[PATCH RFC] Specifying default-disabled devices

[Jeremy Kerr]

Hi Zev,

> Sorry, which semantics exactly do you mean we might not want to
> change? It sounded like Oliver thought that interpretation of
> "reserved" should be viable, modulo some possible bus-specific
> caveats...

At the moment (as you've noticed), status = "reserved" does not
instantiate the device. For what you're proposing here, we'd need to
change that: "reserved" would instantiate the device, but suppress the
probe. I'm not sure what might break if were were to make that change.

> Well, I'm aiming to be able to use a dts fragment looking something
> like (on an ast2500):
> 
>   &spi1 {
>         status = "reserved";
>         pinctrl-names = "default";
>         pinctrl-0 = <&pinctrl_spi1_default>;
>         flash at 0 {
>                 status = "okay";
>                 label = "bios";
>                 m25p,fast-read;
>         };
>   };

[do you want just the flash node to be reserved, or the entire
controller? I assume the controller is always available...]

> ...but I'm wondering about your mention of "rather than instantiating
> entirely from userspace" -- is there some mechanism for
> runtime-materializing a device ex nihilo that I've remained
> (embarrassingly) unaware of?

It depends on the bus; we can instantiate (and bind) i2c devices with
something like:

 # echo tmp75 0x50 > /sys/bus/i2c/devices/i2c-7/new_device
         ^    ^
	 |    i2c addr
	 |
	 i2c device id

- which requires no DT node at all.

But on a quick check, it looks like there's no equivalent facility for
SPI (which makes sense, as there's likely to be additional platform data
required for most devices..)

Cheers,


Jeremy