Support peripheral deallocation at runtime

[gon1332]

Is your feature request related to a problem? Please describe.
Until now, Zephyr allows enabling of the needed peripherals at compile time via Kconfig infrastructure. This is done to reduce code size. In this way the firmware expects that the needed peripheral devices are connected. If the devices are not connected, e.g. a disconnected shield, the firmware can handle this by omitting reading from this port.

The downside to this one is increased power consumption. The GPIO pins have already been initialized in a mode that expects active connection (UART, I2C). These pins are floating and their state is not power efficient. Also clocks for the peripherals have been enabled, so also these should be disabled.

Describe the solution you'd like
I would like to have a driver API like some_peripheral_deinit. This will for sure disable the peripheral's clock. Either called by the user after a failed call to device_get_binding, or automatically in the driver's init(). The last case could be mapped to a Kconfig symbol.

The above is easy for the peripheral's clocks. For the GPIOs needed to be disabled some extra DTS fixups should be added for each peripheral to make them accessible.

Describe alternatives you've considered
Currently I'm doing the whole procedure either at application level, after checking a device_get_binding(). I've also done some of the solution previously described in a private clone.

Additional context
The use case I want to achieve is to have a dynamic firmware that on board power checks ports for known to-be-connected devices (UART, I2C). If there isn't a connected device then disables the port to reduce power consumption. For this kind of application I don't care for code size. I enable a big number of supported peripherals and then sense which are connected.

[tbursztyka]

There is no need to add anything to the device API. This is a PM issue.

However, I would not say it is possible to do it with the PM API at the moment. It's far from being usable (this is a known issue).

[StefJar]

I am agreeing on @gon1332 view.

Like to give some examples where at least a deinit function is quite useful.

1.) sleep modes: To disable the peripherals to save some energy
2.) error states/partial failure of the pcb: After production and delivery the device/pcb gets broken. We can detect the error state and would like to put the device into save operation mode
3.) production/self test: the pcb gets assembled, we have some self test/production state for the firmware. For these tests we like to reconfigure the devices or disable the devices

@tbursztyka even in the linux kernel you can unload the kernel modules/disable them.

I think on the long run this adds well to the Zephyr API.

[pabigot]

API 2020-07-07: Closing as duplicate of #19448 which covers specifying which devices are to be present but not started, and ties to the ability turn devices off (for power management). Please add to that issue any requirements from this that are not implicitly or explicitly addressed.

[carlescufi]

Reopening this issue since this has not been addressed.