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SubjectRe: [PATCH v17 2/3] usb: USB Type-C connector class
Hi,

On Tue, Apr 18, 2017 at 11:52:33AM -0700, Badhri Jagan Sridharan wrote:
> Hi Heikki,
>
> I have a question regarding the preferred_role node.
>
> +What: /sys/class/typec/<port>/preferred_role
> +Date: March 2017
> +Contact: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> +Description:
> + The user space can notify the driver about the preferred role.
> + It should be handled as enabling of Try.SRC or Try.SNK, as
> + defined in USB Type-C specification, in the port drivers. By
> + default the preferred role should come from the platform.
> +
> + Valid values: source, sink, none (to remove preference)
>
> What is the expected behavior when the userspace changes the
> preferred_role node when the port is in connected state ?
>
> 1. the state machine re-resolves the port roles right away based on
> the new state machine in place ? (or)

No! There are separate attributes for sending role swap requests.

The attribute will "enable" Try.SRC/SNK states, i.e. next time the
state machine is executed, those states need to be considered.
Changing the value of this attribute must not affect the current
connection.

> 2. Wait till the subsequent connect for resolving port roles based on the
> new state machine.

Yes.

> For #1 to happen the policy_engine layer would have to reset the port
> to resolve the port roles based on the (Try.SRC /Try.SNK/ Default)
> new state machine preference.
>
> Say for example when two non-PD devices following none (default state
> machine) are connected, the port role resolution is going to be random.
> But, if the userspace in one of the devices later changes the
> preferred_role to source, then that device is most likely to become source
> if the Try.SRC state-machine is re-run.
>
> Does the above question fall under a policy decision ? If so, should there
> be another node to say if the port roles have to re-resolved based on the
> new state machine right away ?

I don't think we should even consider option #1, but just to be sure,
Oliver, what do you say?

I guess we need to say in the documentation explicitly that changing
the value will not affect the current connection.


Thanks,

--
heikki

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