October, 2009


17
Oct 09

Embedded Linux Conference Europe 2009

A couple days ago Juanjo asked me if I wanted to attend the Embedded Linux Conference at Grenoble, France, sponsored by the CE Linux forum and I usually don’t say “no” to travel. So one day after I unpacked from Barcelona, I packed again.

One of the bad sides of living in A Coruña is the fly connections needed to go to anywhere. This time it was A Coruña – Madrid – Lyon (St Exupéry Airport) – Grenoble. Even though I found entertained and challenging all these heterogeneous connections.

In the registration area surprisingly I came across with Rob Taylor and Mark Doffman, from Codethink. Later on I saw also David Neary and Christian Schaller around.

The opening key talk was in charge of Jon Masters, and was about “Porting Linux” to other hardware platforms, where he just highlight some hints and landmarks in the porting process, such as run the vendor tests in the board, port the u-boot, read the hardware erratas (which are more insightful than the rest of the documentation); read the linux kernel mailing lists; use the flattened device tree to specify the device’s addresses (there’s no BIOS neither ACPI in embedded system); gave some recommendations using cscope; and finally insisted in not to take the x86 architecture implementation as references, because is too dirty, he rather recommended use the score arch (“x86 wouldn’t be designed today”). At the end he strongly advised about reading the development-process under the kernel documentation.

The next talk I attended was “Use of the Fast IRQ (FIQ) in ARM-Linux” by Allesandro Rubini. The FIQ is an interrupt request with a higher priority and can be handled while another IRQ. Allesandro use them as near-real-time signal processing under a non-real-time linux kernel. In this talk he showed his project, which hasn’t been accepted in mainstream, because is too task specific.

Then I stayed in the talk “Constrained Power Management” by Patrick Bellasi where he described multiple-policy power management optimization. Patrick distinguish two PM policies: device specific and system-wide. So he propose a coordinator entity among the local policies, and this coordinator uses some kind of linear programming optimization. Cool research stuff.

My objective for the conference was twofold: to learn about embedded linux distributions build systems, and to come aware of the community opinions about Android. So I’d the intention to attend the Matt Porter’s talk, nevertheless Masters recommended the PTXdist one, and I bought it: failed! The Porter’s talk won the “best talk” conference’s award.

PTXdist is just another embeddable-linux-distribution builder, based on Kconfig to “cook” your distro and in makefiles (GAR style) to build the packages and express their dependencies. An interesting feature is the ltmain.sh fixup to avoid path hard-coding in libraries lookups.

Afterward I assisted to Nina Wilner’s talk about “Porting Android to Power Architecture”. She’s technology chief at IBM for the revamped PowerPC platform. Sadly she just make advertising for Mentor Graphics, the guys who employ Matt Porter and actually done the PowerPC port of Linux to Android. A couple interesting insights is that even though Google insist that Android is multi-platform it’s full of ARMisms, and porting its Bionic C Library to other architectures painfully. Nevertheless she stated that Android could make, in the embedded stage, what Linux made for Unix in the servers: do all over again and do it right.

Then I backed to the hardcore topics with “Analyzing Kernel Function Execution with ftrace” by Tom Bird. He’s also one the organizers of the conference. ftrace is the first generic tracing system to get mainlined. In this talk Tom explained his adventures to measure the duration of each kernel’s function at boot. With ftrace you can execute logging functions at every filtered function call. Meanwhile the community is expecting the port of dtrace to Linux.

In the afternoon Marcin Juszkiewicz talked about “Hacking with OpenEmbedded”. It was about different use cases for OpenEmbedded beside the embedded distro building.

Finally, the BoFs of the conference. I stayed at first in the Buildroot, another
embedded-distro-building which is too similar to PTXdist. At the middle of it I ran away to the Android BoF which I catch at the conclusions. Again, Android doesn’t bring nothing new, nothing breakthrough, but a lot of questions and doubts, as its ties with Google, its GPL licensing avoidance efforts, and so on.

And the Small Business BoF, organized by Michael Opdenacker, the owner of the company Free Electrons. Most of the companies there were quite small, several just one-person companies. The hottest topics there were about the lawyers need, the time tracking and billing, the hiring process and local vs abroad clients.

At the night we’d the social event: a cocktail buffet at Restaurant du Téléphérique, with a great panoramic view of the city.

At the next and last day, the opening keynote was in charge of Philippe Gerum: “State of real-time Linux: Don’t stop until history follows”. I arrive late to the talk but as far as I understood, he talked about the advances in the Linux kernel in the Real Time arena. He put special attention to the project Xenomai, which bring hard real-time support in the GNU/Linux user-space.

I flipped a coin and decided to go to “The LLVM MIPS and ARM backends” but I was gladly surprised with the talks swapping to “Coccinelle: A program matching and transformation tool” by Nicolas Palix. I came aware of the powerful concept of semantic patch. The principal use case for it, is to detect the collateral effects of an API breakage, also to fix common programming mistakes. It has been used in the kernel finding tons of potential unseen bugs. Neat!

After a short coffee break I ran into the “Survey of Linux measurement and diagnostics tools” by Frank Rowand. He shown a quick survey of several software tools for performance measurement such as smem (quite interesting); oprofile (quote: “sampling tools are not that good” also is turning obsolete); systemtap (measure the processor use in a non-intrusive way); time (the old & honest); perf (here Frank talked about the interesting problem of cache misses); bootchart; the kernel function trace; the LLTng, Timechart, an finally the famous ftrace.

Meanwhile the lunch time was getting arranged, a showcase took place in a room, where a couple projects were shown. The most impressive IMHO was the Nsec Linux system boot by Montavista. There was also Codethink with a all-microbloggers-on-one application; an LZO compression algorithm for the kernel; a demo of Android in PowerPC, etc.

In the second part of the second day, Gilad Ben-Yossef, from Codefidence, talked about “The good, the bad and ugly: on threads, processes and co-processes”, where he probed that in the Linux realm, threads and processes have no performance differences. In theory the big difference between a process and a thread is that the first owns a memory space and the second share it, so the context switching among process should be more expensive that in threads. So he decided to do some measurements with a modified version of lat_ctx, and surprise! no appreciable differences were shown. Thus, his conclusion, is that the reason why people prefer use threads instead of process is because the API. The mental model offered by the threads is more appeal to the common programmer than the process model, so Gilad decided to create a wrapper library to implement process API in terms of the threads API called coproc.

“A computer is a state machine. Threads are for people who can’t program state machines” — Alan Cox

The last talk I assisted was “Technical features and components of open source build systems” by Alex de Vries from Wind River Systems. It tried to bring a survey of all the requirements of a “perfect” embedded-distro-builder and then a pseudo BoF was raised with the question why we don’t have already one? There are tons of these system on the wild, but every has a very specific and reduced set of features. There was no conclusions but at least the seed was planted.

Finally, the closing keynote was carried on by Tom Bird and Klaas van Gend, in a mixed talk about the purpose of the CE Linux forums its goals and achievements with a contest of the butterfly effect. It was fun.

The conference closed with a round of drinks. I got a nice Chinese beer.


6
Oct 09

GIT quick notes

I want to work on a new plug-in inside of gst-plugins-bad. But also I want to expose my development to a personal GIT repository, so my friends could fetch my changes.

As Linus Torvalds explained once keeping a nice linear regression set of patches above an upstream development implies the use of git-rebase, nevertheless that also implies that I will have to force my pushes and my friends will also have trouble keeping their repositories in sync with mine.

So this is a kind of compromise between be nice with the people who pull your changes, or ease your daily work.