Running/building an 32 bits kernel on A53 CPU?

[Joakim Tjernlund]

On Fri, 2021-10-29 at 11:09 +0100, Catalin Marinas wrote:
> On Fri, Oct 29, 2021 at 08:27:35AM +0000, Joakim Tjernlund wrote:
> > In order to reduce RAM usage we are wondering if running an 32 bits
> > kernel on ARM A53 CPU is a good idea?
> 
> No. Really.
> 
> What's the RAM size on your SoC?

32MB + 4MB SRAM

> 
> > Is it possible to build such a kernel? If so, how to go about it?
> > Should one use ARCH=arm and a 32 bits toolchain or
> > ARCH=arm64 and 64 bits toolchain and adding 32 bits compile options?
> 
> If you want a 32-bit kernel, you'd have to use ARCH=arm and a 32-bit
> toolchain. This would give you an ARMv7 kernel that can run on ARMv8
> hardware. But you'd miss errata workarounds for example and any other
> features that come with ARMv8.
> 
> I don't recommend this at all. If you have some specific embedded
> system, you can disable kernel features you don't need to make it
> smaller. Also note that you can still run 32-bit applications with a
> 64-bit kernel.
> 
I have built the smallest kernel I can(so I think ATM), disabling all the features we
don't need. We do need IPv4/IPv6 though so that builds size.

ATM the my memory consumption is(without starting any user space):
  Memory: 23924K/32768K available (3584K kernel code, 688K rwdata, 568K rodata, 320K init, 244K bss, 8844K reserved, 0K cma-reserved)

The reserved size is about twice the size of static kernel use, any way to lower the dynamic part?