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

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Message-ID: <1463981641.12181.5.camel@suse.com>
Date:	Mon, 23 May 2016 07:34:01 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Andy Shevchenko <andy.shevchenko@...il.com>,
	Rajaram R <rajaram.officemail@...il.com>,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Heikki Krogerus <heikki.krogerus@...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 Sun, 2016-05-22 at 08:54 -0700, Guenter Roeck wrote:
> Hi Oliver,
> 
> On 05/20/2016 11:43 PM, Oliver Neukum wrote:
> > On Fri, 2016-05-20 at 22:51 -0700, Guenter Roeck wrote:
> >> On 05/20/2016 06:37 AM, Oliver Neukum wrote:
> >>> On Fri, 2016-05-20 at 14:24 +0300, Heikki Krogerus wrote:
> >>>> On Thu, May 19, 2016 at 04:47:17PM +0200, Oliver Neukum wrote:
> >>>>>
> >>>>> Please explain. How does that express DRP but prefered master?
> >>>>
> >>>> Sorry but I'm not sure what you mean here. If the port is capable of
> >>>> being used as dual role port (DRP in the supported_data_roles file),
> >>>> that is the only case where you can select the role with this file. So
> >>>> I would imagine that in your case you want to make the port act as
> >>>> DFP only, right? But if the port is capable of acting only as UFP, you
> >>>> are stuck with that role.
> >>>
> >>> How do I trigger that Try.SRC is to be used on a port?
> >>>
> >>
> >> This would be part of the USB PD protocol, ie probably outside the scope
> >> of the class code. In my implementation, I enable Try.SRC or Try.SNK based
> >> on the platform's preferred role.
> >
> > Hi,
> >
> > from a purely formal point of view that makes sense.
> >
> >>From a usability viewpoint I'd ideally want all controls
> > for role at one place. And possibly the controls for modes
> > at the same place.
> >
> > I think part of the problem here is that we lack a statement
> > of mission. What are these controls for?
> > Merely for controlling modes and providing information
> > about modes? And to use neutral terms only for the "master"
> > side or both sides?
> >
> 
> Good question. I originally added a sysfs attribute 'preferred-mode' to
> my code, but then concluded that this is supposed to be provided
> by the platform and added it as platform data instead, with (currently)
> no means to report it to user space.

Mode or role? I would say that the choice of alternate modes belongs to
user space. THere's no point in giving a preference to the kernel.
Role is different. Try.SRC is part of the standard (and Try.SNK has been
added). It needs to be in kernel space.

Now, if you say that this API is for selecting modes only, then I say it
lacks support for cable resets and notifications thereof. Yes, that is a
PD feature, but that doesn't help us. It changes the mode.

> Heikki's current code doesn't really match the semantics of a 'preferred'
> mode, at least not as I read it. In my understanding, 'preferred mode'
> means "this is a DRP port which prefers to be a source/sink". In Heikki's
> code, one can fix the mode to source or sink, but that doesn't support
> situations such as "this port prefers to be a source, but is currently
> a sink because the link partner happens to be a charger or refuses to act
> as sink for some other reason".

Yes. That is why we need a mission statement. We may be talking
a rice cooker not preparing tea here.

> Given that, my working assumption is that preferred mode handling is supposed
> to be outside the scope of the infrastructure. I am happy to be corrected,
> though.
> 
> On a side note, are you working on a USB-PD implementation as well ?

No, one of my task is to to make use of USB PD (and alternate modes)
in a distribution.

	Regards
		Oliver