Re: Re: Possibility for Debian?

On Sun, 06 Mar 2005 12:36:27 -0500, Tommy B 
<gentoo-P1JKnMZDrucAvxtiuMwx3w@xxxxxxxxxxxxxxxx> wrote:
>  John Bowler wrote:
>  
>  >From: Stephen Henson
>  >  
>  >
>  >>I'd been wondering if it might be possible to go one step further and
>  >>boot the kernel from the root drive.
>  >>    
>  >>
>  >
>  >I thought about that too but couldn't see any way of making it happen
>  >without a changed boot loader.  Another interesting possibility though is
> to
>  >run the kernel from flash.  At present the kernel image is compressed into
> a
>  >1MByte slot in the flash memory then decompressed by bootstrap code right
> at
>  >the start of the image.  If, instead, an uncompressed image was built
>  >(xipImage, not zImage) it could be run directly from flash (I believe).
>  >This would free up all the memory used by the kernel code.
>  >
>  >On a minimal OpenSlug there is about 4.5MByte of flash unused, add that to
>  >the 1MByte slot already reserved for the kernel and there is room for a
>  >5.5MByte image.  If the system root is on a hard drive there's not a lot
> of
>  >reason to free space in the flash partition.
>  >
>  >John Bowler <jbowler-HInyCGIudOg@xxxxxxxxxxxxxxxx>
>  >
>  >  
>  >
>  FYI:
>  
>  The option will be for the brave of heart initially and just courageous
>  after that.
>  
>  Beewoolie has been looking at adding ext2 support to APEX for about
>  a week or more.  When APEX is further tested and ext2 support
>  exists,  we should be able to boot from the kernel within the rootfs.
>  We currently plan on having two full rootfs systems each having its
>  own kernel and modules.  
>  
>  The plan is to allow switching between the kernels via the RESET
>  button and front power button and getting feedback from the LEDs.
>  
>  APEX is currently around 16K.  So 1 partition for APEX, 1 for APEX's
>  config data (maybe none) leave 7.75MB for lots of stuff. :)

A couple of things.

  1) XIP is slick from the point of view of saving RAM.  It tends to
     be poor from the POV of performance.  Reads from flash are really
     slow when compared to SDRAM.  Caching helps a bit, but I've seen
     significant performance degradation when I've used XIP on other
     platforms.   

     APEX does support XIP kernels, so it is easy enough to test.

  2) APEX has a driver for reading from ext2 and fat.  There is more
     work to do to get it to properly support the slug.  This allows
     the kernel and a ramdisk to be read from a filesystem at
     boot-time.


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