[PATCH 0/2] i2c: add GPIO buses sharing one SCL line

[Ross Cawston <ross@r-sc.ca>]

Some Realtek RTL930x switches bit-bang several I2C buses on SoC GPIOs
with a single SCL line and one SDA line per bus. The MokerLink
POE-10G07110GSM, for example, has its fan controller, its PoE
controllers and its SFP+ cage on SDA GPIOs 9, 10 and 11, all clocked
by GPIO 8. i2c-gpio cannot describe this: every bus would request the
same SCL GPIO.

This series adds a binding and a small driver that requests SCL once
and registers one i2c-algo-bit adapter per SDA line, serializing the
transfers so that only one bus clocks SCL at a time.

The SoC's own I2C master (i2c-rtl9300) is wired to the same pins, but
it moves at most 16 bytes per transfer, and the PoE controllers on this
board only accept their 25 KiB firmware as one continuous write: sent
in 16-byte writes, they NAK the second one. Tested on the MokerLink
POE-10G07110GSM.

Ross Cawston (2):
  dt-bindings: i2c: add GPIO buses sharing one SCL line
  i2c: add driver for GPIO buses sharing one SCL line

 .../bindings/i2c/i2c-gpio-shared-scl.yaml     |  94 +++++++
 MAINTAINERS                                   |   7 +
 drivers/i2c/busses/Kconfig                    |  12 +
 drivers/i2c/busses/Makefile                   |   1 +
 drivers/i2c/busses/i2c-gpio-shared-scl.c      | 229 ++++++++++++++++++
 5 files changed, 343 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/i2c/i2c-gpio-shared-scl.yaml
 create mode 100644 drivers/i2c/busses/i2c-gpio-shared-scl.c

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2.55.0.windows.5