ERTS 2008 - Using Linux for industrial projects

C. Charreyre

Using Linux for industrial
projects
Return of experience
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TIME SOFTWARE Using Linux in industrial projects - ERTS 2008 - February 1st, 2008

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EMBEDDED REAL

CIO Informatique Industrielle

Company dedicated to industrial software development
Incorporated in 1990
15 engineers to help customers on embedded and real time
software
Great investment on Linux technologies since 2000
Competence center created in 2001

Located in St Etienne and Marseille

Member of Libertis, association of free software service
companies in PACA

4th European
Congress ERTS http://www.libertis.org
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Linux on the embedded market

Linux is a fast growing actor on the embedded market
It is know representing about 50% of the market

Source
Embedded Linux Market Survey 2007 LinuxDevices.com
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Linux on the embedded market

The Arm processor is the most used architecture with Linux
Linux + Arm is a great solution for low price mass market devices

Source
Embedded Linux Market Survey 2007 LinuxDevices.com

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Linux at CIO
Typical projects :
Software developments for industrials or aerospace/defense
With real time and embedded characteristics

OS used until 2000 :
Legacy RTOS like OS9, VxWorks, QNX etc....

Investment on Linux technologies in 2000
A step by step approach :
Use of Linux distribution on PC (desktop, PC104, SBC, Compact PCI)
Home made reduced Linux on same architecture
Use of real time extensions to get same performances as legacy RTOS
Use on other architectures used in projects : PowerPC, Arm
4th European
Congress ERTS We suggest such an approach to new adopters (from easiest to
EMBEDDED REAL most difficult)

Linux for embedded device

Requirements of embedded projects :
Processor architecture can be Intel, but also PPC, Arm, microcontroller ...
Linux available on many architectures used in embedded devices : X86,
Freescale, Arm, SuperH, AVR32, Blackfin ... (24 arch. In 2.6.23)
Low memory and storage requirements
Linux distributions are huge (Gigabytes), and require Megabytes of memory
The good approach is to build a reduced embedded image, with the precise
requirements of the project
By rebuilding a dedicated embedded image, one can reach sizes like 4 MB of
mass storage and 8 MB of memory
Flash support
Compact Flash or Disk on Module emulate IDE, managed by IDE driver
NAND or NOR flash managed by the MTD framework, with a lot of chipsets
supported
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Linux for embedded device

Requirements of embedded projects :
Quick boot
Boot time = Bootloader (+ BIOS if X86) + Linux startup
Bootloader must be tuned to reduce its time
Linux startup can range from few seconds to few tens of seconds depending on
processor speed and quantity of services, for embedded images
Switch off without shutdown
Journaling capabilities of Linux filesystems reduce risks, but are not sufficient in
embedded world
The solution is to use RAM disks as the root of the file system : in case of file
system corruption, the copy in RAM is corrupted, not the file system itself

With an adequate strategy, Linux can deal with the requirements
of embedded devices
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An embedded project
Redundancy
device

duplication

filtering

normal flux
duplicated flux

Customer requirements :
Complete access to source code
Different hardware and form factor
Performant and reliable TCP/IP stack
4th European
Congress ERTS Quick availability after power on
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An embedded project

Linux chosen because :
Linux and basic components governed by GPL, the sources of the
developed application itself were also given to the customer
Linux available on first architectures (X86 on Compact PCI and PPC on
VME)
Linux TCP/IP stack are performant (most Web servers run under Linux)
Very quick availability after power on thanks to very reduced dedicated
image

The project has been delivered successfully for the first 2
architectures, and it will be easy to swap to new ones
The customer performs later evolutions by itself

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Real time with Linux
Types of project :
Embedded only : Linux OK
Soft real time : Linux OK with adapted strategy
Hard real time : Linux not OK, but Linux + Real time extension OK

Linux is a general purpose OS, the scheduler's mission is to
distribute the processor to all processes on a fair base in a large
time scale (no fixed priority).
Soft real time is accessible through the scheduler API :
« Real time » class implements same scheduling principles than RTOS
There is no guarantee on interrupt reaction duration, due to the internal
structure of kernel, so hard real time not possible
Kernel 2.6 can be preemptible, but large sections of the kernel have
interrupt masked or preemption disabled, so even 2.6 is not hard real time
4th European Preemptible kernel 2.6 + real time class scheduling → Soft real
Congress ERTS time
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Real time with Linux

Hard real time needs a real time extension :
RTLinux : GPL, then commercial product, now bought by WindRiver
RTAI : GPL
Xenomai : GPL, skins available

Extension implemented as kernel modules
Extension implements a RTOS
Application shared between Linux (not real time part) and
extension (real time part) with IPC between the 2 worlds.
In the real time part, no access to Linux system calls nor drivers
(a real time driver framework : RTDM)
Possibility to develop the real time part as :
4th European Full kernel modules
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Linux processes managed by the real time extension
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A real time project

The project is to develop a data recorder for high end
automotives (to record data on vehicle's buses).
The hardware is built with PC104 board, with CPU, CAN board
and a DSP board.
Real time requirements : precise time stamp of data, full
determinism in exchanges between CPU and DSP
Kernel 2.4 choosen, due to Linux drivers availability, and
conservative approach when project was launched
RTAI used for real time purposes
CIO's job :
Soft choices
Linux + RTAI integration
4th European Port of Linux drivers towards RTAI
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Application skeleton development
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Porting from RTOS to Linux

Why porting from RTOS to Linux :
Replacing old hardware by newest one, not yet supported by RTOS
Avoiding licence costs ....

To avoid important costs, it is interessant to port as much as
possible at the OS level :
System calls remapping
Compatibility library

The goal is to change the applications sources the less possible,
to avoid development and revalidation costs
A project example : port of a SNMP stack from 68K / OS9
towards PowerPC / Linux
This port was a mock up for a larger application
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Main difficulties encountered :
Compiler change reveals problems in the original sources. Gnu chain
makes much more control than original OS9 compiler :
Quality problems in original sources, syntax errors ....
Bad data alignment of structure members
Consequence : a lot of unforeseen work to correct the sources
Memory model :
OS9 processes shared data structures : members of shared data were pointers
No problem on OS9 with flat memory model
Impossible with Linux with segmented memory through MMU
Solved by using the shared memory API, with fixed mapping of the shared
memory at same address for all processes
Use of signals and alarms :
Cooperation between OS9 process made with a lot of signals
4th European
Congress ERTS Linux offers very few signals available to users
EMBEDDED REAL Development of an emulation API of OS signals based on SIGUSR1


Main difficulties encountered :
Use of signals and alarms :
A lot of alarms running in parallel in the original OS9 application
Linux is not designed to have such an architecture
Solved by developping an alarm emulation API based on dedicated alarm threads

Other system aspects where quite easy to remap with wrappers
(through include files)
The port was finally achieved successfully. It demonstrated the
concept could be extended to the whole application
3 types of difficulties :
Relative easy remapping of system calls (the majority)
More challenging points solved by emulation library (signals, alarms)
4th European
Congress ERTS Most difficult points : memory model and original sources difficulties
EMBEDDED REAL triggered by compiler change.

More information

Web site http://www.cioinfoindus.fr
Contact us :
Tél : +33 4 95 05 19 41
Mail : christian.charreyre@cioinfoindus.fr

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ERTS 2008 - Using Linux for industrial projects

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