[PATCH v9 0/5] I2C Mux per channel bus speed

Marcus Folkesson
2026-03-24T13:54:40Z

This was a RFC on how to implement a feature to have different bus
speeds on different channels with an I2C multiplexer/switch.
As no major complaints on the design came up during the review, I
decided to submit the series without the RFC tag.

The benefit with this feature is that you may group devices after
the fastest bus speed they can handle.
A real-world example is that you could have e.g. a display running @400kHz
and a smart battery running @100kHz using the same I2C controller.

There are many corner cases where this may cause a problem for some
hardware topologies. I've tried to describe those I could think of
in the documentation, see Patch #5.

E.g. one risk is that if the mux driver does not disconnect channels
when Idle, this may cause a higher frequency to "leak" through to
devices that are supposed to run at lower bus speed.
This is not only a "problem" for changing bus speed but could also be
an issue for potential address conflicts.

This patchset has been used and tested heavily the last months
on a custom board based on a da850 (DaVinci) platform.

The implementation is split up into several patches:

Patch #1 Introduce a callback for the i2c controller to set bus speed
Patch #2 Introduce functionality to adjust bus speed depending on mux
channel.
Patch #3 Cleanup i2c-davinci driver a bit to prepare it for set_clk_freq
Parch #4 Implement set_clk_freq for the i2c-davinci driver
Parch #5 Update documentation with this feature

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
---
Changes in v9:
- Fix stray blank line
- Link to v8: https://lore.kernel.org/r/20260314-i2c-mux-v8-0-fb1738a4df0a@gmail.com

Changes in v8:
- Fix gramatics and change %d to %u were appropriate
- Link to v7: https://lore.kernel.org/r/20260223-i2c-mux-v7-0-ec75b214718a@gmail.com

Changes in v7:
- Remove code for finding first mux-locked ancestor
- Introduce a unlocked (i2c_adapter_set_clk_freq) and unlocked
(__i2c_adapter_set_clk_freq) variant
- Let the locking be handled in __i2c_adapter_set_clk_freq
- Use I2C_MAX_STANDARD_MODE_FREQ instead of magic numbers where
appropriate
- Link to v6: https://lore.kernel.org/r/20260216-i2c-mux-v6-0-9be28ecfd7e3@gmail.com

Changes in v6:
- Change logic to find which ancestor to lock with I2C_LOCK_ROOT_ADAPTER
It now find the first mux-locked ancestor and then lock its parent.

- Remove bus_freq_hz in i2c-davinci and only use clock_hz instead
- Mention in commit message that clock_hz can be used to store frequency in an uniform way

- Swap order for change freq/deselect to keep symmetry
- Only allow bus frequency to be lowered in select()
This to not allow an intermediate frequency to be set when it is not
supposed to

- check if(ret) instead of ret(<0) where appropriate
- Fix typos in documentation
- Change i2c_adapter.clock_hz from int to u32
- Simplify i2c_adapter_set_clk_freq() by removing 'ret'
- Link to v5: https://lore.kernel.org/r/20260213-i2c-mux-v5-0-fb2cbf9979b3@gmail.com

Changes in v5:
- Take the lock of the top-most mutex locked mux to make sure that the
root is locked
- Link to v4: https://lore.kernel.org/r/20260128-i2c-mux-v4-0-dee49ce276c0@gmail.com

Changes in v4:
- Rebase on master
- Swap order for printing warning about "channel %u is slower than
parent on a non parent-locked mux\n"
- Fix typo in comment, adaper->adapter
- Link to v3: https://lore.kernel.org/r/20251020-i2c-mux-v3-0-908ac5cf9223@gmail.com

Changes in v3:
- Return -EINVAL if channel is faster than parent (kernel test robot)
- Link to v2: https://lore.kernel.org/r/20251002-i2c-mux-v2-0-b698564cd956@gmail.com

Changes in v2:
- Changed bus_freq field to bus_freq_hz in davinci_i2c_dev (Bartosz Golaszewski)
- Removed idle_state from mux core (Peter Rosin)
- Link to v1: https://lore.kernel.org/r/20250922-i2c-mux-v1-0-28c94a610930@gmail.com

---
Marcus Folkesson (5):
i2c: core: add callback to change bus frequency
i2c: mux: add support for per channel bus frequency
i2c: davinci: calculate bus freq from Hz instead of kHz
i2c: davinci: add support for setting bus frequency
docs: i2c: i2c-topology: add section about bus speed

Documentation/i2c/i2c-topology.rst | 178 +++++++++++++++++++++++++++++++++++++
drivers/i2c/busses/i2c-davinci.c | 43 ++++++---
drivers/i2c/i2c-mux.c | 107 +++++++++++++++++++---
include/linux/i2c.h | 35 ++++++++
4 files changed, 339 insertions(+), 24 deletions(-)
---
base-commit: 1f97d9dcf53649c41c33227b345a36902cbb08ad
change-id: 20250913-i2c-mux-b0063de2ae4d

Best regards,
--
Marcus Folkesson <marcus.folkesson@gmail.com>