linux-kernel - Re: [PATCHv12 2/3] usb: USB Type-C connector class

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Message-ID: <20161128142323.GA32668@kuha.fi.intel.com>
Date:   Mon, 28 Nov 2016 16:23:23 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Badhri Jagan Sridharan <badhri@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCHv12 2/3] usb: USB Type-C connector class

On Mon, Nov 28, 2016 at 11:19:32AM +0100, Oliver Neukum wrote:
> On Thu, 2016-11-24 at 11:57 +0200, Heikki Krogerus wrote:
> > On Wed, Nov 23, 2016 at 09:12:04PM -0800, Guenter Roeck wrote:
> 
> > > In our implementation, the default preferred role is determined by the
> > > low level driver (as, in my understanding, is suggested by the standard).
> > > This means that the ABI will report "no preferred role", unless user space
> > > overwrites it, even though there _is_ in fact a preferred role, and the
> > > low level driver will execute try.src or try.snk based on that role.
> > 
> > I'm not sure which standard are you referring? Try.SNK and Try.SRC are
> > optional mechanisms for *policy-based* role preference according to
> > the USB Type-C spec. The policy really should always come from the
> 
> Not all that obvious. If you are looking at it from a distro view
> point if you know that you are booting on basically a gadget, you'll
> be happy to take the hint. And if the hardware knows it is better
> as a sink or source, we should take the hint.
> 
> > user space in our case, but I don't think that rules out for example
> > initial role preferences coming from the lower level drivers.
> 
> Indeed. That should not be a hindrance to submission and inclusion.
> 
> > We will need a way the OS can set the initial preference for every
> > port. Note that once we can support that, what ever the lower level
> > drivers request will be overridden by it. So if for example the
> > platform has preference for an initial role, we will simply ignore it
> > if the policy says otherwise.
> 
> Again, not obvious in a distro. I would actually prefer a module
> parameter that would allow us to prefer try.src, as we know how
> to be a master.

I would be happy with module parameter.

> None of that should hinder submission and inclusion.

It's already there in v13.

I better ping Greg already after rc1 this time, just so we don't start
the review again when we are already at rc5 (and I have forgotten
about this whole thing). I'll ask about the module parameter idea
then, though I'm guessing he won't like it. But maybe he has some
suggestions.


Thanks Oliver,

-- 
heikki