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    SubjectRe: [PATCH 2/3] usb: type-c: USB Type-C Connector System Software Interface
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    On Thu, Feb 18, 2016 at 4:17 PM, Heikki Krogerus
    <heikki.krogerus@linux.intel.com> wrote:
    > 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@linux.intel.com> wrote:
    >> > On Wed, Feb 17, 2016 at 03:36:46PM +0200, Felipe Balbi wrote:
    >> >>
    >> >> Hi,
    >> >>
    >> >> Heikki Krogerus <heikki.krogerus@linux.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@suse.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

    Yes, not in Spec 1.1 but a new term to differentiate data and power
    roles . All I wanted to bring in some difference between data role
    swap and DRP

    > 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)"

    Ok I get it. Here in a user perspective its a connect and disconnect
    and things are random which user may not prefer.

    >
    >
    > Thanks,
    >
    > --
    > heikki

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