Question: USB serial - device driver debugging

[Aleš Nesrsta]

Hi,

it seems there are some problems in USB serial driver PL2303 and I want
to debug it.

I need to send and receive some data to see what happened, i.e. I need
to enable remote debug via classic serial interface by e.g.
"serial;terminal_output console serial;debug=xxx" and then do something
like "cat some_file > serial_usb0" and "cat serial_usb0 > some file" to
send and receive some data. But redirection is not supported in GRUB...

I cannot use additional terminal redirection like "terminal_output
console serial serial_usb0" to send something into serial_usb0 - it
makes recursive hang/crash when debug output is enabled.

Is there some way how to send/receive data to/from such device from GRUB
command line?
Or do I need to prepare my own testing program as command module?

Regards
Ales