Hi!
I hope this mail is not off-topic for this list :)
I have recently acquired a mini PC equipped with an N97 CPU for use in
my electric guitar setup (I have good experiences with my previous Intel
Celeron J4125-based mini PC), but sadly I can't get it to work reliably
with ALSA/Jack using a USB audio class 2.0 sound card.
The system is running a decently setup rt-kernel:
Linux ogfx97 6.6.44-rt39 #1-NixOS SMP PREEMPT_RT Sat Aug 3 06:54:42 UTC
2024 x86_64 GNU/Linux
Cyclictest gives sufficiently nice results that give me hope that it
should be possible to use this system for realtime-audio. This is just a
short example run but it's quite representative:
[fps@ogfx97:~]$ sudo cyclictest -m -p 90 -S
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 2.07 1.36 0.74 2/644 9634
T: 0 ( 9615) P:90 I:1000 C: 8715 Min: 1 Act: 1 Avg: 2 Max:
14
T: 1 ( 9616) P:90 I:1500 C: 5808 Min: 1 Act: 5 Avg: 2 Max:
28
T: 2 ( 9617) P:90 I:2000 C: 4355 Min: 1 Act: 1 Avg: 2 Max:
15
T: 3 ( 9618) P:90 I:2500 C: 3483 Min: 1 Act: 1 Avg: 3 Max:
24
As usual I have elevated the xhci and ahci IRQ kernel threads to
SCHED_FIFO priority 90 and done some other shenanigans like idle=poll,
disabling C-states in the UEFI firmware, etc..
The xhci in this system is:
00:14.0 USB controller: Intel Corporation Alder Lake-N PCH USB 3.2 xHCI
Host Controller (prog-if 30 [XHCI])
DeviceName: Onboard - Other
Subsystem: Intel Corporation Device 7270
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop-
ParErr- Stepping- SERR- FastB2B- DisINTx+
Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium
>TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0
Interrupt: pin A routed to IRQ 124
Region 0: Memory at 6001100000 (64-bit, non-prefetchable)
[size=64K]
Capabilities: <access denied>
Kernel driver in use: xhci_hcd
Kernel modules: xhci_pci
Sadly there's some built-in junk on the USB besides my keyboard, mouse
and audio interface:
[fps@ogfx97:~]$ lsusb -tv
/: Bus 001.Port 001: Dev 001, Class=root_hub, Driver=xhci_hcd/12p, 480M
ID 1d6b:0002 Linux Foundation 2.0 root hub
|__ Port 001: Dev 004, If 0, Class=Human Interface Device,
Driver=usbhid, 1.5M
ID 046d:c31c Logitech, Inc. Keyboard K120
|__ Port 001: Dev 004, If 1, Class=Human Interface Device,
Driver=usbhid, 1.5M
ID 046d:c31c Logitech, Inc. Keyboard K120
|__ Port 002: Dev 005, If 0, Class=Human Interface Device,
Driver=usbhid, 12M
ID 046d:c07d Logitech, Inc. G502 Mouse
|__ Port 002: Dev 005, If 1, Class=Human Interface Device,
Driver=usbhid, 12M
ID 046d:c07d Logitech, Inc. G502 Mouse
|__ Port 004: Dev 014, If 0, Class=Audio, Driver=snd-usb-audio, 480M
ID 1235:8202 Focusrite-Novation Focusrite Scarlett 2i2 2nd Gen
|__ Port 004: Dev 014, If 1, Class=Audio, Driver=snd-usb-audio, 480M
ID 1235:8202 Focusrite-Novation Focusrite Scarlett 2i2 2nd Gen
|__ Port 004: Dev 014, If 2, Class=Audio, Driver=snd-usb-audio, 480M
ID 1235:8202 Focusrite-Novation Focusrite Scarlett 2i2 2nd Gen
|__ Port 004: Dev 014, If 3, Class=Vendor Specific Class,
Driver=[none], 480M
ID 1235:8202 Focusrite-Novation Focusrite Scarlett 2i2 2nd Gen
|__ Port 005: Dev 002, If 0, Class=Wireless, Driver=[none], 12M
ID 0bda:c821 Realtek Semiconductor Corp.
|__ Port 005: Dev 002, If 1, Class=Wireless, Driver=[none], 12M
ID 0bda:c821 Realtek Semiconductor Corp.
|__ Port 006: Dev 003, If 0, Class=Audio, Driver=snd-usb-audio, 12M
ID 0573:1573 Zoran Co. Personal Media Division (Nogatech)
|__ Port 006: Dev 003, If 1, Class=Audio, Driver=snd-usb-audio, 12M
ID 0573:1573 Zoran Co. Personal Media Division (Nogatech)
|__ Port 006: Dev 003, If 2, Class=Audio, Driver=snd-usb-audio, 12M
ID 0573:1573 Zoran Co. Personal Media Division (Nogatech)
|__ Port 006: Dev 003, If 3, Class=Human Interface Device,
Driver=[none], 12M
ID 0573:1573 Zoran Co. Personal Media Division (Nogatech)
/: Bus 002.Port 001: Dev 001, Class=root_hub, Driver=xhci_hcd/4p, 10000M
ID 1d6b:0003 Linux Foundation 3.0 root hub
OK, with that out of the way, what's the problem? Well, with an USB
audio class 2.0 device I would expect a maximum jitter of a microframe
(125 us) regardless of the buffer size and I have used this audio
interface successfully on different xHCI HCDs with buffer sizes as low
as 24 samples at 48000 Hz sampling rate (0.5 ms period duration).
But in this mini PC I cannot run such low period durations/buffer sizes
at all. Sporadically I see delays of up to 3 or 4 ms - which makes it
absolutely unusable for real-time audio stuff. It can't really be NMIs
from what I can tell - they would show up in the cyclictest results. So
I suspect the xHCI is doing something funky.
Here's another observation: When I run the audio interface with 2
periods of 48 frames at 48000 Hz sampling rate I get constant Xruns from
jack of about 0.004-0.008 ms. It seems that the xHCI is not willing to
deliver all microframes in a timely manner but somehow wants a minimum
time interval of 1 ms between them (I read the term interrupt moderation
somewhere on the web - but I don't know if the linux xhci driver does
something like that at all.)
How could I debug this further? All help appreciated :)
Kind regards,
FPS
