Re: [PATCH v7 2/2] i2c: i2c-gpio: Enhance driver for buses with shared SCL

[Wolfram Sang <wsa+renesas@sang-engineering.com>]

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On Tue, Jul 14, 2026 at 06:29:15PM +0200, Markus Stockhausen wrote:
> Some lower end hardware (especially Realtek based switches) are
> designed with multiple I2C buses that share a single clock line.
> E.g. the D-Link DGS-1250-28X realizes 4 I2C SFP busses with 5 GPIOs.
> 
> Enhance the i2c-gpio driver so it can handle such hardware designs.
> 
> - Detect shared SCL GPIOs that are used by multiple I2C buses in the
>   devicetree by using a "unique identifier". This is basically the
>   phandle and all additional cells.
> 
> - The first probing instance allocates and requests the shared SCL
>   GPIO with an associated rt_mutex. Subsequent instances detect the
>   existing entry via the identifier and increment a reference count
>   to reuse the descriptor.
> 
> - All data transfers are serialized via custom lock_ops that handle
>   both the standard adapter bus lock and the shared SCL mutex. This
>   ensures mutual exclusion across adapters sharing the clock line.
> 
> - This shared SCL detection works only for dts based systems where
>   the GPIO node has at least one cell (usually the pin). GPIOs in
>   legacy systems without devicetree will be handled individudally
>   as before.
> 
> This patch was successfully tested on Linksys LGS310C that has two
> SFP slots with two GPIO based I2C buses that share a single SCL.
> Test environment: OpenWrt snapshot ported to kernel 6.19.14
> including CONFIG_GPIO_SHARED=y and CONFIG_GPIO_SHARED_PROXY=y.
> 
> Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
> Tested-by: Sander Vanheule <sander@svanheule.net>
> Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
> Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Cool stuff! It took a bit of handling gory details, but I could not only
check that there are no regression with non-SCL-shared busses. I could
also try two busses with shared SCL and it worked like a charm.
Congrats!

Andi, from my side, you can pick up these two patches. I will put them
also in my 'for-andi' branch to increase their visibility.


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