Creating armv7 MACHINE_ARCH

[Russell Haley]

On Mon, Jun 12, 2017 at 1:00 PM, Mark Millard <markmi at dsl-only.net> wrote:
> On 2017-Jun-12, at 12:16 PM, Russell Haley <russ.haley at gmail.com> wrote:
>
>> On Mon, Jun 12, 2017 at 10:36 AM, Mark Millard <markmi at dsl-only.net> wrote:
>>>
>>> On 2017-Jun-12, at 8:39 AM, Warner Losh <imp at bsdimp.com> wrote:
>>>
>>>> . . .
>>>>
>>>> Plus, we aren't quite doing what Ian wanted. He wanted a full rename. The
>>>> proposal on the able is to add an armv7 TARGET_ARCH in 12. Not to rename or
>>>> remove armv6. Sadly, that will still be there since the RPI foundation
>>>> keeps finding new ways to repackage the rpi into new boards that are just
>>>> too cheap to ignore.
>>>
>>> On 2017-Jun-12, at 6:59 AM, Andrew Turner <andrew at fubar.geek.nz> wrote:
>>>
>>>> I like this. My understanding is adding armv7 would also fix many of the currently broken ports that assume they are being built for armv7 as many Linux distros target ARMv7+.
>>>>
>>>> It should also be noted the GENERIC kernel is likely to only ever target ARMv7+ even without an armv7 TARGET_ARCH.
>>>
>>>
>>> Hopefully the choices related to TARGET and TARGET_ARCH
>>> for armv7 end up identifying the context to port builds
>>> so that many would just automatically do the right thing.
>>>
>>>
>>> As for GENERIC:
>>>
>>> powerpc has. . .
>>>
>>> TARGET=powerpc TARGET_ARCH=powerpc   and GENERIC
>>> TARGET=powerpc TARGET_ARCH=powerpc64 and GENERIC64
>>>
>>> So there is precedent for more than one GENERIC*
>>> for a family, with which one being appropriate
>>> being based on TARGET_ARCH.
>>>
>>> For powerpc TARGET=powerpc implicitly uses
>>> TARGET_ARCH=powerpc when TARGET_ARCH is not
>>> specified (if I remember right). Which should
>>> be the default for armv6 vs. armv7 might go
>>> the other direction (TARGET_ARCH=armv7 by
>>> default).
>>>
>>>
>>> Side note:
>>>
>>> A caution about talking about "rpi2" as
>>> an example. . .
>>>
>>> Raspberry Pi 2 Model B V1.2 is Cortex-A53 based
>>> (so arm64/aarch64). (A BCM2837, not a BCM2836.)
>>> This dates about to something like 2014 based
>>> on the pictures showing the (c) notice on the
>>> boards.
>>>
>>> V1.1 and before were armv7 (BCM2836) based.
>>>
>>> Unless a kernel and world are made that can
>>> also configure/handle a Cortex-A53 in a
>>> armv7-like manor there will be two different
>>> GENERIC builds in order to span the "rpi2"
>>> family, based on just V1.2+ vs. V1.1 and
>>> before.
>>>
>>> (A single, modern distribution of the official
>>> Raspbian software for the rpi2 does support
>>> all the V1.x boards if I understand right.)
>>
>> I am confused. I don't see any documentation about Raspbian supporting 64 bit?
>
> 64-bit Cortex-A53's can be configure to operate in a
> 32-bit mode (AArch32). Raspian does that for RPI2 V1.2
> and for RPI3.
>
> Raspian does not (yet?) support a 64-bit mode (AArch64).
>
> The Cortex-A53 can support either. As I understand it
> is possible for an OS to allow a user processes to be
> one or the other, different processes using the different
> modes. That does not mean that all operating systems
> bother to.
>
> If I remember right the official Ubuntu for an ODroid-C2
> allows both AArch64 and AArch32 user programs (and
> so processes, with shared library types tracking).
>
>> From Arm at https://www.arm.com/products/processors/cortex-a/cortex-a53-processor.php:
>> "The Cortex-A53 supports the full ARMv8-A architecture. It not only
>> runs 64-bit applications also seamlessly and efficiently runs legacy
>> ARM 32-bit applications."
>>
>> I assume that means it handles armv7-A without issue? (In fact, many
>> on this board know it does)
>
> I've not gone through the details but targeting AArch32
> probably means targeting a subset of armv7. Or may be
> to support both requires targeting a common subset of both.
> (My guess is that AArch32 is the definition of a common
> subset for 32-bit, at least for user processes.)
>
> Raspian targets just AArch32 on armv7 and Cortex-A53
> (user space). (If I've got the definition of AArch32
> right: otherwise a common subset.)
>
> FreeBSD targets armv7 and AArch64 separately (via
> separate GENERIC kernels). I'm not aware of FreeBSD
> targeting AArch32 at all on cores capable of AArch64.
> FreeBSD possibly does not restrict itself to AArch32
> (user processes) on armv7 if AArch32 is really a
> subset.

I thought all 64 bit Arm instructions are defined in armv8?

Russ