Simone
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Posted 2026-07-28: https://lore.kernel.org/linux-i2c/[email protected]/ The badge tracks the new thread, and the local Claude Code settings directory joins the gitignore: it holds machine-local permissions and must never reach the public mirror.
201 lines
6.7 KiB
Markdown
201 lines
6.7 KiB
Markdown
# Intel Skylake-X iMC SMBus I2C Driver
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[](https://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
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[](https://github.com/systempal/i2c-imc-skylake/actions/workflows/ci.yml?query=branch%3Amain)
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[](https://lore.kernel.org/linux-i2c/20260728122423.358416-1-simone.chifari@gmail.com/T/#t)
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An experimental Linux PCI driver for the SMBus engine in the integrated memory
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controller (iMC) of Intel Skylake-X and Cascade Lake-X processors
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(PCU function `8086:2085`).
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The engine reaches the DDR4 DIMMs: SPD EEPROMs at `0x50`-`0x57`, thermal
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sensors (TSOD) at `0x18`-`0x1f`, and whatever else a given module carries on
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those lines. The driver registers it as two standard Linux I2C adapters, one
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per hardware channel, so `i2c-tools`, `decode-dimms` and `lm-sensors` work
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without custom raw PCI writes.
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> [!WARNING]
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> The engine has no arbitration mechanism. SMM, a BMC and the memory
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> controller's own closed-loop thermal throttling (CLTT) can drive the same
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> registers, and the PCI ID does not tell the driver whether any of them is
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> active. The driver is disabled by default and binds only with
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> `allow_unsafe_access=1`. Use it only where concurrent firmware access has
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> been excluded — and check the log for the interference warning described
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> below.
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---
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## Upstream status
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`[PATCH v3]`, a single patch, was posted on 2026-07-28:
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[lore.kernel.org](https://lore.kernel.org/linux-i2c/[email protected]/T/#t).
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See [docs/submission/cover-letter.txt](docs/submission/cover-letter.txt).
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v2 (posted as `i2c-imc-x299`) is archived on
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[lore.kernel.org](https://lore.kernel.org/linux-i2c/[email protected]/T/#t)
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and received no replies.
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**No iMC I2C driver has ever been merged into mainline.** Andy Lutomirski's
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`i2c_imc` (2013-2016) and Stefan Schaeckeler's 2020 Broadwell rewrite both
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stalled on the same firmware-arbitration problem. This driver does not solve it
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either; it detects it. See the cover letter for what that means in practice.
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---
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## Retracted: the CF8/CFC claim
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The v2 posting claimed that firmware filters CF8/CFC config writes to
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the PCU function, and that the registers were reachable only through the
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memory-mapped window. **That was wrong.** It was never tested directly; the
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only evidence was a boot log line naming the access method.
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Measured on this board, driver unloaded:
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```console
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# setpci -s 16:1e.5 9c.L=20085002 # read SPD 0x50, register 0x02
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# setpci -s 16:1e.5 9c.L
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20005002 # GO consumed
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# setpci -s 16:1e.5 a8.L
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0000500c # READ_DONE, no error
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# setpci -s 16:1e.5 b4.L
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0000000c # SPD byte 2: DDR4 type code
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```
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The command executes and returns correct data through the ordinary accessors,
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and `MSR_SMI_COUNT` does not move across it. The driver now uses
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`pci_read_config_dword()` and `pci_write_config_dword()` like any other driver.
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Reproduce with [tools/probe-cf8-transaction.sh](tools/probe-cf8-transaction.sh).
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---
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## What the driver does about concurrency
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* serializes both channels under one mutex and waits for the whole engine to be
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idle before touching any register;
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* saves the command state of both channels, clears TSOD_ACTIVE for the duration
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of the transfer, restores both afterwards;
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* **compares the command registers after every transfer** against what it left
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in them, and fails with `-EAGAIN` if anything else changed;
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* reads the command word back and requires it to be the one just written,
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because the done bits are latched and a stale DONE would otherwise let a
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command that never reached the register look like a completed transfer.
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If your log shows
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```text
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i2c-imc-skylake 0000:16:1e.5: ch0 command changed under us (...): another master is using the engine
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```
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then firmware is using the engine on your system. Unload the driver.
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Not implemented: the Broadwell-E iMC driver additionally stops the PCU's TSOD
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polling for the duration of a transfer. The equivalent register has not been
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identified on this part — see [tools/README.md](tools/README.md).
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---
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## Origin & use case
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This driver started from the need to reach the **ENE KB9012** LED controller on
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Kingston FURY DDR4 RGB modules under Linux on X299, which is on the same iMC
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SMBus lines. It is device-oriented rather than tied to a DIMM brand; the
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practical uses are:
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* reading DDR4 SPD data and DIMM thermal sensors with standard tools;
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* OpenRGB and other userspace daemons for modules whose controllers sit on
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these buses.
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---
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## Installation
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### Prerequisites
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```bash
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# Debian / Ubuntu
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sudo apt-get install build-essential i2c-tools strace linux-headers-$(uname -r)
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# Fedora / RHEL
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sudo dnf install gcc i2c-tools make strace kernel-devel
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```
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### Build & load
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```bash
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make # build
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make checkpatch # style
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make reload ALLOW_UNSAFE=1 # load for testing, after checking arbitration
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sudo make install # install into the kernel tree
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```
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To permit a later `modprobe` or PCI modalias load:
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```bash
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echo 'options i2c-imc-skylake allow_unsafe_access=1' | \
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sudo tee /etc/modprobe.d/i2c-imc-skylake.conf
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```
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This opt-in is intentionally not installed by the project.
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---
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## DKMS
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```bash
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sudo apt-get install dkms
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sudo dkms add .
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sudo dkms install -m i2c-imc-skylake -v "$(cat VERSION)"
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```
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---
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## Verification
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```bash
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i2cdetect -l | grep iMC
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```
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```text
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i2c-6 smbus iMC SMBus Skylake-X channel 0 SMBus adapter
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i2c-7 smbus iMC SMBus Skylake-X channel 1 SMBus adapter
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```
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The driver implements SMBus Receive Byte, Send Byte, BYTE_DATA and WORD_DATA.
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SMBus Quick is deliberately not implemented (a write probe into `0x30`-`0x37`
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is how an EE1004 SPD is write-protected), so scan in read mode:
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```bash
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sudo i2cdetect -y -r 6
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```
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SPD EEPROMs are instantiated automatically at probe (`ee1004`, counted from
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DMI), so `decode-dimms` works with no manual step. `ee1004` owns those
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addresses once bound: read the SPD through the driver, not with raw `i2cget`:
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```bash
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sudo cat /sys/bus/i2c/devices/6-0050/eeprom | hexdump -C | head
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```
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`jc42` is not instantiated (no firmware description of these buses); add it by
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hand where a thermal sensor is populated:
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```bash
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echo jc42 0x18 | sudo tee /sys/bus/i2c/devices/i2c-6/new_device
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```
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Read-only integration test against the module just built:
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```bash
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make
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sudo make test
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```
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Hardware measurements still owed to the upstream submission are in
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[tools/README.md](tools/README.md).
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---
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## License
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GPL v2 only (`GPL-2.0-only`).
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