下面几个链接可以看看,说的比较明白。UBL= user boot loader,即二级boot。
Do I need a secondary bootloader (UBL)?
A secondary bootloader, AKA User Bootloader (UBL), was required on older devices, where the bootloader could not parse AIS files. By using the AISgen tool with the OMAP-L138 bootloader, most of the functions previously performed by the UBL can be done instead by the bootloader.
For a typical Linux application, the old flow looked something like this:
- UBL (sets up DDR, PSC, and copies U-Boot to memory)
- U-Boot (loads Linux and file system)
- Linux
The flow for the OMAP-L138 would look like this:
- AIS-signed U-boot (sets up DDR, PSC, and loads Linux and file system)
- Linux
So in general, there is not a need for a separate UBL, as the AIS functions can perform most of the same tasks.
http://processors.wiki.ti.com/index.php/Main_Page
Think Over Before Asking.
http://www.catb.org/~esr/faqs/smart-questions.html#goal