Thank you for the reply. Ok, I will read the specification about isochronous transfers and check, whether I can do something or not. The specification is indeed quite long. A long time ago I was quite active in open source development, but I didn't expect that I will ever dive so deep into the kernel code. Best regards, Michael Am Donnerstag, 14. Februar 2019, 20:57:25 CET schrieb Alan Stern: > On Thu, 14 Feb 2019, Michael [ISO-8859-1] Häckel wrote: > > Hi, > > > > With kernel 4.20.8 as well as 5.0-rc6 I have the following problem. > > I have an AT2020USBi microphone from audio-technica. This is a full speed > > USB device which supports 24 bit audio at up to 96 kHz. Although the > > hardware is only mono it requestes a wMaxPacketSize of 576 corresponding > > to 24 bit stereo at 96 kHz. According to my understanding this is > > compliant to the USB 2.0 specification, because up to 80% of the > > bandwidth can be used for iso transfers. However the linux kernel does > > not grant this bandwidth although it is the only device connected to one > > port of a USB 2.0 hub. It reports: usb 2-1.3: cannot submit urb 0, error > > -28: not enough bandwidth > > > > I checked the logic in iso_stream_schedule and sitd_slot_ok from > > drivers/usb/ host/ehci-sched.c and found the following. > > - The desired packet size fits into 4 uframes. > > - The variables are calculated to hs_transfers = 4 and stream->ps.cs_mask > > = > > 0xFC01 (line 1110). > > - Searching for a suitable slot starts in iso_stream_schedule. > > - Uframe 7 and 6 are not used. "if ((start % 8) >= 6) continue;" line 1530 > > - Uframe 5 to 1 are rejected, because the condition "mask & ~0xffff" in > > line 1411 ist not met. > > - Uframe 0 does not work, because there is alredy some reserved bandwith > > and tt_available fails. > > > > I am not sure about the intended behavior of the code. If I apply the > > following change then the transfer is scheduled for uframe 5 to 2 and the > > microphone works well. > > > > diff -up linux-4.20.8/drivers/usb/host/ehci-sched.c > > linux-4.20.8.new/drivers/ usb/host/ehci-sched.c > > --- linux-4.20.8/drivers/usb/host/ehci-sched.c 2019-01-31 > > 08:15:47.000000000 +0100 > > +++ linux-4.20.8.new/drivers/usb/host/ehci-sched.c 2019-02-13 > > 22:05:10.622284415 +0100 > > @@ -1408,7 +1408,7 @@ sitd_slot_ok( > > > > return 0; > > > > /* for IN, don't wrap CSPLIT into the next frame */ > > > > - if (mask & ~0xffff) > > + if (mask & ~0x3ffff) > > > > return 0; > > > > /* check bandwidth */ > > As you have seen, the support for isochronous scheduling with > full-speed devices in ehci-hcd is deficient. Improving it would be > extremely difficult; the requirements described in the USB and EHCI > specs are very complex. Nobody has implemented siTD back link > pointers, FSTN nodes, or periodic-schedule rebalancing in Linux. > > For the most part, the code tries to follow the spec by restricting the > use of packets that would be too large for it to handle. The change > you suggest above, for instance, would violate the spec. However now > and then this restriction leads to trouble and unfortunately, you are > the victim. > > If you would like to work on improving the driver, I'd be happy to > help. > > Alan Stern
