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

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Message-ID: <20160602101853.GC25305@kuha.fi.intel.com>
Date:	Thu, 2 Jun 2016 13:18:53 +0300
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Oliver Neukum <oneukum@...e.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 04:29:26PM -0700, Guenter Roeck wrote:
> 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.

The DP alternate mode spec actually separates the display role from
Type-C role. A laptop most likely would only support the modes for
display host roles, but if the port was DRP port then it would still
do so in both Type-C roles.

So basically, even if the display was Type-C host, it would still work
as a display when attached to the laptop.

> Any idea ?

I'm actually not sure this is a problem.


Thanks,

-- 
heikki