the duration of a transfer. The equivalent register has not been identified on this part — see tools/README.md.

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Origin & use case

This driver started from the need to reach the ENE KB9012 LED controller on Kingston FURY DDR4 RGB modules under Linux on X299, which is on the same iMC SMBus lines. It is device-oriented rather than tied to a DIMM brand; the practical uses are:

• reading DDR4 SPD data and DIMM thermal sensors with standard tools;
• OpenRGB and other userspace daemons for modules whose controllers sit on these buses.

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Installation

Prerequisites

# Debian / Ubuntu
sudo apt-get install build-essential i2c-tools strace linux-headers-$(uname -r)

# Fedora / RHEL
sudo dnf install gcc i2c-tools make strace kernel-devel

Build & load

make                          # build
make checkpatch               # style
make reload ALLOW_UNSAFE=1    # load for testing, after checking arbitration
sudo make install             # install into the kernel tree

To permit a later modprobe or PCI modalias load:

echo 'options i2c-imc-skylake allow_unsafe_access=1' | \
  sudo tee /etc/modprobe.d/i2c-imc-skylake.conf

This opt-in is intentionally not installed by the project.

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DKMS

sudo apt-get install dkms
sudo dkms add .
sudo dkms install -m i2c-imc-skylake -v "$(cat VERSION)"

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Verification

i2cdetect -l | grep iMC

i2c-6   smbus           iMC SMBus Skylake-X channel 0                SMBus adapter
i2c-7   smbus           iMC SMBus Skylake-X channel 1                SMBus adapter

The driver implements SMBus Receive Byte, Send Byte, BYTE_DATA and WORD_DATA. SMBus Quick is deliberately not implemented (a write probe into 0x30-0x37 is how an EE1004 SPD is write-protected), so scan in read mode:

sudo i2cdetect -y -r 6

SPD EEPROMs are instantiated automatically at probe (ee1004, counted from DMI), so decode-dimms works with no manual step. ee1004 owns those addresses once bound: read the SPD through the driver, not with raw i2cget:

sudo cat /sys/bus/i2c/devices/6-0050/eeprom | hexdump -C | head

jc42 is not instantiated (no firmware description of these buses); add it by hand where a thermal sensor is populated:

echo jc42 0x18 | sudo tee /sys/bus/i2c/devices/i2c-6/new_device

Read-only integration test against the module just built:

make
sudo make test

Hardware measurements still owed to the upstream submission are in tools/README.md.

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License

GPL v2 only (GPL-2.0-only).