Open Firmware use on OLPC.

[Mitch Bradley]

Carl-Daniel Hailfinger wrote:
> Peter Lorenzen wrote:
>   
>>>> why not just LZMA the whole image?
>>>>         
>> ofw is like a "tar" of mostly gzipped files
>>
>> ofw as distributed here  -> 338 kB
>> that image throught lzma -> 300 kB
>> compress idividual files -> 225 kB
>>
>> It could make sense to include VSA and .. in this kind of
>> tar like a minifilesystem to avoid duplication of code.
>>     
>
> OK, so the easy fix is to remove compression from the individual
> files and instead use lzma on the uncompressed image. LinuxBIOS
> will decompress the whole image into memory anyway, so I can't
> see any disadvantage of this method.
>   

The disadvantages:

a) It's not necessarily the case that LB decompresses the image into 
memory.  OFW is currently set up to stay in ROM and just decompress 
parts of itself as necessary.  That's fairly easy to do, just set the 
entry point in the ELF header and don't include any pheaders.

b) OFW gets a fair amount of reuse from its embedded decompressor.  It 
uses not only for bits of itself, but also for loaded os images, .zip 
archives, and JFFS2 file fragments.

c) The "tar-like minifilesystem" (the real name is "dropin format") is 
constructed by simple concatenation of individual "dropin modules".  
That's useful, because an OEM who is creating a system that's a small 
variation on an existing system can easily add their own stuff just by 
appending a new module at the end.  The SPARCstation clone market used 
that capability to good advantage, to do stuff like add vendor-specific 
customizations without having to muck around with the base code.  A lot 
of small-time hardware vendors have very limited on-staff software 
capabilities, so not having to rebuild entire bootroms from scratch can 
be a big win.