[Arm-dev] Why RPi3 under Arm32 rather than AArch64?

[Jim Perrin]

On 01/04/2018 10:26 AM, Christopher Ursich wrote:
> Hi, all.  First-timer here.
> 
> I am setting up a new Raspberry Pi 3.  When I review the AltArch pages,
> I see that most of the RPi3 coverage is categorized under Arm32, including
> 
>   https://wiki.centos.org/SpecialInterestGroup/AltArch/Arm32/RaspberryPi3
> 
> I'm under the impression that the RPi3 is actually a AArch64 system. 
> I'm not sure whether I should use
> 
>  
> http://mirror.centos.org/altarch/7/isos/armhfp/CentOS-Userland-7-armv7hl-Minimal-1708-RaspberryPi3.img.xz
> 
> versus
> 
>  
> http://mirror.centos.org/altarch/7/isos/aarch64/CentOS-7-aarch64-rootfs-7.4.1708.tar.xz
> 
> Can a more-experienced person clarify this for me?
> 

The aarch64 build we produce was aimed at server systems like Cavium's
ThunderX initially. There are some limitations with the aarch64
distribution kernel (currently based on RH's kernel-alt 4.11) which make
it terrible to run on the rpi.

1. It's set up for 64k pagesize. This, combined with some CMA math in
the kernel means that you'll take a pretty severe memory hit when
running the CentOS aarch64 build on the rpi. In some cases this memory
hit consumes nearly all of the available ram.

2. The distro is built to expect ACPI. While you can disable it and
it'll work okay, it generates quite a number of errors in the logs.


In short, the rpi currently works better (by default) as a 32bit setup
and we want users to have a better experience.


-- 
Jim Perrin
The CentOS Project | http://www.centos.org
twitter: @BitIntegrity | GPG Key: FA09AD77