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

[Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>]

On Sun, Jun 12, 2011 at 04:15:23PM +0200, Arnd Bergmann wrote:
> On Sunday 12 June 2011 13:58:20 Russell King - ARM Linux wrote:
> > Exactly my point - I have quite a number of platforms here which will
> > never be able to have a boot loader capable of modifying a DT blob for
> > the kernel.
> > 
> > One of the points of Nicolas' patch set is to allow existing boot loaders
> > to boot kernels where the hardware description is contained in a DT blob
> > encapsulated with the kernel.  That's great but the way things are currently
> > setup, it means that the boot loader does nothing more than loading and
> > executing - and we lose the existing flexibility for the boot loader to
> > pass platform specific information to the kernel.
> > 
> > So, what I'm considering to do is update the boot protocol such that the
> > base address of the DT blob is provided in r3, separately from the ATAG
> > pointer in r2.
> >
> > This means that boot loaders can provide a DT blob (r2 = 0 r3 => DT) or
> > they can provide ATAGs (r2 => atag, r3 = indeterminant).  We can then
> > cater for the situation where we have an ATAG boot loader, but a kernel
> > with an appended DT description (r2 => atag, r3 => DT) and have the ATAG
> > information override the DT for things like memory layout and the command
> > line string.
> 
> But when you have both atag and DT and the atag overrides the DT, that
> means we have incorrect information in the DT, and code might later
> rely on that information.
> 
> IMHO when we allow passing a DT to a kernel while booting from an
> existing boot loader that only knows about atag, the code that loads
> the DT should be responsible for updating the DT with the atag information,
> not pass two conflicting sets of data into the actual kernel.

I completely agree here.  I /started/ from the position that ATAGs and
DTB would coexist, and after extensive debate[1] my opinion turned around
to it should be one or the other.  Otherwise there are all kinds of
questions about accuracy of the information and which takes
precedence.

[1]http://lists.ozlabs.org/pipermail/devicetree-discuss/2010-May/002130.html

> > Let me give a situation where this matters: you have a boot loader which
> > loads a kernel from disk and executes it.  You have 256MB of RAM fitted
> > to the machine.  You replace this kernel with a DT-enabled kernel which
> > has the DT blob appended to the kernel.  The DT blob says you have 256MB
> > of RAM.
> > 
> > You remove a 128MB DIMM because its gone faulty.  You try to boot.  The
> > boot loader provides the kernel with an ATAG telling it that there's
> > 128MB of RAM.  However, the kernel ignores the ATAGs and instead looks
> > at the encapsulated DT information which tells it that there's 256MB
> > of RAM.  Your kernel OOPSes.  You reboot, and try passing a command
> > line argument of 'mem=128M'. The kernel again ignores this because its
> > an ATAG.
> > 
> > The result: you can't boot the platform.
> >
> > Another case: your flash has become corrupted, and the kernel won't mount
> > the flash based rootfs.  You want to boot from a root-NFS export to sort
> > the problem out, but the kernel ignores your new command line telling it
> > to do so.
> > 
> > The result: you can't boot the platform.
> > 
> > Another case: you have a Thecus N2100 acting as a server, with a pair
> > of drives setup as raid 1.  You reboot it one day and it refuses to build
> > the raid 1 rootfs, and so panics at boot.  You want to change the kernel
> > command line so that it mounts root from somewhere else, but because
> > you're using a DT based kernel, it ignores you.
> > 
> > The result: you can't boot the platform.
> 
> So we need to at least the command line and the memory layout to be adapted
> in the in-memory DT, from the atags. Any other tags?

initrd location.

There already exists a prototype patch that loads some atag data into
the dt, and it is exactly the mechanism used by powerpc to boot kernels
on non-dt firmware.

g.