linux-kernel - Re: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software Interface

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Message-ID: <20160218104742.GD1859@kuha.fi.intel.com>
Date:	Thu, 18 Feb 2016 12:47:42 +0200
From:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:	Rajaram R <rajaram.officemail@...il.com>
Cc:	Felipe Balbi <balbi@...nel.org>, Oliver Neukum <oneukum@...e.com>,
	Felipe Balbi <balbif@...il.com>,
	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software
 Interface

On Thu, Feb 18, 2016 at 04:07:54PM +0530, Rajaram R wrote:
> On Wed, Feb 17, 2016 at 7:58 PM, Heikki Krogerus
> <heikki.krogerus@...ux.intel.com> wrote:
> > On Wed, Feb 17, 2016 at 03:36:46PM +0200, Felipe Balbi wrote:
> >>
> >> Hi,
> >>
> >> Heikki Krogerus <heikki.krogerus@...ux.intel.com> writes:
> >> > On Wed, Feb 17, 2016 at 11:36:52AM +0100, Oliver Neukum wrote:
> >> >> On Wed, 2016-02-17 at 12:29 +0200, Felipe Balbi wrote:
> >> >> > Hi,
> >> >> >
> >> >> > Oliver Neukum <oneukum@...e.com> writes:
> >> >> > > On Wed, 2016-02-17 at 09:58 +0200, Heikki Krogerus wrote:
> >> >> > >> On Tue, Feb 16, 2016 at 02:39:47PM +0100, Oliver Neukum wrote:
> >> >>
> >> >> > >> > Yes, but we need an API. We can't keep adding to it. So if that
> >> >> > >> > is to be supported, it needs to be defined now.
> >> >> > >>
> >> >> > >> When you say API, do you mean the API the class provides to the
> >> >> > >> drivers? Or did you mean ABI which would be the sysfs in this case?
> >> >> > >
> >> >> > > The API to user space. That is the point. We cannot break user space.
> >> >> > > Once this sysfs API is upstream we are stuck with it.
> >> >> >
> >> >> > yeah, in fact I have been wondering if sysfs is the best interface to
> >> >>
> >> >> That is the discussion we must have.
> >> >>
> >> >> > userspace. I talked with Heikki a few days back about this; I was
> >> >> > wondering if something like what the NFC folks did with netlink would be
> >> >> > better here.
> >> >>
> >> >> I doubt that, because the main user is likely to be udev scripts.
> >> >> They can easily deal with sysfs attributes.
> >> >
> >> > IMHO for high level interface like this, sysfs is ideal because of the
> >> > simple fact that you only need a shell to access the files. netlink
> >> > would make us depend on custom software, no?
> >> >
> >> > I'm not against using netlink, but what would be the benefit from it
> >> > in this case?
> >>
> >> With HW we see nowadays, CC stack is hidden on some microcontroller, but
> >> is it too far-fetched to consider a system where this is not the case ?
> >
> > There already are several USB PD stacks out there, like also Greg
> > pointed out.
> >
> >> Specially when we consider things like power delivery which, I know, you
> >> wanted to keep it out of this interface, however we would have two
> >> 'stacks' competing for access to the same pins, right ?
> >
> > No. This class would be the top layer for the coming stack, where ever
> > it ends up coming. The class is only the interface to the user space
> > and nothing else.
> >
> > By saying we need to keep USB Type-C separate from USB PD I meant that
> > the userspace access can not be mixed somewhere in layers of the USB
> > PD/CC stack like it has been in the USB PD stacks I've seen so far.
> > They assume that we always use the software USB PD stack with USB
> > Type-C, which as we can see is not true when the stack is implemented
> > in EC or firmware or some complex USB PD controller or what ever.
> > However, the operations the userspace needs to do are exactly the same
> > in both cases.
> >
> > - data role swapping
> > - power role swapping (depends on USB PD)
> > - Alternate Modes (depends on USB PD)
> >
> > And we really should not forget that we actually also have USB Type-C
> > PHYs that can't do any USB PD communication over the CC pin, so USB PD
> > is simply not always going to be available. But the data role swapping
> > and also accessories are still available with them, as the do not need
> > USB PD.
> >
> > This was the whole point with the class. It allows the different ways
> > of dealing with Type-C ports to be exposed to userspace in the same
> > way.
> >
> >> IIRC mode and role negotiation goes via CC pins using the power delivery
> >> protocol. If I misunderstand anything, let me know.
> >
> > The data role swap with USB Type-C connectors is in no way tied to USB
> > Power Delivery. The USB Type-C spec defines that when USB PD is
> 
> Its not data role swap i guess its dual role, A Data role swap is tied
> with USB PD,
> 
> > available, DR_Swap USB PD function is used to swap the role, otherwise
> > emulated disconnect will do the trick.
> 
> I doubt a USB host with no device capability implement DRP ?? Also
> emulated trick(??) is not spec requirement rt ?
> 
> >
> > Data role swapping is a must thing to have with USB Type-C connectors
> 
> I guess you are referring to Dual role (DRP) and not data role (DRD).

There is no term "DRD" in USB Type-C spec. A quote from Type-C spec
ch. 2.3.3:

"Two methods are defined to allow a USB Type-C DRP to functionally
swap data roles, one managed using USB PD DR_Swap and the other
emulating a disconnect/reconnect sequence (see Figure 4-16)"


Thanks,

-- 
heikki