Is it possible to lose a xfer request queued using usbd_transfer()?

[Rita Lin]

Hello,

I stumbled on a problem that I need some suggestion to continue tracking
where the problem is.
I started a tip session to the usb port, after typing two characters, any
more input returned TS_BUSY.
I noticed that this problem also exists when I do a 'cat junk > /dev/ucom0'.
After two consecutive usbd_transfer(), it sits there.

After some code tracking, I found that the ucomwritecb() for the third
usbd_transfer() never gets called in uhci_idone(). I haven't dig further
down yet. I'm going to do that tomorrow. If you have some idea, or have some
suggestion how I can tackle this problem, please shine some light on me. The
first two usbd_transfer are successful, and I could see the packets going
out to the USB port with a USB analyzer.

Thank you so much.
Rita