[PATCH 0/3] patches to allow DTB to be appended to the ARM zImage

[Nicolas Pitre]

On Mon, 13 Jun 2011, David Brown wrote:

> On Sun, Jun 12 2011, Russell King - ARM Linux wrote:
> 
> > On Sun, Jun 12, 2011 at 06:42:16PM +0800, Shawn Guo wrote:
> >> On Sun, Jun 12, 2011 at 10:52:17AM +0100, Russell King - ARM Linux wrote:
> >> > On Sun, Jun 12, 2011 at 05:38:23PM +0800, Shawn Guo wrote:
> >> > > On Sun, Jun 12, 2011 at 10:21:31AM +0100, Russell King - ARM Linux wrote:
> >> > > > What if your platform doesn't use uboot?
> >> > > 
> >> > > Add dtb parsing support with the help from libfdt, I guess.  It is
> >> > > some amount of work, but it's not a rewrite of bootloader, IMHO.
> >> > 
> >> > I guess you're suggesting that this wrapper uses libfdt to merge
> >> > the ATAGs with the DT info?
> >> > 
> >> No, ATAGs does not play at all in this case.  For u-boot example
> >> again, if it boots a dt kernel, dtb will be parsed to get cmdline
> >> node overwritten as bootargs env value, and then it boots the dt
> >> kernel with this updated dtb.
> >
> > You've missed my point entirely.  What you're saying is that we have to
> > re-build and replace the boot loader in order to pass a command line into
> > a kernel using the DT wrapper.
> >
> > I'm saying that you shouldn't have to, and the kernel should accept the
> > memory size and command line from the ATAGs _in addition_ to the
> > appended DT blob, and the ATAGs in that case should take precidence.
> 
> Having just now booted an MSM target using device tree, I'm going to
> agree with Russell here.  I don't really see a clean transition path for
> us to hard cut over to DT, at least on existing targets.
> 
> The MSMs use a custom bootloader.  It builds ATAGS to pass in memory
> information, command line arguments, and the ramdisk address.  Working
> with the bootloader team just to do this was difficult; trying to
> support DT in the bootloader would be even more difficult.
> 
> But, beyond this, we have to still be able to boot our production
> kernels (which don't have DT support, and probably won't for quite some
> time).
> 
> The steps I had to go through to boot a DT kernel were kind of
> complicated:
> 
>   - Boot kernel in a debugger, and inspect the ATAGS.
> 
>   - Edit my DTS file to match the current values.
> 
>   - Compile the DTS file.
> 
>   - Load the dtb on top of the ATAGS.
> 
> Unless I'm missing something, I don't see a clean way of supporting this
> that doesn't involve the kernel being able to parse the ATAGS as well.

FYI: I've dug up the patch from John Bonesio doing just that.  While the 
patch doesn't apply anymore, it looks trivial enough.  I should have it 
working by tomorrow.


Nicolas