[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[Russ Dill]

On Tue, Nov 6, 2012 at 10:35 AM, Tony Lindgren <tony at atomide.com> wrote:
> * Grant Likely <grant.likely at secretlab.ca> [121106 03:16]:
>> On Tue, Nov 6, 2012 at 10:30 AM, Pantelis Antoniou
>> <panto at antoniou-consulting.com> wrote:
>> >
>> > Another can of worms is the pinctrl nodes.
>>
>> Yes... new pinctrl data would need to trigger adding new data to
>> pinctrl. I don't know if the pinctrl api supports that.
>
> The actual pins stay the same, just their configuration
> changes. AFAIK all that is already supported using the
> pinctrl framework.
>
> For example, considering hotplugging capes on the beaglebone:
>
> 1. You need to map all the sensible modes for the pins exposed
>    to the capes in the board specific .dts file. This will
>    add roughly 4 x nr_capbus_pins named modes in the .dts file
>    so not too bad.
>
> 2. Claim all the capebus pins during the capbus driver probe
>    and set them to some safe mode.
>
> 3. Try to detect the connected cape(s) over i2c.
>
> 4. Use pinctr_select_state to set the desired modes for
>    the pins used by the cape(s).
>
> 5. Enable capebus regulators and clocks etc.
>
> 6. Load the driver modules for whatever omap internal
>    devices the cape supports.
>
> You could also claim the pin for the omap internal
> devices instead of claiming them in the capebus, but then
> things can get messy with binding and unbinding the
> drivers. So just claiming all the pins in the capebus
> probably keeps things simpler.

That assumes that for a particular external bus, certain pins aren't
already shared with functions already on the board, for instance if an
I²C bus brought out to the external bus already has a chip connected
to it.