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 -- 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
