[PATCH v2] ARM: new platform for Energy Micro's EFM32 Cortex-M3 SoCs

[Arnd Bergmann]

On Saturday 28 September 2013, Uwe Kleine-König wrote:
> Hi Arnd,
> 
> On Fri, Sep 27, 2013 at 11:44:01PM +0200, Arnd Bergmann wrote:
> > On Thursday 26 September 2013, Uwe Kleine-König wrote:
> > > I made that work now and can prepare a patch. I had to drop "depends on
> > > !ARCH_MULTIPLATFORM" from XIP_KERNEL. That's because my machine only
> > > works with XIP_KERNEL as it only has 4 MiB of RAM.
> > 
> > Ok, cool. We might run into a few problems with 'make randconfig' and
> > 'make allyesconfig' when it becomes possible to enable XIP_KERNEL then.
> > IIRC, there is no fundamental reason to disallow XIP_KERNEL with
> > ARCH_MULTIPLATFORM, but I added the dependency because it causes
> > build errors in combination with other options.
> ah, OK. Do you have an idea to fix both?

No, I don't actually remember what problems I ran into. It may be anywhere
from trivial to impossible to fix.

> > * Do you think 4MB is now a strict lower bound for running a modern
> > kernel? It would be a good data point if we could show that any target
> > with less than that is by definition broken and could get removed
> > from the kernel. What is the size of your kernel and user space?
> $ objdump -p vmlinux
> 
> vmlinux:     file format elf32-littlearm
> 
> Program Header:
>     LOAD off    0x00000000 vaddr 0x88020000 paddr 0x88020000 align 2**15
>          filesz 0x00000094 memsz 0x0000a9f4 flags rw-
>     LOAD off    0x00008000 vaddr 0x8c000000 paddr 0x8c000000 align 2**15
>          filesz 0x001679b0 memsz 0x001679b0 flags rwx
>     LOAD off    0x00170000 vaddr 0x88008000 paddr 0x8c1679b0 align 2**15
>          filesz 0x00018d2c memsz 0x00018d2c flags rw-
> private flags = 5000002: [Version5 EABI] [has entry point]
> 
> my rootfs (busybox, no init) is 153600 bytes big.
> 
> After booting I get:
> 
> 	/ # free
> 		     total         used         free       shared      buffers
> 	Mem:          3892         1428         2464            0            0
> 	-/+ buffers:               1428         2464
> 
> but it doesn't run anything but a busybox shell ATM. Assuming the next
> smaller configuration is 2 MiB of RAM I'd say that machine can maybe
> boot, but cannot do anything sensible after that.

Ok, thanks for that data! I can still imagine embedded applications where
you have a custom /sbin/init that does just one thing even with 2MB, but
it's good to know that a 4MB system is still basically usable with some
free memory left for workloads.

> > * What user space are you running? Anything that's easy to build
> > for testing? Should that run with a mach-virt kernel built for
> > ARMv7-A NOMMU?
> There is a BSP publically available at
> 
> 	http://git-public.pengutronix.de/?p=OSELAS.BSP-EnergyMicro-Gecko.git;a=summary
> 
> which also includes a README file. For troubleshooting /join #efm32 on
> freenode.

I've never tried ptxdist, but if that is known to work fine with NOMMU,
I might just try building the base distro and running it on mach-virt.

Does this work with ELF FDPIC or do you need binfmt-flat?

> > * An ARMv7-M kernel cannot run on either ARMv4/v5 nor ARMv6/v7-A, right?
> The entry convention is different (ARMv7-M doesn't support the ARM
> instruction set but you need to jump into the kernel in ARM mode for
> v4-v7). Other that that I don't know if there is a problem. Maybe
> Jonathan can say anything here? Or alternatively if you want an efm32
> devboard, just tell me.

I haven't started a test farm like Olof has yet. I'll have to think about
whether I want to have my own, but he might also be interested in adding
a NOMMU target to his collection.

> > Do you prevent building such a kernel in Kconfig?
> I'm sure my Kconfig magic isn't waterproof. It took me a few tries to
> expand the multiarch architecture selection to make v7-m selectable at
> all.

Ok, I can have a look and give you suggestions for how it needs to be
phrased if it currently allows broken (non-building) combinations.

	Arnd