[U-Boot] how to support usbtty

[Bryan O'Donoghue <bodonoghue@codehermit.ie>]

On Fri, 29 Aug 2008 11:53:45 -0400
"mike zheng" <mail4mz@gmail.com> wrote:

> Hi Bryan,
> 
> The USB/RS232 cable is a USB device, it shall has firmware in it,
> which works as a USB slave. What I need to do is have some code talk
> to this device via the Host controller - ISP1561. So, I need some code
> as usb-storage.c, maybe called usb-serial.c.

Almost certainly.

> I am not sure how "usbtty" being used. Is it used when there is a
> USB/USB cable connected between the target and PC?

Yes. Think of usbtty being equivalent to the firmware that runs inside of that
USB/RS232 cable you have.

It allows u-boot to be a USB slave and provide a console interface over the
USB, to a USB host somewhere.

> Both ends of the
> cable are USB port, the PC end is a master, the target end is the
> slave. So usbtty running on the target is acting as the USB device.
> However it is not my case. The cable I have has one USB end, and one
> RS232 end. 

Understood.

> The USB end is connected to the target. I assume there is
> something in the chip within the cable acts as a USB slave device
> already.

A bit of firmware implementing either some FDTI-specific USB protocol stuff to
emulate UART over the USB, or just simply following the USB standard for this
=> cdc-acm.

And again you're right - you'll need to be implementing a host side USB serial
driver for u-boot.

Alternatively if the ISP1561 can do device mode - you could have your PC be
master and just implement the usbdcore_isp1561.c for u-boot and then use either
the usbserial or cdc-acm USB protocol to talk to the USB host. In this case you
could use the code in usbtty to plug into the u-boot console system - and just
implement whatever is required to make the isp1561 be a well behaved u-boot USB
slave.

Most embedded USB transceivers these days to both host and slave - so it'd
probably be worth looking into this option ?

In either case it might be interesting to plug your host-side stuff into
usbtty.c in u-boot - though it's unclear to me if there'd be more or less pain
for you in the long run !

 --
Bryan