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

[Tom Rini <trini@konsulko.com>]

On Fri, Dec 02, 2016 at 04:25:37PM +0000, Ryan Harkin wrote:
> On 2 December 2016 at 15:41, Tom Rini <trini@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.

Maybe this gets to the heart of the problem then, and we should
re-structure and fix this.  If you look in board/raspberrypi/rpi/ we
support rpi1 2 and 3, and that includes rpi3 in 64bit mode.  So if we
want to re-work board/armlt/vexpress/ to support the various ways the
base hardware can be (/ has been over the years), lets.  Does that sound
like a plan?

-- 
Tom
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