Simone
973e173a6d
The CI badge now points explicitly at main, the retracted CF8/CFC claim is attributed to v2 (the only revision ever posted), the next posting is v3, the DKMS version follows the VERSION file instead of a hardcoded 1.0.0, and the verification section catches up with the driver: Receive Byte and Send Byte exist, i2cdetect scans in read mode, the SPD EEPROMs are instantiated at probe and owned by ee1004, jc42 stays manual.
200 lines
6.6 KiB
Markdown
200 lines
6.6 KiB
Markdown
# Intel Skylake-X iMC SMBus I2C Driver
|
|
|
|
[](https://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
|
|
[](https://github.com/systempal/i2c-imc-skylake/actions/workflows/ci.yml?query=branch%3Amain)
|
|
[](https://lore.kernel.org/linux-i2c/20260620144131.415559-1-simone.chifari@gmail.com/T/#t)
|
|
|
|
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.
|
|
|
|
---
|
|
|
|
## Upstream status
|
|
|
|
Patch series v2 is archived on
|
|
[lore.kernel.org](https://lore.kernel.org/linux-i2c/[email protected]/T/#t)
|
|
and received no replies.
|
|
|
|
The next posting is `[PATCH v3]`, a single patch. See
|
|
[docs/submission/cover-letter.txt](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.
|
|
|
|
---
|
|
|
|
## 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:
|
|
|
|
```console
|
|
# 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](tools/probe-cf8-transaction.sh).
|
|
|
|
---
|
|
|
|
## 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
|
|
|
|
```text
|
|
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](tools/README.md).
|
|
|
|
---
|
|
|
|
## 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.
|
|
|
|
---
|
|
|
|
## Installation
|
|
|
|
### Prerequisites
|
|
|
|
```bash
|
|
# Debian / Ubuntu
|
|
sudo apt-get install build-essential i2c-tools strace linux-headers-$(uname -r)
|
|
|
|
# Fedora / RHEL
|
|
sudo dnf install gcc i2c-tools make strace kernel-devel
|
|
```
|
|
|
|
### Build & load
|
|
|
|
```bash
|
|
make # build
|
|
make checkpatch # style
|
|
make reload ALLOW_UNSAFE=1 # load for testing, after checking arbitration
|
|
sudo make install # install into the kernel tree
|
|
```
|
|
|
|
To permit a later `modprobe` or PCI modalias load:
|
|
|
|
```bash
|
|
echo 'options i2c-imc-skylake allow_unsafe_access=1' | \
|
|
sudo tee /etc/modprobe.d/i2c-imc-skylake.conf
|
|
```
|
|
|
|
This opt-in is intentionally not installed by the project.
|
|
|
|
---
|
|
|
|
## DKMS
|
|
|
|
```bash
|
|
sudo apt-get install dkms
|
|
sudo dkms add .
|
|
sudo dkms install -m i2c-imc-skylake -v "$(cat VERSION)"
|
|
```
|
|
|
|
---
|
|
|
|
## Verification
|
|
|
|
```bash
|
|
i2cdetect -l | grep iMC
|
|
```
|
|
|
|
```text
|
|
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:
|
|
|
|
```bash
|
|
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`:
|
|
|
|
```bash
|
|
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:
|
|
|
|
```bash
|
|
echo jc42 0x18 | sudo tee /sys/bus/i2c/devices/i2c-6/new_device
|
|
```
|
|
|
|
Read-only integration test against the module just built:
|
|
|
|
```bash
|
|
make
|
|
sudo make test
|
|
```
|
|
|
|
Hardware measurements still owed to the upstream submission are in
|
|
[tools/README.md](tools/README.md).
|
|
|
|
---
|
|
|
|
## License
|
|
|
|
GPL v2 only (`GPL-2.0-only`).
|