Re: [PATCH v4] i2c: i2c-gpio: Enhance driver for buses with shared SCL

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

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On Thu, Jul 09, 2026 at 08:25:33AM +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 GPIOs that are used by multiple I2C buses in the dts by
>   using a unique identifier for each managed SCL.
> 
> - The first probing instance allocates and requests the shared SCL
>   GPIO plus 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 patch was successfully tested on Linksys LGS310C that has two
> SFP slots with two GPIO based I2C buses that share a sinlge 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>

Well, what I saw looks good to me. I have to admit that I don't have the
time to dive into the corner cases of its complexity. So, I wonder if
this patch should be resend with lkml in CC, so Sashiko can have a look
at it, too? This is optional, though, if Bartosz is also happy, we can
surely try it as is.

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

Oh, there is a checkpatch warning:

WARNING: please, no spaces at the start of a line
#225: FILE: drivers/i2c/busses/i2c-gpio.c:440:
+    return a->fw_node == b->fw_node && a->fw_pin == b->fw_pin && a->fw_flags == b->fw_flags;$


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