[BUG] ALSA: usb-audio: Scarlett 2 mixer driver fails on ehci-pci

[Geoffrey D. Bennett]

Hi there,

I'm trying to track down an issue with my Scarlett Gen 2 mixer driver
that has been reported by a few people, and I can now reliably
reproduce the issue, but I need some help in figuring out where
exactly the issue is and how to fix it please.

The issue only occurs when attempting to use the interface on a USB
port using ehci-pci. xhci_hcd USB ports work fine.

The issue occurs when sending the first vendor-specific USB command,
but only when sending from the kernel driver. Sending the same USB
packets from user-space works fine(!).

I did initially think that the fault could have been due to earlier
USB messages putting the device into a state where it would reject the
vendor-specific USB commands, but I have carefully ruled that out &
have gotten identical usbmon traces from device power-on up until the
device responds differently, the only difference beforehand being
whether the USB packet was sent from the kernel driver or user-space.

The messages look like this in "usbmon -s 10000 -fu" when sent from
user-space (or when sent from the kernel driver when the interface is
plugged in to an xhci_hcd port):

ffff888125855200 1006026497 S Ci:2:040:0 s a1 00 0000 0005 0018 24 <
ffff888125855200 1006026680 C Ci:2:040:0 0 24 = 66191018 73190604 01000000 01000000 00040000 00000000

And like this when sent from the kernel driver when the interface is
plugged in to an ehci-pci port:

ffff88810487a300 3686673995 S Ci:2:036:0 s a1 00 0000 0005 0018 24 <
ffff88810487a300 3692178724 C Ci:2:036:0 -2 0

Identical messages sent according to usbmon, but they must be
different somehow!

The kernel code to send that message looks like this:

  return snd_usb_ctl_msg(
    dev, usb_sndctrlpipe(dev, 0),
    usb_req,
    USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_IN,
    0, interface, buf, size);

and helper.c snd_usb_ctl_msg() then calls usb_control_msg().

For sending arbitrary USB packets from user-space for testing, I'm
using libusb and:

  int ret = usb_control_msg(
    devh, reqtype, request, value, index, buf, size, 1000
  );

So, I presume usbmon isn't giving me the full story? How can I
determine the difference between the kernel and the user-space
usb_control_msg() functions? I see that I can #define EHCI_URB_TRACE
in ehci-hcd.c. Can anyone with more experience than me let me know if
I'm going in the right direction to track this down?

Thanks,
Geoffrey.