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 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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
