linux-kernel - Re: [RFC PATCHv2] usb: USB Type-C Connector Class

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Date:	Wed, 1 Jun 2016 16:29:26 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Oliver Neukum <oneukum@...e.com>
Cc:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Rajaram R <rajaram.officemail@...il.com>,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [RFC PATCHv2] usb: USB Type-C Connector Class

On Wed, Jun 01, 2016 at 11:26:09AM +0200, Oliver Neukum wrote:
> On Thu, 2016-05-19 at 15:44 +0300, Heikki Krogerus wrote:
> > Just noticed that the "active" file is for now read only, but it needs
> > to be changed to writable. That file will of course provide means for
> > the userspace to Exit and Enter modes. But please note that the
> > responsibility of the dependencies between the modes, say, if a plug
> > needs to be in one mode or the other in order for the partner to enter
> > some specific mode, will fall on the Alternate Mode specific drivers
> > once we have the altmode bus. I remember there were concerns about
> > this in the original thread.
> 
> There's one thing we haven't touched upon yet. And I cannot really find
> an answer in the spec.
> 
> What do we do if we return from S4 or S3? I think we need to restore
> the ALternate Mode because our display may be running over that
> Alternate Mode.
> If we want to support USB persist we also need to restore data role
> after S4.
> 
I don't have an answer ... but another interesting question.

How do we distinguish between alternate modes supported by a host vs.
alternate modes supported by a sink ? typec_capability includes a pointer
to alternate modes supportedf by the connector, but it is not clear if
those are alternate modes supported as host, or alternate modes supported
as device, or alternate modes supported by both.

This doesn't matter much if only a fixed role is supported, but it does matter
for dual role ports. A laptop will typically only support DisplayPort as host,
for example.

Any idea ?

Thanks,
Guenter