Linux seamless booting

[Fortini Matteo <matteo.fortini@mta.it>]

Hi all,
we've been working on a PPC512x board booting with u-boot + linux 
2.6.24.6, and one major issue for our application is boot time.

Right now, we went down to less than 6s boot time (i.e. time from power 
off to launch of the user app), even with some drivers installed, i.e.:

    * CAN bus
    * GPU on PCI bus
    * USB host
    * UBI

with r/o squashfs root and r/w ubifs configuration filesystem mounted 
from a NOR flash memory.

This time could be enough for the high-level application (we're showing 
a splash screen in about 2-3s so it's ok if the main app takes a little 
longer to come up), but the whole system is connected to others via CAN 
and we can't afford the "black out" period between u-boot and linux.

Some other operating systems (e.g. QNX) have this "seamless booting" 
function, in which drivers are started in stages, so that a (minimal) 
functionality can be given in milliseconds, and more functionalities 
come up as time goes by.

Is there a way to support something similar with the u-boot -> linux 
solution?

Thank you,
Matteo