On 20.5.2020 14.26, Rik van Riel wrote:
After a few more weeks of digging, I have come to the tentative conclusion that either the XHCI driver, or the USB sound driver, or both, fail to handle USB errors correctly.
I have some questions at the bottom, after a (brief-ish) explanation of exactly what seems to go wrong.
TL;DR: arecord from a misbehaving device can hang forever after a USB error, due to poll on /dev/snd/timer never returning.
The details: under some mysterious circumstances, the PCM290x family sound chips can send more data than expected during an isochronous transfer, leading to a babble error. Those circumstances seem to in part depend on the USB host controller and/or the electrical environment, since the chips work just fine for most people.
Receiving data past the end of the isochronous transfer window scheduled for a device results in the XHCI controller throwing a babble error, which moves the endpoint into halted state.
This is followed by the host controller software sending a reset endpoint command, and moving the endpoint into stopped state, as specified on pages 164-165 of the XHCI specification.
Note that isoch endpoints should generate buffer overrun instead of babble detect error on TD babble conditions. See xHCI spec 6.4.5 additional note 115.
Or maybe a frame babble could halt an isoc endpoint, see xhci 4.10.2.4.1 but then you should see a port error and port going to disabled state.
Any logs of this?
mount -t debugfs none /sys/kernel/debug echo 'module xhci_hcd =p' >/sys/kernel/debug/dynamic_debug/control echo 'module usbcore =p' >/sys/kernel/debug/dynamic_debug/control echo 81920 > /sys/kernel/debug/tracing/buffer_size_kb echo 1 > /sys/kernel/debug/tracing/events/xhci-hcd/enable < trigger the issue > Send output of dmesg Send content of /sys/kernel/debug/tracing/trace
However, the USB sound driver seems to have no idea that this error happened. The function retire_capture_urb looks at the status of each isochronous frame, but seems to be under the assumption that the sound device just keeps on running.
The function snd_complete_urb seems to only detect that the device is not running if usb_submit_urb returns a failure.
err = usb_submit_urb(urb, GFP_ATOMIC); if (err == 0) return; usb_audio_err(ep->chip, "cannot submit urb (err = %d)\n", err); if (ep->data_subs && ep->data_subs->pcm_substream) { substream = ep->data_subs->pcm_substream; snd_pcm_stop_xrun(substream); }However, the XHCI driver will happily submit an URB to a stopped device. Looking at the call trace usb_submit_urb -> xhci_urb_enqueue -> xhci_queue_isoc_tx_prepare -> prepare_ring, you can see this code:
/* Make sure the endpoint has been added to xHC schedule */ switch (ep_state) {... case EP_STATE_HALTED: xhci_dbg(xhci, "WARN halted endpoint, queueing URB anyway.\n"); case EP_STATE_STOPPED: case EP_STATE_RUNNING: break;
This leads me to a few questions:
- should retire_capture_urb call snd_pcm_stop_xrun, or another function like it, if it sees certain errors in the iso frame in the URB?
- should snd_complete_urb do something with these errors, too, in case they happen on the sync frames and not the data frames?
- does the XHCI code need to ring the doorbell when submitting an URB to a stopped device, or is it always up to the higher-level driver to fully reset the device before it can do anything useful?
xhci driver should ring the doorbell.
xhci_queue_isoc_tx() giveback_first_trb() xhci_ring_ep_doorbell()
when we are talking about babble or transaction errors the device might be completely unaware of the situation. Device side of the endpoint is not necessarily halted. xHC host will only halt its internal endpoint state, and it needs a reset endopoint command from the xhci driver to clear the internal halt state.
-Mathias
- if a device in stopped state does not do anything useful, should usb_submit_urb return an error?
- how should the USB sound driver recover from these occasional and/or one-off errors? stop the sound stream, or try to reinitialize the device and start recording again?
I am willing to write patches and can test with my setup, but both the sound code and the USB code are new to me so I would like to know what direction I should go in :)