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

[Rajaram R]

On Thu, Feb 18, 2016 at 4:17 PM, Heikki Krogerus
<heikki.krogerus@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> wrote:
>> > On Wed, Feb 17, 2016 at 03:36:46PM +0200, Felipe Balbi wrote:
>> >>
>> >> Hi,
>> >>
>> >> Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx> 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@xxxxxxxx> 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