[PATCH v2 0/2] i2c: Add i2c-shared-gpio driver

[Markus Stockhausen <markus.stockhausen@gmx.de>]

This series adds support for hardware designs where multiple I2C
gpio based busses are realized with dedicated SDA lines and a
shared SCL line. This way N busses can be realized with N+1 gpios.

Currently there are several Realtek switches that make use of 
this design. Samples are:

HPE 1920-48G
Linksys LGS310C
Zyxel GS1920-24
Engenius EWS2910
D-Link DGS-1250

While at first glance this might be a usecase for the new shared 
gpio architecture, discussion and testing shows that this is not 
trivial at all. A shared gpio is handled by a voting system. If 
there at least one user that votes for the GPIO to be "high", 
it stays high.

So the implementation manages several classic bitbang buses with
locking/unlocking in the pre_xfer/post_xfer handlers.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

---

v1 -> v2:
 - Adapted the dts so that all SDA/SCL gpios are defined in the 
   parent node. This way the i2c child nodes are kept clean. 
   Their <reg> property is used as an index into the parent SDA 
   list.
 - Adapted the driver to make use of the new dts structure.
 - Hardened the driver by adding multiple consistency checks
   during probing (number of busses, duplicate dts nodes, ...).
 - Added recovery handlers that are synchronized via the central
   mutex like normal bus transfers.
v1: https://lore.kernel.org/linux-i2c/20260507181711.2696783-1-markus.stockhausen@gmx.de/