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.
199 lines
6.8 KiB
Markdown
199 lines
6.8 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)
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[](https://lore.kernel.org/linux-i2c/20260620144131.415559-1-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 series v2 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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The next posting is an RFC in **two patches**:
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1. a PCI/x86 change adding `pci_mmcfg_read_config()` / `pci_mmcfg_write_config()`,
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which force config access through the memory-mapped (ECAM) path;
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2. this driver, which uses them.
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The split exists because those are two decisions belonging to two different
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sets of maintainers. See [docs/submission/cover-letter.txt](docs/submission/cover-letter.txt).
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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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## Why the ECAM path
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The engine's registers are at config offsets `0x9C`-`0xB8`, inside the first
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256 bytes. On x86 that range is dispatched to `raw_pci_ops`, which is CF8/CFC
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whenever configuration type 1 is usable; the memory-mapped path is only used
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from `0x100` upwards. On the tested X299 board a dword written through CF8/CFC
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to the command register does not take effect: the register reads back
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unchanged, the GO bit is never consumed, no transaction is issued. The same
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write through ECAM works.
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What drops the write is **not known**. Earlier revisions of this project
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attributed it to System Management Mode; the only evidence was a boot log line
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naming the access method, which says nothing about who filters the write, so
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that claim has been removed. [tools/probe-cf8-write.sh](tools/probe-cf8-write.sh)
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is there to settle it with an `MSR_SMI_COUNT` measurement.
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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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* requires a positive sign that the engine accepted the command, because the
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done bits are latched and would otherwise let a never-executed command look
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like a completed one.
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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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The driver calls `pci_mmcfg_read_config()`, which is added by patch 1 of the
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upstream series and is not in any released kernel yet. When the target kernel
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lacks it, the build force-includes
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[compat/pci-mmcfg-compat.h](compat/pci-mmcfg-compat.h), which reimplements the
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two accessors out-of-tree so the driver source stays byte-identical to the
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submitted patch. The build prints a warning when the shim is in use. The shim
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is **not** part of the upstream submission.
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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 1.0.0
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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 advertises SMBus BYTE_DATA and WORD_DATA only, so `i2cdetect` cannot
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scan the bus. Read a device directly — SPD byte 2 (DDR4 type code `0x0c`) from
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EEPROM `0x50` on channel 0:
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```bash
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sudo i2cget -y 6 0x50 0x02
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```
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The adapter does not instantiate `ee1004` or `jc42` clients. Check first that
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the address is not already bound, then:
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```bash
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echo ee1004 0x50 | sudo tee /sys/bus/i2c/devices/i2c-6/new_device
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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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