On Sat, 25 Jul 2015, Robert Jarzmik wrote: > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes: > > Hi Alan, > > > On Sat, 25 Jul 2015, Robert Jarzmik wrote: > > > >> Petr Cvek <petr.cvek@xxxxxx> writes: > >> > >> > On 23.7.2015 21:46, Alan Stern wrote: > >> >>> It seems that it allows using a BULK endpoint for requested INT > >> >>> endpoint. For my PXA27x machine the original code returns BULK EP > >> >>> even with valid INT endpoint definition (because BULK EPs are defined > >> >>> earlier than INT EPs). > >> >> > >> >> Yes, it does allow a bulk endpoint to be used when an interrupt > >> >> endpoint was requested. However, it won't return a bulk endpoint if > >> >> all the bulk endpoints are already in use. > >> This cannot work for pxa27x. > > > > Do you mean that on pxa27x, a bulk endpoint cannot be used as an > > interrupt endpoint? Why not? From the device controller's point of > > view, there is no difference between bulk and interrupt (except > > possibly for the maxpacket sizes and high-bandwidth usage when running > > at high speed). > That's the point, maxpacket size and priority. > > As you said, it's not that it won't work, it won't work with the priority > expected by the software stack, ie. higher priority for ISO endpoint. As Robert Baldyga pointed out, this isn't relevant to a UDC driver. Only to a host controller driver. > >> The pxa27x IP has a hardware limitation which prevents an endpoint from changing > >> its type once the UDC is enabled (see the comment at the beginning of > >> pxa27x_udc.c). > >> > >> If that patchset implies that for a requested INT endpoint a BULK endpoint can > >> be returned, that won't work. Felipe and Robert, is that what this patchset > >> implies ? > > > > Sort of. The matching code has always behaved that way and this > > patchset does not change the behavior. > Then all is fine I suppose, if it was working before and nothing changes, it > will continue to work, won't it ? It should. > >> > A default PXA27x configuration returns BULK for requested INT. Which is > >> > unfortunate, because PXA27x supports INT endpoints and has one predefined, but > >> > this function find BULK first (one BULK is allocated and INT is never used). > >> See above. > > > > See response above. > > > > Besides, let's say the pxa27x has one bulk and one interrupt endpoint. > > Now suppose the gadget driver requests a bulk endpoint first. The > > matching code will allocate the single bulk endpoint. Then the gadget > > driver requests an interrupt endpoint. The matching code cannot > > allocate the bulk endpoint, because that endpoint is already allocated. > > So it will allocate the interrupt endpoint. > > > > Thus, as you can see, under the right conditions everything will work > > as desired. > > Let me give you another example : > - pxa27x_udc offers 3 endpoints : ep-in, ep-out, ep-iso-in > - a gadget driver does : > - request an ep-in > - request an ep-out > - request an ep-in > - request an ep-iso-in > In that case, the ep-iso-in request will fail, right ? Yet I would have expected > the second ep-in request to fail, as that's the one which cannot be serviced. In this example, the second ep-in request _will_ fail. An isochronous endpoint will not be allocated when the gadget driver requests a bulk endpoint. The bahavior that Petr didn't like was quite different: The matching code will sometimes allocate a bulk endpoint when the gadget driver requests an interrupt endpoint. > Of course, this hypothetical case implies that pxa27x_udc is not compatible with > this gadget driver, so it's not really relevant, is it ... > > >> > Because if they do, the ep_matches() function works poorly. It returns a BULK > >> > for device (gadget) side, but host side (PC) thinks that this endpoint is an INT > >> > and handles it in this way. But the PXA27x thinks the endpoint is a BULK and > >> > handles it in its way (according to datasheet, settings for a BULK and an INT > >> > transfers are not 100% compatible). > > > > How do they differ? > One example I have in mind is chapter 12.4.2 of pxa27x developer manual > "Endpoint Memory Configuration", quote follows : > If the USB host controller transmits more OUT data than the maximum > size packet for a bulk or interrupt endpoint, the UDC does not send > any handshake to the host controller causing the host controller to > time-out. If the USB host controller transmits more OUT data than the > maximum size packet for an isochronous endpoint, the UDC sets the data > packet error (DPE) bit in the Endpoint Control/Status register, > UDCCSRx[DPE]. That's a difference between isochronous and bulk/interrupt. We are talking about the difference between bulk and interrupt. Alan Stern -- 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
