kernel entry for thumb2-only cpus

[Matt Sealey]

On Mon, Aug 6, 2012 at 5:36 PM, Nicolas Pitre <nico at fluxnic.net> wrote:
> On Mon, 6 Aug 2012, Matt Sealey wrote:
>
>> Right now there's already a fixed entry point - ARM kernels are fixed
>> entry point of relative 0 to the start of the loaded image. Okay, so
>> we can't format a totally valid ELF header, but in the event that we
>> want to encode all this information yet again, why reinvent the wheel?
>
> That's the point.  We do _not_ want to encode anything.
>
> By convention, if your CPU is ARM mode capable, then it must call the
> kernel in ARM mode, period.  Can't be simpler than that, right?  The
> bootloader shouldn't care if the kernel is itself ARM or Thumb, and the
> kernel shouldn't care if the bootloader is ARM or Thumb.
>
> If you have a real problem with this then let's hear it so it can be
> addressed.

I don't have a real *problem* with it, I just think since the kernel
can be compiled in both modes, but you may have some kind of remit to
not run ARM code (even if it's just a couple instructions at the
start) or some toolchain requirement or.. something.. it would be
entirely the bootloader's job to determine what mode it is in when it
jumps into the kernel. If you boot a Thumb bootloader and load an ARM
kernel, you'll get a crash. If you boot an ARM bootloader and a Thumb
kernel (with the current ARM instructions in head.S), you'll get a
crash. This is well defined, perfectly obvious, and I realise that..

But if you would need to micromanage the system such that they both
matched, or were perverse enough to build a Thumb bootloader and a
Thumb kernel, you might be counfounded that you actually created a
99.99% Thumb bootloader which switched to ARM mode before going into
the kernel, and a 99.99% Thumb kernel but needed to jump into Thumb
mode after the first part of the kernel code? It should be possible
(with that kernel config switch it would be) if you're into that kind
of configurability to build your Thumb bootloader, build your Thumb
kernel and have it never exit Thumb mode for any reason, even if it is
just for some few bytes of very minor bootstrapping.

The same perfectly obvious caveats apply, but it's possible. The
question is, if you could do it, how would you make sure bootloaders
booted the right way? I would hazard a guess that given your opinion
on it, it would be just as good if they all just crashed.

So let's just patch out the ARM parts if you're building a Thumb
kernel on a Thumb-only device, and assume it was in Thumb mode all
along if another subordinate option was selected.. that way the kernel
can automatically fulfill one side of the contract in that you
configured "build thumb2 kernel" and it would contain 0% normal arm
code for ARMv7-M (or -R) and be able to be prompted to do so if you
were building for ARMv6k/ARMv7-A. Ordinary people would not turn it
on.. whacky weirdos like you and me might do it for proof of concept
;)

All it needs is a bootloader able to fulfill it's side of the contract
(build a thumb2 bootloader..) and jump while in Thumb mode, which I am
100% certain exists. It'd be a 20 line patch maximum to make U-Boot do
this (it already can build as Thumb2 I think.. if not, there are
patches around) through an environment variable and distros that knew
they built in Thumb mode could simply modify the variable in boot.scr
or so, and the jump would end up being done in Thumb. That way you
have to kick it, but no changes are required in the kernel format
(just in the packaging of the kernel and configuration of the
bootloader). Guessing would be a bad idea...

I still say considering you thought of a workable solution and your
problem is only the linker in use (seems like a bug report to the gas
guys is in order..) and there are probably ways to get around that
anyway, why not do it? I find it odd that Linux needs a "hard customer
requirement" for it to need it to be capable of doing it, if it's just
*possible*.

-- 
Matt Sealey <matt at genesi-usa.com>
Product Development Analyst, Genesi USA, Inc.