[PATCH v3 0/2] mux: gpio: Add optional enable gpio

[Tapio Reijonen]

Analog multiplexers have an enable input that puts all channels in the
high-impedance off state independent of the address inputs - the E input
of a 74HC4051, for example. gpio-mux has no way to describe it, so it can
neither reach the idle state of disconnect that the mux-controller
binding defines, nor keep the mux disabled while the address gpios are
updated one gpio controller at a time.

Add enable-gpios for that input, and use it in the driver.

Without it the driver rejects MUX_IDLE_DISCONNECT outright.
mux-controller.yaml allows it - idle-state is an int32 with a minimum of
-2 - but mux_gpio_probe() treats every negative value other than
MUX_IDLE_AS_IS as invalid, so a device tree asking for the disconnect
idle state does not probe at all. On the board below that takes out the
i2c-mux behind the mux controller, both SFP i2c buses and both sfp
devices with it:

  gpio-mux muxes:mux-controller-slot@0: invalid idle-state 4294967294
  gpio-mux muxes:mux-controller-slot@0: probe with driver gpio-mux failed with error -22

4294967294 is MUX_IDLE_DISCONNECT, -2, printed through %u.

Tested on an i.MX6SX board where two SFP cages share one SMBus behind
DG9411 analog switches: the address input selects the cage and the
enable input gates the bus, both driven by a PCA9501 I2C gpio expander.
With enable-gpios and an idle state of disconnect, the parent bus scans
empty between transfers while each cage stays reachable through its own
child bus. A logic analyser on the enable, address and bus lines over a
full boot shows 97 enable windows and 82 address changes, with no bus
activity outside a window and no address change inside one. Dropping
enable-gpios while keeping the disconnect idle state makes the driver
refuse to probe, as intended.

Signed-off-by: Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>
---
Changes in v3:
- Address Peter Rosin's review of 2/2. Cosmetic, except for the last
  item below.
- Say "Some analog multiplexers", not all of them, and note that the mux
  gpios are not guaranteed to be updated atomically and that only
  controllers implementing set_multiple() can possibly update theirs in
  a single write.
- Drop the _gpio suffix from the struct member: enable_gpio -> enable.
- Reflow the comment in mux_gpio_set() and soften it to "might not be
  updated atomically".
- Flatten the idle-state handling in probe, read the property into the
  existing default when it is absent, and assign mux->idle_state once at
  the end. Use dev_err() and return -EINVAL rather than dev_err_probe(),
  as suggested. Written without the empty if branches of the suggested
  snippet, but with the same behaviour.
- Beyond the review: return the status of the gpio writes from
  mux_gpio_set() instead of zero. The other mux drivers pass the write
  status back, and mux_control_set() only invalidates the cached state
  when set() reports an error, so a failed write reported as success is
  not retried. Happy to drop this hunk if it should be a separate patch.
- Link to v2: https://lore.kernel.org/r/20260831-add-external-mux-enable-gpio-v2-0-f6027a4afe61@xxxxxxxxxxx

Changes in v2:
- Describe the enable input in terms of real multiplexers (the E input of
  a 74HC4051) instead of repeating the binding text, and say what needs
  it, as requested by Krzysztof.
- Add the missing gpio-mux: prefix to the binding patch subject.
- Limit enable-gpios to one entry, and require it for an idle state of
  disconnect in both the schema and the driver: without an enable gpio
  mux_gpio_set() drove the address gpios to the bit pattern of
  MUX_IDLE_DISCONNECT instead of disconnecting.
- Deassert the enable gpio while the mux gpios are updated, not only for
  the disconnect idle state, since they are not updated atomically.
- Show enable-gpios in the binding example.
- Link to v1: https://lore.kernel.org/r/20251105-add-external-mux-enable-gpio-v1-0-e59cba6f9e47@xxxxxxxxxxx

---
Tapio Reijonen (2):
      dt-bindings: mux: gpio-mux: Add enable-gpios
      mux: gpio: Add optional enable gpio

 .../devicetree/bindings/mux/gpio-mux.yaml          | 22 +++++++++++
 drivers/mux/gpio.c                                 | 43 ++++++++++++++++++----
 2 files changed, 57 insertions(+), 8 deletions(-)
---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20250925-add-external-mux-enable-gpio-c9ba5beddd49

Best regards,
-- 
Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>