Simone
3cd1d6e0a6
The driver reached the PCU function's config space by walking the ACPI MCFG
table and mapping the ECAM page itself. That is one leaf driver deciding
config-space policy, and no other driver in mainline does it, so it was the
single most likely reason for a NAK.
Move that decision where it belongs: the upstream series is now two patches,
a PCI/x86 accessor that forces the memory-mapped path with the core's
locking, and this driver on top of it. Reviewers on each side now get
something they can act on, and get_maintainer.pl selects the PCI and x86
people by itself.
Correctness work in the same pass:
- the completion bits are latched, so a command the engine never accepted
was indistinguishable from a completed one and returned the previous
transaction's data as valid. Require a positive sign that the engine
acted on the command;
- compare the command registers after every transfer against what the
driver left in them and fail with -EAGAIN on a mismatch. The original
iMC driver had this check and it is what makes an unarbitrated system
detectable instead of silently wrong;
- use the i2c core's suspended-adapter mechanism rather than a private
flag and -EHOSTDOWN;
- use devm_mutex_init(), drop the unnecessary pci_enable_device(), and
return -ENODEV rather than -EPERM from the default-off gate, which was
printing a driver-core error on every boot;
- mask GO when restoring a saved command word.
Stop attributing the dropped CF8/CFC write to System Management Mode. The
only evidence was a boot log line naming the access method, which says
nothing about who filters the write. tools/probe-cf8-write.sh is there to
settle it with an MSR_SMI_COUNT measurement.
Add Documentation/i2c/busses/i2c-imc-skylake.rst, which every other PC SMBus
driver has and which was explicitly asked for on the 2020 iMC thread, the
tools/ scripts for the hardware measurements the submission still owes, and
compat/pci-mmcfg-compat.h so the driver still builds out-of-tree against a
kernel without the new accessor while the source stays byte-identical to the
submitted patch.
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 an RFC in two patches:
- a PCI/x86 change adding
pci_mmcfg_read_config()/pci_mmcfg_write_config(), which force config access through the memory-mapped (ECAM) path; - this driver, which uses them.
The split exists because those are two decisions belonging to two different sets of maintainers. 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.
Why the ECAM path
The engine's registers are at config offsets 0x9C-0xB8, inside the first
256 bytes. On x86 that range is dispatched to raw_pci_ops, which is CF8/CFC
whenever configuration type 1 is usable; the memory-mapped path is only used
from 0x100 upwards. On the tested X299 board a dword written through CF8/CFC
to the command register does not take effect: the register reads back
unchanged, the GO bit is never consumed, no transaction is issued. The same
write through ECAM works.
What drops the write is not known. Earlier revisions of this project
attributed it to System Management Mode; the only evidence was a boot log line
naming the access method, which says nothing about who filters the write, so
that claim has been removed. tools/probe-cf8-write.sh
is there to settle it with an MSR_SMI_COUNT measurement.
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; - requires a positive sign that the engine accepted the command, because the done bits are latched and would otherwise let a never-executed command look like a completed one.
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
The driver calls pci_mmcfg_read_config(), which is added by patch 1 of the
upstream series and is not in any released kernel yet. When the target kernel
lacks it, the build force-includes
compat/pci-mmcfg-compat.h, which reimplements the
two accessors out-of-tree so the driver source stays byte-identical to the
submitted patch. The build prints a warning when the shim is in use. The shim
is not part of the upstream submission.
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).