Running/building an 32 bits kernel on A53 CPU?

[Joakim Tjernlund]

On Fri, 2021-10-29 at 14:00 +0100, Robin Murphy wrote:
> On 2021-10-29 11:32, Joakim Tjernlund wrote:
> > 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?
> 
> FWIW, if you haven't already I'd certainly reduce CONFIG_NR_CPUS to the 
> minimum you need, as oversized percpu structures can add up. I'd also 
> try hacking ARCH_DMA_MINALIGN down to 64 - provided your SoC doesn't 
> have a system cache with larger lines - as that should save a bit of 
> wastage on smaller allocations.
> 

Right, I already had CONFIG_NR_CPUS=2 (minimum allowed, why is SMP forced? I got only one core ATM).
The ARCH_DMA_MINALIGN didn't help Reserved but I guess it will help runtime later, thanks

Memory: 23924K/32768K available (3584K kernel code, 682K rwdata, 560K rodata, 320K init, 242K bss, 8844K reserved, 0K cma-reserved)