[U-Boot] Segmented RAM, big userspace.

[Sergey Lapin <slapin@ossfans.org>]

On Tue, Dec 13, 2011 at 05:24:22PM +0100, Wolfgang Denk wrote:
> Dear Sergey Lapin,
> 
> In message <20111213160810.GA27440@build.ihdev.net> you wrote:
> > 
> > We have a board, based on s3c2416, with 128MB of RAM
> > and 1GB of NAND flash.
> > RAM is organized as 2 memory banks with far placed bases:
> > TOP
> > UNUSED 64MB
> > 64MB SDRAM
> > --- 128MB segment base 1 -----
> > UNUSED 64MB
> > 64MB SDRAM
> > -- 128MB segment base 0 ------
> > BOTTOM
> > 
> > So we could have only 64MB as one piece; Due to u-boot reloaction code we
> > need to have u-boot in first 64MB also. ...
> 
> Why would that be the case?

If we specify memory size as 128MB (gd->ram_size) u-boot reloacates itself to unused
part of first memory bank instead of the end of second bank, which
actually makes it fail badly.
If we specify memory size to 64MB, u-boot works, but we lose
access to second segment (segment 1). Or at least have not really
comfortable placement.

> 
> I would expect that U-Boot (with all it's heap and stack and
> everything) sits only at the upper end of the upper bank of memory (in
> your sgment 1).
> 
> >                                   ... . And now we have root filesystem
> > which we need to flash using u-boot, which is a little over this 64MB limit.
> 
> Split it?
There comes issue of NAND bad blocks.
> 
> > Is there some way to use second memory bank from u-boot?
> > Is it possible to tftp file in parts and flash it on NAND in parts?
> > (This requires handling of bad blocks too)
> > 
> > We need to flash using u-boot, that's requirement for speedy production.
> 
> I somewhat doubt that. Booting Linux is probably a matter of 2 or 3
> seconds, or less.  You migth save that time again by using Linux' much
> better performing network stack, together with interleaving network
> traffic and flash writing.
> 
> If I were in this situation, I'd probably run this under Linux.
Well, we already running production using u-boot.
And on production devices we have no (fast) network access.
We boot u-boot from mmc, which flashes images on the same card
and then boots device. I can't see how faster we can be with Linux,
which will add several more seconds to boot.

tftp is needed for development and images actually can be split, but
it is quite hard to flash them properly.

Thanks a lot,
S.