[RFC PATCH 11/11] ARM: Allow ARCH_MULTIPLATFORM to be selected for NOMMU

On 22/11/16 10:17, Arnd Bergmann wrote:

On Tuesday, November 22, 2016 9:26:08 AM CET Vladimir Murzin wrote:

quoted

With this patch applied potentially any platform can be built in NOMMU
configurations if CONFIG_EXPERT is selected. However, there is no
guaranty that platform can successfully run such Image. So the main
motivation behind of this patch:
- bring build coverage for NOMMU configurations
- allow known working NOMMU platforms (like R-class) to be used
- pave a way to add support for single address space (aka 1:1 mapping)
  for MMU platforms, so they can be usable in NOMMU configurations

Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ryan Mallon <redacted>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Thierry Reding <redacted>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Vladimir Murzin <redacted>

I'd have to give this a spin with my randconfig build setup, I'd
rather not introduce build regressions. Have you tried an
allmodconfig build with CONFIG_MMU disabled?

I used defconfigs and just got results for allmodconfig apart of complain on
isb instruction in arch/arm/kernel/head-nommu.S [1] there are several link
time errors [2].

Can you provide a git tree that I can try pulling in?

Unfortunately, I can't provide you with git tree at the moment I'll try to
do something around this before proposing the next version.

Another question is what architecture levels and what platforms
we want to support without MMU. The only ARMv4/v5 platform we
still have that can actually use NOMMU cores is Integrator
with its ARM7TDMI, ARM920T and ARM966E core tiles (and possibly
others I couldn't immediately find). Do we actually care about
them any more now that all the NOMMU world is ARMv7-M? Are
there any benefits in running an ARM920T or ARM926E core
with MMU disabled, and does this work with your patches?

I don't have such hardware, so I can't acctually test it - it is why "there is
no guaranty" :( OTOH, if sombody has these platforms these pathces is a good
start to try NOMMU.

If not, we could limit it to ARMv7-A/R and possibly ARMv6.
Depending on how the build tests go, a per-platform opt-in
might be easier than having an opt-out for things that
don't work.

	Arnd

[1]
  AS      arch/arm/kernel/head-nommu.o
arch/arm/kernel/head-nommu.S: Assembler messages:
arch/arm/kernel/head-nommu.S:223: Error: selected processor does not support ARM mode `isb'
arch/arm/kernel/head-nommu.S:231: Error: selected processor does not support ARM mode `isb'
arch/arm/kernel/head-nommu.S:235: Error: selected processor does not support ARM mode `isb'
arch/arm/kernel/head-nommu.S:244: Error: selected processor does not support ARM mode `isb'
arch/arm/kernel/head-nommu.S:248: Error: selected processor does not support ARM mode `isb'
arch/arm/kernel/head-nommu.S:258: Error: selected processor does not support ARM mode `isb'
arch/arm/kernel/head-nommu.S:265: Error: selected processor does not support ARM mode `isb'
make[1]: *** [arch/arm/kernel/head-nommu.o] Error 1

[2]
arch/arm/kernel/head-nommu.o: In function `secondary_startup':
(.text+0x1c): undefined reference to `__setup_mpu'
arch/arm/kernel/head-nommu.o: In function `stext':
(.head.text+0x30): undefined reference to `__setup_mpu'
arch/arm/kernel/built-in.o: In function `setup_arch':
arch/arm/kernel/smccc-call.o:(.init.text+0xa50): undefined reference to `erratum_a15_798181_init'
kernel/built-in.o: In function `kimage_free_entry':
memremap.c:(.text+0xd3d9c): undefined reference to `arch_phys_to_idmap_offset'
kernel/built-in.o: In function `kimage_alloc_page':
memremap.c:(.text+0xd4338): undefined reference to `arch_phys_to_idmap_offset'
kernel/built-in.o: In function `kimage_alloc_control_pages':
memremap.c:(.text+0xd4ac8): undefined reference to `arch_phys_to_idmap_offset'
kernel/built-in.o: In function `kimage_load_segment':
memremap.c:(.text+0xd4f40): undefined reference to `arch_phys_to_idmap_offset'
kernel/built-in.o: In function `crash_free_reserved_phys_range':
memremap.c:(.text+0xd50bc): undefined reference to `arch_phys_to_idmap_offset'
arch/arm/mach-mediatek/built-in.o: In function `__mtk_smp_prepare_cpus':
mediatek.c:(.init.text+0xe8): undefined reference to `secondary_startup_arm'
arch/arm/mach-qcom/built-in.o: In function `qcom_smp_prepare_cpus':
platsmp.c:(.init.text+0xe8): undefined reference to `secondary_startup_arm'
mm/built-in.o: In function `do_mmu_notifier_register':
usercopy.c:(.text+0x34d10): undefined reference to `mm_take_all_locks'
usercopy.c:(.text+0x34d9c): undefined reference to `mm_drop_all_locks'
usercopy.c:(.text+0x34de4): undefined reference to `mm_take_all_locks'
make: *** [vmlinux] Error 1

Cheers
Vladimir