Appended DTB files for multi-machine kernels

[Nicolas Pitre]

On Fri, 26 Jul 2013, Daniel Mack wrote:

> On 04.07.2013 19:28, Nicolas Pitre wrote:
> > On Thu, 4 Jul 2013, Daniel Mack wrote:
> >> In short, devices that have been deployed in quantities come in three
> >> hardware variants, which all boot with a unique machine-id. We ship
> >> kernel images that have board support for all three machine types, and
> >> do minor fixups to platform data of some drivers at runtime, depending
> >> on the board revision number (passed in via ATAGs).
> >>
> 
> > What you describe above more or less fits the definition of what I 
> > called the "impedance matcher".  However it doesn't need to be part of 
> > the kernel at all.  But you should make it into a separate binary.
> > 
> > Please have a look at the bottom of this post for a more comprehensive 
> > description: http://article.gmane.org/gmane.linux.ports.arm.kernel/242929
> 
> FWIW, I gave that approach a try today for a PXA based hardware
> platform, and in fact it seems to work very well. Thanks again for
> sharing the idea.
> 
> In case anyone is interested, the repository is here:
> 
>   https://github.com/zonque/pxa-impedance-matcher
> 
> As the README says, it's completely specific to my use case, but maybe
> anyone can use the code base to spin a different version for a different
> board.

That looks nice, but I'd suggest going a step further if possible.

If you could load the kernel zImage independently with a separate load 
command in u-boot, then people could update their kernel only without 
having to relink with this shim.  The load address for that zImage can 
be totally arbitrary as longas it is known to be the same from within 
this shim and in the u-boot script.

Also, more related to your code, you apparently link and load it to run 
at 0xa0008000.  This is also where the zImage code is going to 
decompress the final kernel image after it has relocated itself.  I'm 
afraid the DTBs you point to might be overwritten at that point since 
zImage is not aware of them (in fact I'm surprized this works at all as 
you say).  If I were you I'd use 0xa1000000 instead (and make sure the 
initrd is out of the way as well if you have one).


Nicolas