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

[Andi Shyti <andi.shyti@kernel.org>]

Hi Wolfram,

On Wed, Jul 15, 2026 at 10:36:45PM +0200, Wolfram Sang wrote:
> 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>

Thanks for testing this series!

> 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.

are you going to send a pull request at the merge window or can I
pull it? Either way works.

Thanks,
Andi