The document discusses porting Linux to microcontrollers with low memory and storage. It describes how Linux can leverage the microcontroller development environment and avoid fragmentation by using the device tree to describe hardware instead of coding it directly into the kernel. The document recommends starting with a known Linux configuration like stm32_defconfig and using the Kconfig menuconfig tool to customize it for the specific microcontroller.

1. Linux 4.6 em
Microcontroladores:
Um caso prático.
CPS | CERTI 2016
COPYRIGHT 2016 – Fundação CERTI

2. COPYRIGHT 2016 – Fundação CERTI
DISCLAIMER
The information in this presentation was
compiled from sources believed to be
reliable for informational purposes only.
Content includes opinions, presentations,
articles, hyperlinks or other third party
content (“Third Party Material”) that is not
intended to, nor constitutes an
endorsement by CERTI of the author or the
Third Party Materials. The content and views
within the Third Party Material are solely
those of the third party and do not reflect
the opinions of CERTI.
The opinions expressed in this presentation
and on the following slides are solely those
of the
presenter and not necessarily those of
CERTI. CERTI does not guarantee the
accuracy or
reliability of the information provided
herein.
Presentation Notes

3. WARM
UP
www.linkedin.com/in/bherrera
COPYRIGHT 2016 – Fundação CERTI
Present how Linux can be ported to
MCUs with low memory and low
storage how can it leverage the
MCU development environment
and avoid fragmentation(kind of).
Goal
BrunoMHerrera

4. WARM
UP
www.linkedin.com/in/bherrera
COPYRIGHT 2016 – Fundação CERTI
I’m a Computer Engeneer
I’m a Computer Enginere
I’m a Computer Engenere
I’m good with Computers ….
Who I Am
BrunoMHerrera

5. MCU
Chapter 1
Action
Chapter 4
Linux
Chapter 2
Wrap up
Chapter 5
WIP
Chapter 3
AG
EN
DA
COPYRIGHT 2016 – Fundação CERTI

6. COPYRIGHT 2016 – Fundação CERTI
Photo credit: Adam Greig via VisualHunt / CC BY-SA
MCU – Microcontroller Unit
“Microcontrollers are
designed for embedded
applications, in contrast to the
microprocessors used in
personal computers or other
general purpose applications
consisting of various discrete
chips.”
-Wikipedia

7. COPYRIGHT 2016 – Fundação CERTI
Microcontrollers
usually contain MPU
(memory protection
unit)
MMU-less
Reduced RAM <192k
Flash (ROM) <2Mb
ASP 2014 - $0.83
ASP 2015 - $0.65
*7 to 8 times in Brazil
http://www.icinsights.com/news/
bulletins/Microcontroller-Unit-
Shipments-Surge-But-Falling-Prices-
Sap-Sales-Growth/
Microcontrollers usually
contain from several to
dozens of general
purpose input/output
pins (GPIO). GPIO pins
are software
configurable to either
an input or an output
state.[1]
Microcontrollers
usually contain
several peripherals
integrated (U[S]ART,
ADC, USB, SDIO,
etc…)
Automation
Healthcare
Automotive
Aerospace
Wellness
Internet of Things
MCU - Traits
Small Footprint Low Price
ApplicationsSoCGPIO

8. COPYRIGHT 2016 – Fundação CERTI
BSP(board support package) / HAL
(hardware abstraction layer)
provided by the “Chip” vendor
Hardware dependent code, not
portable
No operating system
Hard to port applications
Hard to update on the field
Do not forget: it is all about
registers!!
without
addition; basic
and simple
Chapter 1 | MCU
Bare Metal

9. COPYRIGHT 2016 – Fundação CERTI
eLua www.eluaproject.net
Embedded power, driven by Lua
Quickly prototype and develop
embedded software applications
with the power of Lua and run
them on a wide range of
microcontroller architectures
MicroPython micropython.org
MicroPython is a lean and efficient
implementation of the Python 3
programming language that includes a
small subset of the Python standard
library and is optimised to run on
microcontrollers and in constrained
environments.
Netduino www.netduino.com
Netduino, an open-source
electronics platform using
the .NET Micro Framework.
Cont: JavaME,
Chapter 1 | MCU
Bare Metal
Not so bare

10. COPYRIGHT 2016 – Fundação CERTI
ATmega328P
14 Digital I/O Pins
6 PWM Digital I/O Pins
6 Analog Input Pins
32 KB Flash Memory of which 0.5 KB
used by bootloader
2KB SRAM
1KB EEPROM
16MHz Clock Speed
U$ 3.38
Chapter 1 | MCU
Arduino Uno
“The UNO is the best board to
get started with electronics
and coding. If this is your first
experience tinkering with the
platform, the UNO is the most
robust board you can start
playing with”
https://www.arduino.cc/en/Main/
ArduinoBoardUno

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Real Time
Operating
System
FreeRTOS www.freertos.org
FreeRTOS is the market leading real
time operating system (or RTOS), and
the de-facto standard solution for
microcontrollers and small
microprocessors.
Nuttx nuttx.org
NuttX is a real-time operating system
(RTOS) with an emphasis on standards
compliance and small footprint. Scalable
from 8-bit to 32-bit microcontroller
environments, the primary governing
standards in NuttX are Posix and ANSI
standards.
Zypher www.zephyrproject.org
Zephyr Project is a small, scalable real-
time operating system for use on
resource-constrained systems supporting
multiple architectures.
Cont: Contiki (IoT), BROS, ChibiOs
Chapter 1 | MCU
RTOS

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VxWorks windriver.com/products/vxworks
VxWorks is designed for use in embedded
systems requiring real-time, deterministic
performance and, in many cases, safety
and security certification, for industries,
such as aerospace and defense, medical
devices, industrial equipment, robotics,
energy, transportation, network
infrastructure, automotive, and consumer
electronics.[X]
Nucleus mentor.com/embedded-software/nucleus/
The Nucleus RTOS is designed for deeply
embedded systems applications
including consumer electronics, set-top
boxes, cellular phones, and other
portable and handheld devices. For
limited memory systems Nucleus RTOS
can be scaled down to a memory
footprint as small as 13 KB for both code
and data.
Cont: QNX, SAFERTOS,
Chapter 1 | MCU
RTO$
Real Time
Operating
System

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“as one of key benefits of an MMU:
an operating system can use it to
protect against errant programs by
disallowing access to memory that
a particular program should not
have access to”
“Given the need for a high security
bar in IoT we try and design Brillo in
way that promotes a strong security
architecture in the final products.
I’m afraid if we offered a MMU-less
variant of Brillo we'd be promoting
a very insecure architecture, and for
that reason its not something we'd
consider.”
[Brillo Mailist]
Chapter 1 | MCU
MMU-less
WARNING:
No memory
management!

14. COPYRIGHT 2016 – Fundação CERTI
you-see-linux
www.uclinux.org
The Embedded Linux/
Microcontroller project is a port of
Linux to systems without a
Memory Management Unit
(MMU).
No elf support
Not in mainstream(2.6.x)
uClibc
Emcraft (Commercial and
Opensource)
https://github.com/
EmcraftSystems/linux-emcraft/
Chapter 1 | MCU
uClinux

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Photo credit: Derek K. Miller via VisualHunt.com / CC BY-NC
Linux
“That's what makes
Linux so good: you put
in something, and that
effort multiplies. It's a
positive feedback
cycle.”
- Linus Torvalds

16. COPYRIGHT 2016 – Fundação CERTI
Linus was
wrong
Chapter 2 | Linux
History
https://groups.google.com/forum/#!msg/comp.os.minix/
dlNtH7RRrGA/SwRavCzVE7gJ

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Video
Chapter 2 | Linux
History
The Story of Linux:
Commemorating 20 Years of the
Linux Operating System

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Chapter 2 | Linux
History
1995
> BusyBox a collection of command
line utilies - Bruce Perens (1.44Mb)
Debian Installer
> MIPS
1996
> M68k, PPC
1998
> uClinux at m68k DragonBall on
3Com palm pilot
1999
> ARM
2000
> iPaq H3600 running X11
2005
> Nokia 770 Internet Tablet
running Maemo Linux
embedded

19. COPYRIGHT 2016 – Fundação CERTI
Chapter 2 | Linux
History
1995
> BusyBox a collection of command
line utilies - Bruce Perens (1.44Mb)
Debian Installer
> MIPS
1996
> M68k, PPC
1998
> uClinux at m68k DragonBall on
3Com palm pilot
1999
> ARM
2000
> iPaq H3600 running X11
2005
> Nokia 770 Internet Tablet
running Maemo Linux
embedded

20. COPYRIGHT 2016 – Fundação CERTI
Pros
Chapter 2 | Linux
Why Linux?
Strong Community / Big Players
contributing / Stable versions and
release cycles
Portable (ARM Cortex M3/M4
support is now mainstream)
POSIX Compliant
OSS Environment and Tools
Huge drivers support
XIP (execution in Place)
Free

21. COPYRIGHT 2016 – Fundação CERTI
Cons
Chapter 2 | Linux
Why not
Linux?
Big footprint (linux-tiny)
Latency / Not real time (linux-rt)
Bloated subsystems (procps,
printk, comand line parser)
Not designed for MCU : “The Linux
community is skeptical about
MCUs “[x]
Too many config parameters, hard
to find the optimal configuration
Do try this without and external
RAM (8Mb)

22. COPYRIGHT 2016 – Fundação CERTI
www.devicetree.org
The devicetree is a data structure
for describing hardware. Rather
than hard coding every detail of a
device into an operating system,
many aspects of the hardware can
be described in a data structure that
is passed to the operating system at
boot time.
The kernel no longer contains the
description of the hardware,
it is located in a separate binary: the
device tree blob
The Device Tree Blob is produced by
the compiler, and is
the binary that gets loaded by the
bootloader and parsed by
the kernel at boot time.
model = “TDC 2016 Linux Embarcado”;
Chapter 2 | Linux
Device Tree

23. COPYRIGHT 2016 – Fundação CERTI
www.devicetree.org
The devicetree is a data structure
for describing hardware. Rather
than hard coding every detail of a
device into an operating system,
many aspects of the hardware can
be described in a data structure that
is passed to the operating system at
boot time.
The kernel no longer contains the
description of the hardware,
it is located in a separate binary: the
device tree blob
The Device Tree Blob is produced by
the compiler, and is
the binary that gets loaded by the
bootloader and parsed by
the kernel at boot time.
model = “TDC 2016 Linux Embarcado”;
Chapter 2 | Linux
Device Tree

24. COPYRIGHT 2016 – Fundação CERTI
make menuconfig
Chapter 2 | Linux
KConfig
The configuration database is a
collection of configuration
options
organized in a tree structure
[Kernel Doc]
“Can't use strategy of manual
tuning(i.e.config options)
3.9 has about 13,000 options”
[X Tim Bird]
Start from one known
configuration
stm32_defconfig
Don’t Panic

25. COPYRIGHT 2016 – Fundação CERTI
Chapter 2 | Linux
BootLoader
Essentially, the boot loader should
provide (as a minimum) the
following:
1. Setup and initialize the RAM.
2. Initialize one serial port.
3. Detect the machine type.
4. Setup the kernel tagged list.
5. Load initramfs.
6. Call the kernel image.
https://www.kernel.org/doc/Documentation/arm/Booting
Run Lola Run

26. COPYRIGHT 2016 – Fundação CERTI
fast
Chapter 2 | Linux
BootLoader
Forget about u-Boot! It is big!
Directly load Linux kernel by
really small program
AFBoot (only works with XIP now)
Tiny Linux bootloader for the
STM32F429-Discovery
https://github.com/mcoquelin-
stm32/afboot-stm32
Bootloader + Kernel in less than
1sec. (I can prove)

27. COPYRIGHT 2016 – Fundação CERTI
Chapter 2 | Linux
BootLoader
Essentially, the boot loader should
provide (as a minimum) the
following:
1. Setup and initialize the RAM.
2. Initialize one serial port.
3. Detect the machine type.
4. Setup the kernel tagged list.
5. Load initramfs.
6. Call the kernel image.
https://www.kernel.org/doc/Documentation/arm/Booting
Eclipse

28. COPYRIGHT 2016 – Fundação CERTI
Chapter 3 | Wip
mainstream
STM32F429 support added to
mainstream kernel by Maxime
Coquelin at 2015
Peripherals already supported:
RCC
GPIO
RNG
UART
Ethernet
DMA
On Going:
USB
I2C
SDIO
WIP

29. COPYRIGHT 2016 – Fundação CERTI
Chapter 3 | Wip
mainstream
https://patchwork.kernel.org/
project/linux-arm-kernel/list/?
q=stm32
WIP

30. COPYRIGHT 2016 – Fundação CERTI
80386DX
32Bits MPU
1985
12 MHz to 40 MHz
FPU Co Processor 80387
STM32F429
32Bits MCU
??
Up to 180MHz
FPU
192Kb RAM
512Kb to 2Mb FLASH
MAC(Ethernet), SDIO, TFT, USB, SERIAL
tale of the tape
Clash of Titans

31. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
Target
Action

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Chapter 4 | Action
Tools
Toolchain
https://launchpad.net/gcc-arm-
embedded/4.9/4.9-2014-q4-major
sudo add-apt-repository ppa:terry.guo/
gcc-arm-embedded
sudo apt-get update
sudo apt-get install gcc-arm-none-eabi
Open OCD
git clone git://git.code.sf.net/p/openocd/
code openocd
cd openocd
git submodule init && git submodule
update && ./bootstrap && ./configure --
enable-stlink && make && sudo make
install
Action

33. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
Talk is cheap…
git clone git://git.kernel.org/pub/scm/linux/
kernel/git/torvalds/linux.git
cd linux
make ARCH=arm CROSS_COMPILE=arm-
none-eabi- stm32_defconfig
make ARCH=arm CROSS_COMPILE=arm-
none-eabi-
Action

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Chapter 4 | Action

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Chapter 4 | Action

36. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action

37. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
GPIO
What is a GPIO?
===============
A "General Purpose Input/Output" (GPIO) is
a flexible software-controlled
digital signal. They are provided from many
kinds of chip, and are familiar
to Linux developers working with
embedded and custom hardware. Each
GPIO
represents a bit connected to a particular
pin, or "ball" on Ball Grid Array
(BGA) packages. Board schematics show
which external hardware connects to
which GPIOs. Drivers can be written
generically, so that board setup code
passes such pin configuration data to
drivers.
https://www.kernel.org/doc/Documentation/gpio/gpio.txt
Action

38. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
GPIO
The famous hello world “led blink”
at user space
gpio = (bank * 16) + pin
gpio = 6 * 16 + 14(LD4) = 110
cd /sys/class/gpio
echo 110 > export
echo out > gpio110/direction
echo 1 > gpio110/value
Action

39. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
GPIO
The famous hello world “led blink”
at user space (cont)
while echo blink
> do
> echo 1 > gpio110/value; sleep 1
> echo 0 > gpio110/value; sleep 1
> done
Action

40. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
LEDs
LED handling under Linux
========================
In its simplest form, the LED class
just allows control of LEDs from
userspace. LEDs appear in /sys/
class/leds/. The maximum
brightness of the
LED is defined in max_brightness
file. The brightness file will set the
brightness
of the LED (taking a value 0-
max_brightness). Most LEDs don't
have hardware
brightness support so will just be
turned on for non-zero brightness
settings.
https://www.kernel.org/doc/Documentation/leds/leds-class.txt
Action

41. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
LEDs
The famous hello world “led blink”
using the DT
Action

42. COPYRIGHT 2016 – Fundação CERTI
Chapter 4 | Action
Optimization
The tip of the iceberg
./scripts/bloat-o-meterAction

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Photo credit: Derek K. Miller via VisualHunt.com / CC BY-NC
Wrap Up
“People who are really
serious about software
should make their own
hardware”
- Alan Kay

44. COPYRIGHT 2016 – Fundação CERTI
Chapter 5 | Wrap Up
When developing your hardware
select components that are
supported (device drivers)
Linux has a lot of space for
optimization and tinyfication
Cannot run without external
RAM(yet)
Hardware is hard, so take care of
your design.
Try first in development boards
One size does not fit all! Linux may
not be the best option for your
case
Take away

45. COPYRIGHT 2016 – Fundação CERTI
References
https://en.wikipedia.org/wiki/Microcontroller
https://events.linuxfoundation.org/sites/events/files/slides/
petazzoni-device-tree-dummies.pdf
http://2net.co.uk/embedded-history
http://elinux.org/STM32
http://www.emcraft.com/stm32f429discovery/controlling-gpio-
from-linux-user-space
http://elinux.org/Kernel_Size_Tuning_Guide

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Thanks!
Obrigado!

47. COPYRIGHT 2016 – Fundação CERTI
Sorteio
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