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

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:

quoted

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.

I don't think you're considering real-world usage scenarios, but instead
concentrating on the use issues.  If you only do that you're boxing
yourself into a corner and will cause a world of pain for folk who would
just like the kernel to be usable.

The information in ATAGs which will either never be in DT or which should
override what is in DT is:

- memory parameters
- command line
- initrd location
- system serial number
- system revision number

Things like the serial number will _never_ be in DT (do you want to build
a DT image unique to each platform?)  initrd location will never be in
DT either (it depends where the boot loader placed it.) ... and so forth.

This is the kind of information which should override whatever is in
DT because the DT contained information could well be wrong or outdated,
and its not the kind of information that anything other than core code
should be dealing with in any case.

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.

So, that means Nicolas' patches which allow a DT to be appended to the
image need to have DT and ATAG parsing in them to update the DT stuff
with the ATAG information.

quoted

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?

See above.