linux-kernel - [PATCH RFC 0/7] I2C Mux per channel bus speed

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Message-Id: <20250922-i2c-mux-v1-0-28c94a610930@gmail.com>
Date: Mon, 22 Sep 2025 08:20:55 +0200
From: Marcus Folkesson <marcus.folkesson@...il.com>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>, 
 Peter Rosin <peda@...ntia.se>, 
 Michael Hennerich <michael.hennerich@...log.com>, 
 Bartosz Golaszewski <brgl@...ev.pl>, Andi Shyti <andi.shyti@...nel.org>
Cc: linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, 
 Marcus Folkesson <marcus.folkesson@...il.com>
Subject: [PATCH RFC 0/7] I2C Mux per channel bus speed

This is an RFC on how to implement a feature to have different bus
speeds on different channels with an I2C multiplexer/switch.

The benefit with this approach 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 #7.

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.

The implementation is split up into several patches:

Patch #1 Introduce a callback for the i2c controller to set bus speed
Patch #2 Introduce idle state to the mux core.
Patch #3 Introduce functionality to adjust bus speed depending on mux
         channel.
Patch #4 Set idle state for an example mux driver
Patch #5 Cleanup i2c-davinci driver a bit to prepare it for set_clk_freq
Parch #6 Implement set_clk_freq for the i2c-davinci driver
Parch #7 Update documentation with this feature

[1] https://elixir.bootlin.com/linux/v6.16.7/source/Documentation/i2c/i2c-topology.rst#L240
[2] https://elixir.bootlin.com/linux/v6.16.7/source/Documentation/i2c/i2c-topology.rst#L298
[3] https://elixir.bootlin.com/linux/v6.16.7/source/Documentation/i2c/i2c-topology.rst#L322

Signed-off-by: Marcus Folkesson <marcus.folkesson@...il.com>
---
Marcus Folkesson (7):
      i2c: core: add callback to change bus frequency
      i2c: mux: add idle_state property to i2c_mux_core
      i2c: mux: add support for per channel bus frequency
      i2c: mux: ltc4306: set correct idle_state in i2c_mux_core
      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  | 176 ++++++++++++++++++++++++++++++++++++
 drivers/i2c/busses/i2c-davinci.c    |  37 ++++++--
 drivers/i2c/i2c-mux.c               | 127 +++++++++++++++++++++++---
 drivers/i2c/muxes/i2c-mux-ltc4306.c |   3 +
 include/linux/i2c-mux.h             |  21 +++++
 include/linux/i2c.h                 |  13 +++
 6 files changed, 357 insertions(+), 20 deletions(-)
---
base-commit: 22f20375f5b71f30c0d6896583b93b6e4bba7279
change-id: 20250913-i2c-mux-b0063de2ae4d

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