Arnd Bergmann, LCU14

LCU14-109: Embedded Systems:How Low Can You Go?

Small ARM Systems past and present

Sony XPeria Z2:3GB RAM, Krait

$33 Intex Cloud FX128MB RAM, Cortex-A5

WiFI SDIO card32MB RAM, ARM926

Psion Series 5,8-16MB RAM, ARM710

Linksys NSLU2,32MB RAM, XScale

Apple iPod, 32MB RAM,ARM7TDMI (no MMU)

D-Link DIR-61532 MB RAM, Feroceon

Typical embedded system• Classic use case for ARM (also MIPS, PowerPC, …)• Driving factor: cost reduction• Power consumption: 1W-10W

Typical Implementation:• ARM926EJ or Cortex-A5 CPU• Standard kernel, custom configuration• Busybox, uhttpd, dropbear• Distro: OpenEmbedded, OpenWRT, Buildroot, …• libc: uClibc, musl, dietlibc, newlib, …

Scaling down to 32MB of RAM

Large microcontroller• Upstream support for EFM32 and AT91x40• Driving factor: Power usage• Power consumption: 100mW-1W

Typical Implementation:• ARM7TDMI or Cortex-M3 CPU, no MMU!• (P)SRAM rather than DRAM• Customized user space• Execute-in-place kernel in NOR flash

Scaling down to 2MB of RAM

Single-chip microcontroller system• Experiment done by Vitaly Wool• Running on STM32F29 internal SRAM+NOR• Power consumption: < 100mW

Implementation:• Highly customized kernel• XIP kernel and R/O file system• RW data compressed to fit on flash• Very little user space

Scaling down to 256KB of RAM

Alternatives:• Open source: FreeRTOS, eCos, RTEMS, Minix3, BSD…

• Proprietary RTOS

Linux advantages:• Developer familiarity• Driver support• Network stack

Linux Disadvantages:• Code size

Why Linux

Alternatives:• With MMU: MIPS, PowerPC, OpenRISC, x86, m68k, ...• Without MMU: C6x, Blackfin, Xtensa, Microblaze, …

ARM Advantages:• Widely available from many vendors• Developer community• Linaro

ARM Disadvantages:• No MMU on low-power cores

Why ARM

ARMv4T/V5E:• ARM7TDMI almost dead• ARM9TDMI, ARM740T, ARM940T, ARM946E unused• Out-of-tree users on 2.4.xx

ARMv7-R/M:• EFM32 support recently added• ARMv7-R patches existed but were never merged• Out-of-tree users on 2.6.33

NOMMU mode for normal v5/v6/v7-A CPUs• Sometimes builds and works, largely untested

State of NOMMU-Linux on ARM

Kernel size growth on ARMvm

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• What is the smallest ARM system that Linaro and/or the upstream kernel community should care about?

• Should we work on actively making kernels smaller?

• Should we do more testing on MMU-less systems or remove the ones we don’t use?

• Are there any Cortex-M platforms that need upstream support?

• Is anybody still using ARM7TDMI on recent kernels?

Discussion

Embedded Linux Wiki page on small systems: http://elinux.org/System_Size

Linux-2.6.33 on various Cortex-M3/M4: https://github.com/EmcraftSystems/linux-emcraft

Historic uClinux-2.4 patches for ARM7TDMI: http://www.uclinux.org/pub/uClinux/uClinux-2.4.x/

PTXDIST for linux-3.16 with EFM32/Cortex-M3: http://git-public.pengutronix.de/?p=OSELAS.BSP-EnergyMicro-Gecko.git

Resources

More about Linaro Connect: http://connect.linaro.orgMore about Linaro: http://www.linaro.org/about/

More about Linaro engineering: http://www.linaro.org/engineering/Linaro members: www.linaro.org/members