linux-kernel - Re: [PATCH v3 0/5] I2C Mux per channel bus speed

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Message-ID: <aRGrIRNgNlRqDLzr@gmail.com>
Date: Mon, 10 Nov 2025 10:06:41 +0100
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,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3 0/5] I2C Mux per channel bus speed

Hi all,

On Mon, Oct 20, 2025 at 09:17:23AM +0200, Marcus Folkesson wrote:
> 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.
> 
> 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@...il.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 | 176 +++++++++++++++++++++++++++++++++++++
>  drivers/i2c/busses/i2c-davinci.c   |  41 ++++++---
>  drivers/i2c/i2c-mux.c              | 116 +++++++++++++++++++++---
>  include/linux/i2c.h                |  13 +++
>  4 files changed, 324 insertions(+), 22 deletions(-)
> ---
> base-commit: 22f20375f5b71f30c0d6896583b93b6e4bba7279
> change-id: 20250913-i2c-mux-b0063de2ae4d
> 
> Best regards,
> -- 

A friendly ping on this series.

Thank you in advance,
Marcus Folkesson


> Marcus Folkesson <marcus.folkesson@...il.com>
> 

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