[Bug 220748] usb: xhci_queue_isoc_tx_prepare ignore start_frame and always assumes URB_ISO_ASAP is set

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=220748

--- Comment #4 from Michał Pecio (michal.pecio@gmail.com) ---
As for debugging, you can obviously print anything to dmesg with
printk()/pr_info()/xhci_info(). But this may generate quite a noise on isoc
endpoints if you print in every interval.

If you mount debugfs and go to

/sys/kernel/debug/usb/xhci/<controller-pci-address>/devices/<device-slot-id>/

you will find epXX directories and trbs file in each. The file contains a
human-readable dump of the transfer ring written by the driver for given
endpoint, so you can inspect if the TRBs make sense. This only works while the
endpoint is enabled, i.e. if you stop streaming and driver switches to an empty
altsetting zero, the endpoint will disappear from debugfs.

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