Where to put a large bootloader-supplied device tree on ARM ?

[Nicolas Pitre]

On Fri, 6 Jul 2012, Mitch Bradley wrote:

> On 7/6/2012 3:23 PM, David VomLehn (dvomlehn) wrote:
> > The kernel *must* go where it is linked, but the FDT contains only relative
> > references and is thus free to go anywhere. The same is true of ramdisks,
> > which
> > are usually placed after the kernel.

The kernel must go where it is linked *only* if you are using the 
'Image' output.  When using 'zImage' you can put the kernel anywhere in 
memory, or in the first 128MB of RAM if CONFIG_AUTO_ZRELADDR is used.

> Right, but the kernel image is compressed, so after decompression it expands
> into the area just after it.  Also, the .bss segment is in that vicinity.

To be exact, the compressed kernel moves itself out of the region where 
the decompressed kernel will end up before doing the decompression, but 
only if necessary.  So it is a good idea to load zImage away from the 
decompressed kernel area to avoid this extra move and save some fraction 
of a second on boot time.

> There's some code in arch/arm/boot/compressed/head.S to relocate
> device tree blobs, but it requires CONFIG_ARM_APPENDED_DTB which
> is not recommended - arch/arm/Kconfig recommends using the
> documented boot protocol istead .

This is in case a DTB is appended to zImage.  When the DTB is detected, 
the moving of zImage out of the decompressed area must take care of 
moving the DTB as well.

> Documentation/arm/Booting says
> to put the dtb "in a region of memory where the kernel decompressor
> will not overwrite it", further recommending the first 16KiB.
> 
> As noted, the first 16KiB loses if the dtb is too large.  And
> "where the kernel decompressor will not overwrite it" says what
> won't work, not what will.  It appears that the decompressor works
> out its addresses dynamically, so there's no hard prescription even
> for what to avoid.

A good rule of thumb is to take the size of the decompressed kernel and 
multiply this by 3.  Rounding up is also fine.  So for example if your 
arch/arm/boot/Image is 5MB, then putting anciliary data such as a 
ramdisk or a large DTB from 16MB into RAM or above should be fine.

> For now, I'm putting the initrd at the end of memory and the dtb
> below that.  That seems to work, but I'm unsure whether or not
> I'm just "getting lucky".

That's also perfectly fine.


Nicolas