Simone
c9b8799f69
Two things came out of preparing the suspend test, and one of them is a correction. ee1004 does not bind to this adapter. Both functionality combinations it accepts include I2C_FUNC_SMBUS_BYTE (drivers/misc/eeprom/ee1004.c:301), which this driver does not implement, so the probe fails with -EPFNOSUPPORT. The cover letter claimed BYTE_DATA and WORD_DATA "cover ee1004 and jc42" and the .rst told the reader to instantiate it; both were wrong and are corrected. jc42 is fine, it only needs BYTE_DATA and WORD_DATA, but no TSOD is populated here to bind to. The practical cost is that decode-dimms does not work through this adapter. SPD bytes stay readable byte by byte with i2cget. Receive Byte is what PawnIO implements with its pointer-select bit and what this project chose not to advertise for want of validation; that choice was not free, and adding it validated is the most useful thing left. Suspend and resume tested with continuous traffic on both adapters, s2idle and S3. Transfers recover, nothing complains. Across S3 the engine comes back reprogrammed on both channels: DATA 0x2000b600 -> 0x20005000 STATUS 0x0000500c -> 0x00005004 The command word addresses SPD 0x50 for a read and the status drops to READ_DONE alone. Linux issued none of it: the adapter was marked suspended and userspace frozen. This is consistent with firmware reading SPD during memory training on resume, and it is the first direct observation of a second master on this engine. It is also the concrete justification for restoring command state per transfer and comparing it afterwards. Add tools/test-suspend.sh, which drives that test and reports the register delta across the sleep. Traffic goes through i2c-dev rather than a bound client: with ee1004 unable to attach and no TSOD present, there is no in-kernel client to use, and the PM path exercised is the same either way -- i2c_smbus_xfer() consults __i2c_check_suspended() before reaching the adapter.
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.
Upstream status
Patch series v2 is archived on lore.kernel.org and received no replies.
The next posting is [PATCH v4], 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.
Retracted: the CF8/CFC claim
Up to v3 this project 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.
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
-EAGAINif 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.
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
# 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.
DKMS
sudo apt-get install dkms
sudo dkms add .
sudo dkms install -m i2c-imc-skylake -v 1.0.0
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 advertises SMBus BYTE_DATA and WORD_DATA only, so i2cdetect cannot
scan the bus. Read a device directly — SPD byte 2 (DDR4 type code 0x0c) from
EEPROM 0x50 on channel 0:
sudo i2cget -y 6 0x50 0x02
The adapter does not instantiate ee1004 or jc42 clients. Check first that
the address is not already bound, then:
echo ee1004 0x50 | sudo tee /sys/bus/i2c/devices/i2c-6/new_device
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.
License
GPL v2 only (GPL-2.0-only).