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 */
This is the relevant information sent by the microphone and captured with
wireshark.
Class-specific Audio Streaming Interface Descriptor: Format type descriptor
bLength: 20
bDescriptorType: 0x24 (audio class interface)
Subtype: Format type descriptor (0x02)
FormatType: 1
Number Channels: 2
Subframe Size: 3
Bit Resolution: 24
Samples Frequence Type: 4
Samples Frequence: 32000
Samples Frequence: 44100
Samples Frequence: 48000
Samples Frequence: 96000
ENDPOINT DESCRIPTOR
bLength: 9
bDescriptorType: 0x05 (ENDPOINT)
bEndpointAddress: 0x82 IN Endpoint:2
bmAttributes: 0x0d
wMaxPacketSize: 576
bInterval: 1
Best regards,
Michael Häckel
