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SubjectRe: [sodaville] [PATCH 02/11] x86: Add device tree support
On 12/30/2010 12:26 AM, Grant Likely wrote:
> Since Linux on x86 has pretty much always depended on a two stage boot
> (firmware boots a bootloader like grub which in turn boots the
> kernel), then what is the use case for pursuing an in-kernel dtb
> linkage? simpleimage was used on powerpc for the use-case where there
> is no 2nd stage bootloader, but instead only the kernel which is
> booted from some firmware that is non-upgradeable (or at least too
> risky to upgrade). Same with the cuImages. The wrapper is
> effectively a 2nd stage bootloader to adapt from what older u-boot
> provides and what the kernel needs.
> What is the boot sequence for the embedded x86 platforms? Is there
> still a bootloader? If so, what prevents always depending on the
> bootloader to pass in the device tree blob? If the bootloader is
> software (not firmware) then it should be something we have control
> over when shipping a distribution.
> BTW, don't take microblaze as the example to be emulated. Some of
> the things it does for device tree support is not scalable, like
> linking the .dtbs directly into the kernel.
> John Bonesio has also prototyped doing a similar zImage bootwrapper on
> arm which allows a dtb to be concatenated to the kernel image and
> updated before passing it to the kernel. As it stands, there are no
> plans to use in-kernel .dtb linking on ARM.
> I know it's not very fair to bring up these issues again right before
> the merge window opens. I got myself overcommitted and dropped the
> ball over the last 1.5 months and I beg forgiveness. However, I do
> want to make sure that the right decision is made and I'd be happier
> if a consistent scheme is used for passing the .dtb on all
> architectures.

There are a number of different boot loader solutions in use on embedded
platforms, as much as we would like to avoid it.

However, the ability to link in the dtb will provide a
architecture-neutral option of last resort. I'm not saying it's a good
option, but it's better than random ad hoc stuff, and if that means that
it will only ever be used during in-lab platform bringup, *that is still
a huge win*.


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