Re: USB role switches, usb-connector, typec and device trees

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On 06.06.2018 23:36, Mats Karrman wrote:
> Hello Gentlemen,
>
> I'm trying to get my head around USB role switches in connection with Type-C ports
> and device-trees. So far I have not found much documentation, e.g. there are no
> device-tree bindings documented and really no good examples in existing device
> trees, although there has been some attempts, e.g. [1] and [2]. Anyway, so I send
> you a couple of questions instead:
>
> 1) tcpm uses the port device struct to find a single usb_role_switch but there is
> room for three USB busses in the Type-C connector; one high speed and two (?) super-
> speed. These would not all come from the same controller (there might even be
> separate controllers for host and device mode for each bus).
> The case I am working on now only have a single USB2 otg controller so it should
> be possible to make that driver register a role switch but for other cases?
> I imagine it would be possible to create a composite driver as a proxy for all role
> switches but that would probably be different for every platform/product - not
> very elegant. Could the role switch infrastructure be extended to handle arbitrary
> sets of coordinated switches?
>
> 2) How should the connection between the Type-C port and the switches best be
> expressed in a device tree? Using graph I presume, but should it be mixed into the
> existing "usb-connector" or should this be a separate block?
> I think it is unfortunate that the graph use numeric addresses that need to be
> fixed by documentation 

I also do not like port numbers, I even have argued for using names
during multiple discussions, but the discussion ended as is :(

> and already I see problems with the current assignment
> (0=HS, 1=SS, 2=SBU), e.g. if the host and device mode are handled by different
> controllers. Graph do support multiple endpoints for one port but then we have
> another level of magic numbers which does not exactly make things easier
> (e.g. 0=dual or host controller, 1=device controller, 2=mode switch).

Where do you want to use these magic numbers? To describe endpoints? I
guess there should be controllable mux/switch behind the port, it could
be standalone, or a part of bigger IP.
Anyway it should be described by device node, probably parent of USB-C
device-node, in such case only links from USB connector going to
different IPs should be described using OF graphs.
And in such case drivers of these devices should
ask/determine/change/communicate their role using in-kernel frameworks
(for example extcon).

There are many different configurations of USB-C ports
InterfaceControllers, Muxes/Switches, USB related devices, Alternate
Mode devices, PMICs, USB-PDs, ....
Could you describe particular one you have problem with, to make the
discussion more specific.

Regards
Andrzej

>
> What are your thoughts on this? Please tell me I missed something and that there is
> a simple solution :-)






>
> BR // Mats
>
> [1] https://www.spinics.net/lists/linux-usb/msg168071.html
> [2] https://www.spinics.net/lists/linux-usb/msg168072.html
>
>
>
>

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