linux-kernel - Re: [PATCHv3 1/2] usb: USB Type-C connector class

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Date:	Wed, 22 Jun 2016 14:44:58 +0300
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Oliver Neukum <oneukum@...e.com>
Cc:	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Guenter Roeck <linux@...ck-us.net>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCHv3 1/2] usb: USB Type-C connector class

On Wed, Jun 22, 2016 at 12:14:55PM +0200, Oliver Neukum wrote:
> On Wed, 2016-06-22 at 12:50 +0300, Heikki Krogerus wrote:
> > On Tue, Jun 21, 2016 at 10:25:05PM +0200, Oliver Neukum wrote:
> > > On Tue, 2016-06-21 at 17:51 +0300, Heikki Krogerus wrote:
> > > > +What:          /sys/class/typec/<port>/supported_data_roles
> > > > +Data:          June 2016
> > > > +Contact:       Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > > > +Description:
> > > > +               Lists the USB data roles, host or device, the port is
> > > > capable
> > > > +               of supporting.
> > > 
> > > On third thought, this is a problem. Looking at 4.4.8.1
> > > DEVICE_CAPABILITIES (Required) of USB Type-C Port Controller
> > > Interface Specification we lack capability.
> > > 
> > > A port that can do DRP is not the same thing as a port that
> > > can be switched between DFP and UFP. We cannot express that.
> > 
> > What do you mean? DRP means we support and are able to swap the data
> 
> No. That is the error. We support them concurrently. And that is not
> obvious. It is perfectly possible to support both but not concurrently.
> 
> > role, but it just does not mean we can act as both source and sink. And
> > that information we already get from separate attribute:
> > "supported_power_roles".
> 
> But it is different. Suppose we have a port that can be switched between
> UFP and DFP, as the spec defines. If it is switched to DFP and we plug
> in a DFP it will not work. UFP into UFP has the same result.
> 
> Plugging it into a DRP will always work.
> 
> It is true that both support host and device, but the capability of
> the ports is different. And that is not expressed.

Sorry but I don't think I understand?

So if we can act only as UFP, the supported_data_roles will list:

        device

If we can act only as DFP, the supported_data_roles will list:

        host

If our port is DRD (which would be DRP in the port controller spec),
the supported_power_roles will list:

        device, host

And the power role, if the port is Source only, the
supported_power_roles will list:

        source

If the port is Sink only, the supported_power_roles will list:

        sink

If our port is DRP, the supported_power_roles will list:

        source, sink

What is there that is missing? We are able to express all the types of
"Roles Supported" that the DEVICE_CAPABILITIES define, no?


Thanks,

-- 
heikki