[U-Boot] mx28: booting u-boot via USB

[Matthias Fuchs <matthias.fuchs@esd.eu>]

Hi,

I have a problem booting the current u-boot-imx version
on a MX28EVK via USB. The i.MX28 CPU supports a special boot mode
where the image (sb boot stream) is downloaded via USB device
(all DIP switches in position 'off' on the EVK). The
CPU's internal bootrom implements a HID device in this mode.
Freescale provides a download tool "sb_loader" for this purpose.
There is also a GPL'd implementation based on libusb.

So back to my problem:

I build u-boot from the u-boot-imx repository. This runs fine
when booting from SD/MMC or SPI-flash. But the console stays dark
when using the USB download mode. USB download works fine with the
Freescale provided U-Boot sources together with the imx-bootlets
SPL code.

Now I digged a little bit deeper to find out the cause:

When downloading via USB I saw the SPL running correctly.
board_init_ll() (arch/arm/cpu/arm926ejs/mx28/start.S) runs through
correctly. Then control is passed back to the bootrom. That's
where things end as far as I can see.

The 2nd stage of u-boot is not started.

The USB boot mechanism is not documented that detailed.
Is it possible that the SPL code overwrites the bootroms ivt and
in consequence crashes the bootrom's USB code? Any further ideas?

Matthias