[U-Boot] [RFC PATCH 0/2] ARMv8 Aarch32 support

[Ryan Harkin]

On 2 December 2016 at 15:41, Tom Rini <trini at konsulko.com> wrote:
> On Fri, Dec 02, 2016 at 11:51:07AM +0000, Ryan Harkin wrote:
>
>> I've been working with Soby Mathew to get U-Boot booting on ARM's
>> AEMv8 FVP model in Aarch32 mode.
>>
>> Soby worked out what needed to be changed and I'm refining the changes
>> into patches that can be built for both Aarch64 and Aarch32 mode.
>>
>> There are two patches for discussion:
>>
>> [RFC PATCH 1/2] Add Aarch32 option for ARMv8 CPUs
>> [RFC PATCH 2/2] Add vexpress_aemv8a_aarch32 variant
>>
>> I expect the first patch to be controversial.  I also don't expect it to
>> be accepted, but to demonstrate what changes we needed to make to get an
>> ARMv8 platform to boot in Aarch32 mode when selecting CPU_V7 instead of
>> ARM64 as the CPU type.  This in itself may be the wrong approach.
>>
>> It adds an ARMV8_AARCH32 config option and some checks in generic code
>> for that option to allow the code to differentiate between the two
>> modes.
>>
>> The second patch should be less controversial.  It adds support for a
>> new AEMv8 variant that runs in 32-bit mode.  The most awkward part is
>> that it defines itself not as ARM64, but as CPU_V7.  I expect this to
>> change based on feedback from patch 1/2.
>>
>> The Aarch32 code runs on the same AEMv8 model as the Aarch64 code, but
>> takes an extra per-core model launch parameter to switch the cores into
>> Aarch32 mode, eg. "-C cluster0.cpu0.CONFIG64=0".
>
> So my first and slightly ignorant question is, why isn't this just a new
> regular ARMv7 board being added rather than a special cased ARMv8?
>

That's a valid question.

I guess it could be either.  At the moment, it's a bit of both.
arch/arm/Kconfig says it's an ARMv7, but then it's added to
board/armltd/vexpress64/Kconfig to re-use vexpress_aemv8a.h.

But there's no reason it couldn't be added to
board/armlt/vexpress/Kconfig and have a copy of vexpress_aemv8a.h that
isn't special cased at all.  That approach seems more copy/paste-y
than what I've done in this series, though.

I think the whole setup for vexpress/vexpress64 and AEMv8/Juno is
confused.  Really, all of these armlt boards are the same with minor
variations, even if the minor variation could be ARMv7 vs ARMv8.

I'd quite like to address that, but I'm unsure how to approach the
problem in the most flexible way.


> --
> Tom