[PATCH 3/4] arm64: export effective Image size to bootloaders

[Rob Herring]

On Wed, Jun 18, 2014 at 11:49 AM, Mark Rutland <mark.rutland at arm.com> wrote:
> On Mon, Jun 16, 2014 at 09:27:12PM +0100, Geoff Levand wrote:
>> Hi Mark,
>
> Hi Geoff,
>
>> Sorry for such a delay in my reply.
>>
>> On Fri, 2014-05-16 at 10:50 +0100, Mark Rutland wrote:
>> > Both the image load offset and the effective image size are forced to be
>> > little-endian regardless of the native endianness of the kernel to
>> > enable bootloaders to load a kernel of arbitrary endianness. Bootloaders
>> > which wish to make use of the load offset can inspect the effective
>> > image size field for a non-zero value to determine if the offset is of a
>> > known endianness.
>>
>> Doing this conversion in the linker script seems complicated.  My
>> thought was to just have an image header field that specifies the
>> byte order, in the same way that the EI_DATA part of the ELF
>> e_ident field does.
>
> While the conversion in the linker script is a little ugly, it does
> work, and that complexity is hidden behind the macro I added.
>
> While I initially considered having a field to specify byte order, it's
> incredibly likely that bootloaders will not use it. Maintaining a fixed
> endianness everywhere makes it simpler for bootloaders to do the right
> thing, and matches what existing bootloaders are already doing. That's
> less pain for loaders and less pain for the kernel, as things are less
> likely to go wrong.
>
> To me it makes more sense to ensure these fields have a consistent
> endianness, rather than adding more room for possible mistakes.
>
>> Another advantage of having the byte order in the header is that
>> tools other than a boot loader that need to know the byte order
>> can get that info from the header, otherwise they would need to
>> guess the order with some kind of inspection.
>
> What kind of tools do you envision which would need to know the
> endianness of the kernel but would be looking at the Image rather than
> the vmlinux?

Seems like this has the same problem as being discussed for arm32:

http://permalink.gmane.org/gmane.linux.kernel/1727993

Rob