Pushing The Limits OfLinux On ARMLinuxCon Japan 2015

Andreas Färber afaerber@suse.de

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Overview

• Definition of “Linux on ARM”• Transitioning from Embedded to Servers & Desktops• Pushing the limits

‒ in general

‒ for embedded

Linux On ARM

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Quick Introduction

• Architecture by ARM Ltd. / ARM Holdings plc (UK)‒ ARM7, ARM9 used in embedded devices “for ages”

‒ ARMv7 with Cortex-A8, etc. also popular with hobbyists

‒ ARMv8 offers 64-bit AArch64 mode, attractive for servers

• Development boards (for ARMv7) abundant‒ Reminder: Android is based on the Linux kernel, but

‒ Too often unofficial “downstream” kernel forks are used

• Goal: Get support for ARM boards into the official kernel.org Linux tree (“mainline” or “upstream”)‒ To run a standard userspace distribution, e.g. openSUSE

From Embedded And MobileTo Desktop And Servers

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From Embedded To Desktop & Servers

• Traditionally used in resource-constrained devices‒ Cross-built with configuration optimized for target device

‒ Device drivers built into kernel (e.g., exynos_defconfig)

• Multi-platform support allows to build support for multiple chip platforms into a single kernel‒ Allows for generic kernel with most drivers as modules

‒ Less tested, some build failures caught via randconfig

‒ Occasional runtime failures only found when testing on device!

‒ Requires such devices to be supported by one kernel source

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The openSUSE Kernel

linux.git

linux-stable.git

kernel-source.git

kernel.git

Kernel:HEAD

Kernel:stable

openSUSE:FactoryopenSUSE:Factory:ARM

Pushing The Limits

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Where Are Limits Being Pushed?

• New ARM IP‒ Handled by ARM themselves (except for Mali graphics)

• New chip vendors‒ Linaro can help ARM licensees get started properly

• New chips, new boards‒ Vendors often distribute a Board Support Package (BSP)

including Linux kernel but don't submit their patches‒ Little or no security fixes, no new upstream features

• Old chips and boards people didn't need Linux on yet

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Simplest: Board Support

• Prereq.: SoC (platform and drivers) supported• Device Tree (.dts) is used as hardware description

‒ Either supplied to firmware (U-Boot), or by firmware (UEFI)

• ACPI upcoming as alternative hardware description‒ Supplied by firmware (UEFI)

• No .dts file for your board? Then add it yourself!‒ Peek at downstream .dts and/or similar upstream .dts files

‒ Read schematics, if available; fallback: trial-and-error

‒ Examples: exynos5250-spring.dts, zynq-parallella.dts

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Advanced: System-On-Chip Support

• Prereq.: SoC family already supported• .dtsi file will reference at least one new driver• Technical Reference Manual or vendor tree needed

‒ Compare chip-specific bits to those of one already supported

‒ Peek at earlier and later .dtsi files and drivers? (Careful!)

• Example: Samsung Exynos 5410

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Expert: New System-on-Chip Support

• Prereq.: CPU architecture already supported (ARM)• Technical Reference Manual really needed

‒ Serial console driver – to get output

‒ Clocksource driver(s) – for timers

‒ Clock driver

‒ Reset driver?

‒ Memory controller?

• Example: STM32 F4

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“Some things have to be done just because they are possible.Not because they make sense.”

- SUSE Hackweek T-Shirt

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The Final Frontier (1/2)

Cortex-A8Cortex-A9

Cortex-A5Cortex-A7

Cortex-A15

Cortex-A17

Cortex-A12

Cortex-A53

Cortex-A57Cortex-A72

ARM11ARM9

ARM7

BCM2835OMAP3xxx……

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The Final Frontier (2/2)

Cortex-M3Cortex-M1

Cortex-M0

Cortex-M4

Cortex-M7

Cortex-M0+

EFM32 GG

LPC18xx

STM32 F4

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Address Size Usage

08000000 16 KB Bootloader

08004000 16 KB Device Tree

08008000 16 KB Kernel

0800c000 16 KB

08010000 64 KB

08020000 128 KB

08040000 128 KB

08060000 128 KB

08080000 128 KB

080a0000 128 KB

080c0000 128 KB

080e0000 128 KB

08100000 16 KB

08104000 16 KB

08108000 16 KB

0810c000 16 KB

08110000 64 KB

08120000 128 KB

08140000 128 KB Root filesystem

... ...

STM32 F429 – Flash Layout

• Custom bootloader (1.6 / 16 KB)‒ github.com/afaerber/afboot-stm32

‒ Compare: U-Boot ~75 KB / 128 KB

• Device Tree (2.9 / 16 KB)• Kernel (1235.5 / 1248 KB)

‒ Execute-in-place (XIP) image‒ Me: clk, clocksource, serial, fbdev

‒ Maxime: clocksource, reset, serial, pinc.

‒ Daniel: clk

• romfs (293.1 / 768 KB)

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Bootloader tasks on ARMv7-M

• Usually runs from flash, using SRAM• Initializes clocks• Configures pins• Sets up serial port (for debug output)• Sets up memory controller for SDRAM or PSRAM

• If necessary, takes timers out of reset• Jumps to the kernel entry point

‒ Start of xipImage plus 1 for THUMB mode

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Summary

• ARMv7-A chips usually well supported these days‒ But boards not always!

• ARMv7-M chips often not supported by vendors• What ARMv7-M boards may be able to run Linux:

‒ 4+ MB of SDRAM/PSRAM‒ Chip needs some external memory interface to enable it

‒ 1+ MB of flash‒ Memory-mapped flash allows execution in place, saves RAM

‒ Secondary storage needed?‒ SPI, MMC, network, …

One More Thing …

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Spansion FM4 – StatusAddress Size Usage

00000000 8 KB Bootloader

00002000 8 KB Device Tree

00004000 8 KB Kernel

8 KB

32 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

64 KB

8 KB

... ...

• OpenOCD 0.9.0‒ But no working flash driver yet

• Custom bootloader (1.5 / 8 KB)‒ github.com/afaerber/afboot-fm4

• Device Tree (0.8 / 8 KB)• Kernel (1070 / 2032 KB)

‒ clocksource (WIP)

• 2 MB of on-chip flash available

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Infineon XMC4500 – Status

Address Size Usage

0c000000 16 KB Bootloader

0c004000 16 KB Device Tree

0c008000 Kernel

• OpenOCD 0.9.0‒ Flash driver by Jeff Ciesielsky

• Custom bootloader (2.4 / 16 KB)‒ github.com/afaerber/afboot-xmc4000

• Device Tree (3.7 / 16 KB)• Kernel (~986.6 / 992 KB)

‒ clk, clocksource, reset, pinctrl, serial, sdhci drivers

• 1 MB of on-chip flash available only

Questions?

Thank you.

Join us on #opensuse-arm oropensuse-arm@opensuse.org!en.opensuse.org/Portal:ARM

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Have a Lot of Fun, and Join Us At:www.opensuse.org

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