linux-kernel - Re: [PATCH v17 2/3] usb: USB Type-C connector class

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Message-ID: <20170420115529.GA4769@kuha.fi.intel.com>
Date:   Thu, 20 Apr 2017 14:55:29 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Badhri Jagan Sridharan <badhri@...gle.com>
Cc:     Oliver Neukum <oneukum@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Mats Karrman <mats.dev.list@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        USB <linux-usb@...r.kernel.org>
Subject: Re: [PATCH v17 2/3] usb: USB Type-C connector class

Hi,

On Wed, Apr 19, 2017 at 07:45:00AM -0700, Badhri Jagan Sridharan wrote:
> >> 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.
> 
> Right. But, that might not be helpful in cases when PD is not implemented.
> and Implementing PD is not mandatory according the spec :/
> 
> FYI quoting from the Type-C specification release(page 24),
> role swaps are not limited to devices that only support PD.
> 
> "Two independent set of mechanisms are defined to allow a USB Type-C
> DRP to functionally swap power and data roles. When USB PD is
> supported, power and data role swapping is performed as a subsequent
> step following the initial connection process. For non-PD implementations,
> power/data role swapping can optionally be dealt with as part of the initial
> connection process."
> 
> But, the current interface definition actually prevents current/data role
> swaps for non-pd devices.

No it doesn't. When USB PD is not supported, you can still use
data_role to swap the role. Of course, the lower level driver may
not support any role swapping in that case.


Thanks,

-- 
heikki