[PATCH v3 0/2] GPIO-based hotplug i2c bus

[Svyatoslav Ryhel]

ASUS Transformers require this driver for proper work with their dock.
Dock is controlled by EC and its presence is detected by a GPIO.

The Transformers have a connector that's used for USB, charging or
for attaching a keyboard (called a dock; it also has a battery and
a touchpad). This connector probably (I don't have the means to verify
that) has an I2C bus lines and a "detect" line (pulled low on the dock
side) among the pins. I guess there is either no additional chip or
a transparent bridge/buffer chip, but nothing that could be controlled
by software. For DT this setup could be modelled like an I2C gate or
a 2-port mux with enable joining two I2C buses (one "closer" to the
CPU as a parent).

In this case it's hard to tell the difference if this is real or virtual
hardware.

This patchset is a predecessor of a possible larger patchset which
should bring support for a asus-ec, an i2c mfd device programmed by
Asus for their Transformers tablet line. Similar approach is used in
Microsoft Surface RT for attachable Type Cover.

> What is this actually doing?
Basically it duplicates the parent i2c bus once detection GPIO triggers
and probes all hot-pluggable devices which are connected to it. Once
GPIO triggers a detach signal all hot-pluggable devices are unprobed and
bus removed.

> Is the GPIO an irq line for signalling hoplugging and can be used by
> any driver or just this one?
It can be shared if necessary but usually all hot-pluggable devices
are gathered in one container and are plugged simultaneously.

---
Changes from v2:
- expanded descryption of driver implementation commit
- expanded descryption in patchset cover
- no changes to code or yaml from v2

Changes from v1:
- documentation changes:
  - dropped | from description
  - dropped nodename
  - unified use of quotes
  - used GPIO_ACTIVE_LOW define
  - used phandle instead of path
---

Michał Mirosław (1):
  i2c: Add GPIO-based hotplug gate

Svyatoslav Ryhel (1):
  dt-bindings: i2c: add binding for i2c-hotplug-gpio

 .../bindings/i2c/i2c-hotplug-gpio.yaml        |  65 +++++
 drivers/i2c/Kconfig                           |  11 +
 drivers/i2c/Makefile                          |   1 +
 drivers/i2c/i2c-hotplug-gpio.c                | 266 ++++++++++++++++++
 4 files changed, 343 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/i2c/i2c-hotplug-gpio.yaml
 create mode 100644 drivers/i2c/i2c-hotplug-gpio.c

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2.39.2