On Sun, Aug 24, 2014 at 7:26 PM, Daniel Mack <daniel@xxxxxxxxxx> wrote: > Hi Jassi, > > On 08/19/2014 11:52 AM, Jassi Brar wrote: >> Its been quite some time now, but I think we designed the uac2 to rely >> on USB's ISO packets' rate control to send and receive audio data at >> announced sampling rate. > > I'm still thinking how that setup should have ever been possible with > snd-usb-audio on the host side. > > snd-usb-audio prepares its capture USB ISO frames with a pre-calculated > value of how many bytes per packet to expect on the capture endpoint. > That value is derived from the currently configured sample rate, and if > the sound card fills each of its buffers with that number of bytes or > less, everything's fine. Also, each urb is resubmitted immediately after > reception, and there is no delay or timing or anything. > Doesn't .bInterval, set to get one ISO packet per millisec, play any role? > f_uac2, however, currently always completes its buffers with 512 bytes > packets, Current uac2 uses the max packset size (which may not be 512 for every udc) for the EP. Probably it (wMaxPacketSize) should be set to bytes/ms for the sampling rate. > which causes two problems: a) it leads to -EOVERFLOW errors on > the host side, as the host doesn't expect such big packets, and b) audio > is transported as fast as possible, and nothing ties the actual rate to > any clock on either side. In my tests, audio was transported roughly at > 3x the actual sample rate. While this works fine if only files are in > the game on both sides > f_uac2 is a virtual sound card that can not play/record sound to/from a physical codec ... it's basically just a way to capture audio data in digital form ;) so 'files' was assumed to be the primary way of using it. Now for USB-IN data flow control, maybe we should play with bInterval, wMaxPacketSize and an hrtimer for some sampling rates. > Which OS did you test with on the host side, and what type of gadget > hardware was in use? I'll send out the patches once I have confidence > that I'm not missing anything essential :) > The host was Ubuntu latest at that time. Testing method and 'quirks' are documented in archives http://www.spinics.net/lists/linux-usb/msg50855.html We didn't have access to a Mac and Windows wouldn't support UAC2. -jassi -- 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
