Real-Time Summit 2018

This year, Bootlin engineer Maxime Chevallier attended the Real-Time Summit, which took place after ELCE in Edinburgh. In a similar fashion to the Linux Media Summit, this was a 1-day conference dedicated to Real-Time topics in the Linux ecosystem, more specifically the Linux Kernel.

The future of Preempt-RT

Real-Time (or should we say, deterministic) behavior in the Linux kernel has been pursued for a long time, the most famous effort being the Preempt-RT patch. As Steven Rostedt announced during his talk at ELCE 2018, the Preempt-RT patch is close to being fully merged in mainline Linux, we can expect to see this happen in 2019.

Some of the maintainers of the Preempt-RT patch were present at the Real-Time summit, including Thomas Gleixner who lead the discussion throughout the day.

This was the occasion to discuss the remaining points to be addressed for Preempt-RT to make it into mainline Linux :

  • Printk : As Steven Rostedt explained at ELCE 2017, printk is not very real-time friendly. The main issue was worked around, but John Ogness presented his current work of fully redesigning printk’s behaviour.
  • Thomas Gleixner talked about the current state of softirq handling, which is also a critical point for determinism. They work by “stealing” some irq context time, falling back to ksoftirqd when necessary. This is particularly problematic for networking drivers that heavily rely on softirq.
  • Peter Zijlstra exposed the different scheduler related issues that needs to be addressed, focusing on SCHED_DEADLINE.

Modeling and analyzing the kernel behavior

All the talks weren’t about the Preempt-RT match merging effort. Daniel Bristot de Oliveira presented his ongoing academic work on modeling the Linux task model. The idea here is to build a formal model that doesn’t take shortcuts or idealize the way tasks are handled in the kernel, so that this can be used as a basis for academic research on topics such as scheduling.

One of the main arguments is that there’s a gap in terms of language and methodology used between kernel developers and the academic world. Daniel explained how he managed to build a huge state-machine representing the task model, and how he uses it now to verify that tasks behave how they should by running trace events in the state machine.

This talk sparked a lot if interesting discussions, for example Peter Zijlstra suggested to compile the state machine into eBPF code and run it live in the kernel.

Julia Lawall was present in the room, and improvised a talk inspired by Daniel’s presentation. She presented DSAC, a static analysis tool dedicated to finding Sleeping in Atomic Context bugs. Julia is involved in the development and use of the coccinelle tool, and explained that it is quickly limited when trying to find that categories of bugs, where sleeping calls can be deeply nested in a call stack protected by spinlocks. Using LLVM, DSAC can analyze complex scenarios with multiple level of nesting and indirect calls to detect SAC bugs. After analyzing the v4.17 kernel sources for only a few hours, the tool was able to detect more than 1000 bugs, 220 of which were confirmed.

The overall technical level of the different talks was high, leading to passionate discussions and suggestions on every topic that was brought during the day.

Feedback from Bootlin at the Linux Plumbers Conference 2018

The Linux Plumbers Conference (LPC) was held a few weeks ago in Vancouver, BC. As always there were several tracks where contributors gave a presentation of on-going or future work, and discussed it with the audience, on specific topics such as thermal, containers, real time, device tree and many more. For the first time at LPC a 2-day networking track took place. As we work on a diversity of networking projects at Bootlin we decided to attend.

Networking track at LPC. Photo @linuxplumbers.

The hot topic of the last couple of years in conferences in the network subsystem is XDP, so the conference was not exception. We saw a handful of talks and discussions about the on-going work and support of XDP within the kernel. XDP provides a programmable network data path (using eBPF) in the Linux kernel to process bare metal packets at the lowest point in the network stack. Packets are processed directly in the drivers’ Rx queues, before any allocation happen (such as socket buffers). Facebook is one well known heavy user of this technology (every packet toward Facebook is processed by XDP) and its engineers gave feedback about how they use XDP and the issues they faced. Other projects and companies are currently evaluating and starting to use XDP as well: we also saw presentations about XDP/eBPF in Open vSwitch, DPDK or kTLS.

While XDP/eBPF was featured in most of the discussions, other interesting topics where brought up. Andrew Lunn gave a presentation about the current need to go beyond 1G copper PHYs for many Linux enabled embedded devices. This was very interesting for us as we used and worked on the technologies used within the Linux kernel to address this, such as Phylink and the SFP bus (we used those when enabling 10G interfaces in the Marvell MacchiatoBin board).

Another presentation caught our attention as the topic was related to what we do at Bootlin. Jesse Brandeburg from Intel talked about the networking hardware offloads and their APIs. He exposed a brief history of the offloads supported by NICs and then showed some issues with the current APIs, where some use cases or behaviors are not clearly defined and sometimes overlap. This is a feeling we share as we experienced it while implementing some of those hardware networking offloads. Jesse’s idea was to open a discussion to come up with better solutions within the next years, as NICs offloading continue to grow.

The Linux Plumbers Conference was very pleasant and well organized. We had the chance to attend the networking track, seeing lots of great cutting-edge topics being discussed; as well as other interesting tracks.

We’d like to thank the conference and track organizers, we had a great time! Videos, slides and papers are now available on the official website or on Youtube.

Buildroot 2018.11 released, Bootlin contributions inside

Buildroot logoThe 2018.11 release of Buildroot was published a few days ago. As is well-known, Bootlin is a strong contributor to this project, and this blog post proposes a summary of the new features provided by 2018.11, and highlights the contributions made by Bootlin.

What’s new in 2018.11 ?

  • From a CPU architecture support point of view, by far the most important addition is support for the RISC-V 64 architecture. For now, only the 64-bit version of the architecture is supported, but the patches for the 32-bit version have been posted already, and will hopefully be merged for the next release. It is worth mentioning that we have already used the RISC-V 64 support in Buildroot to provide a pre-built toolchain for this architecture on our site.
  • In the toolchain support area, the most important change is that glibc was upgraded to version 2.28. This caused a number of build issues with various packages, which were detected by the project autobuilders and fixed. musl was bumped to version 1.1.20, and the ARM (formerly Linaro) pre-built toolchains for ARM and AArch64 were updated.
  • The support for hardening flags, i.e flags passed to gcc to improve the “security” of programs, has been changed, and is now done directly as part of the compiler wrapper that Buildroot has. Indeed, when using Buildroot, arm-linux-gcc is not directly the usual gcc compiler, but a small wrapper program that Buildroot uses to make sure we always pass the appropriate flags when calling the cross-compiler. Passing those hardening flags through the wrapper allowed to solve a number of build issues. Options such as BR2_RELRO_PARTIAL, BR2_RELRO_FULL, BR2_SSP_REGULAR, BR2_SSP_STRONG and BR2_SSP_ALL should therefore work better now.
  • In terms of filesystem images, while Buildroot already supports the most popular filesystem types, two additional filesystems are now supported: btrfs and f2fs.
  • A number of new default configuration for various boards have been added: Amarula a64-relic, Bananapi m2 ultra, Embest riotboard, Hardkernel Odroid XU-4, QEMU riscv64-virt.
  • A number of packages have been added: bird, bluez5_utils-headers, btrfs-progs, checksec, davici, duktape, ell, haproxy, libclc, libcorrect, libopencl, libopenresolv, nss-myhostname, riscv-pk, sedutil, spandsp, tini, waffle, xapian, not counting the dozens of new Perl and Python modules that have been added.

Bootlin contributions

By far and large, the most significant contribution from Bootlin to Buildroot is the activity of Thomas Petazzoni as a co-maintainer for the project. Out of the 1366 commits made between the 2018.08 and 2018.11 release, Thomas authored 126 commits, but more importantly reviewed and merged 883 patches from other developers, or in other words 64% of the commits that have been made.

As part of the 127 commits Bootlin contributed, we:

  • Fixed a large number of build issues reported by the autobuilder, or by the CI testing the build of our defconfigs.
  • Introduced a make check-package target to more easily use Buildroot check-package tool to verify the coding style of packages.
  • Updated the musl C library to 1.1.20.
  • Updated the Solidrun MacchiatoBin defconfigs (board powered by a Marvell Armada 8K processor) to use the most recent kernel, U-Boot and ATF versions.
  • Started adding a virtual package for opencl.
  • Fixed a number of missing dependencies on host-pkgconf (i.e pkg-config) which were detected by our work on per-package directories, that we will discuss in a future blog post.

Here is the detailed list of our contributions to this release:

Bootlin at the Linux Media Summit 2018

This year’s edition of the Linux Media Summit happened a month ago, in Edinburgh, right after the Embedded Linux Conference. Since we were already at the ELCE, and that we’ve been more and more involved in the media community thanks to our work on the Allwinner CSI driver and more importantly the Cedrus driver, it was natural for us to attend.

The media summit is usually a meeting to discuss the hot topics, so the whole day was a mix and match of various status updates and discussions on the future needs and developments around the Video4Linux2 framework.

Linux Media Summit attendees

Most of the discussion was about how to improve the contributor’s experience and improve the maintenance. The DRM subsystem was used as an example, since the number of patches are in the same order of magnitude, and a number of v4l2 contributors are also contributing to DRM drivers. Part of the improvement of both the maintenance and contribution experience will also come through some CI work, so there was a lot of discussions on how to improve the already existing tools (such as v4l2-compliance) but also how to setup some automatic tooling to run those tests as early as possible.

A good part of the day was also spent on dealing with the current developments, such as the Request API we’ve used in the Cedrus driver, and how to integrate that API into popular multimedia frameworks like gstreamer or ffmpeg. It looks like our libva implementation was well received, so it will probably be made standard and hosted on in the near future. Other developments discussed were fault tolerant v4l2, in order to deal with video pipelines where one or several components might not work anymore, and storing the v4l2 controls state in a persistent way.

It was overall a very productive day, and it’s always nice to meet people you interact with over mailing list and IRC on a regular basis. If you want more information, you can read the extensive report.

Bootlin adds SPI NAND support to U-Boot

Bootlin is proud to announce that it has contributed SPI NAND support to the U-Boot bootloader, which is part of the recently released U-Boot 2018.11. Thanks to this effort, one can now use SPI NAND memories from U-Boot, a feature that had been missing for a long time.

State of the art: Linux support

A few months ago, Bootlin engineer Boris Brezillon added SPI-NAND support in the Linux kernel, based on an initial contribution from Peter Pan. As Boris explained in a previous blog post, adding SPI NAND support in Linux required adding a new spi-mem layer, that allows SPI NOR and SPI NAND drivers to leverage regular SPI controller drivers, but also to allow those SPI controller drivers to expose optimized operations for flash memory access. The spi-mem layer was added to the SPI subsystem by a first series of patches, while the SPI NAND support itself was added to the MTD subsystem as part of another patch series.

The spi-mem framework in Linux
The spi-mem framework in Linux

Moving to U-Boot

Since accessing flash memories from the bootloader is often necessary, Bootlin engineer Miquèl Raynal took the challenge of adding SPI NAND support in U-Boot. Miquèl did this by porting the SPI-mem and SPI-NAND subsystems from Linux to U-Boot. The first challenge when porting the SPI-mem and SPI-NAND code from Linux to U-Boot was that the U-Boot MTD stack hadn’t been synchronized with the one of Linux for quite some time. Thus a number of changes in the Linux MTD subsystem had to be ported to U-Boot as well, which was a fairly time-consuming effort. The SPI NAND code has been imported in drivers/mtd/nand/spi, while the spi-mem layer is in drivers/spi/spi-mem.c.

Once the core code was ready, we had to find a way to let the user interact with the SPI NAND devices. Until now, U-Boot had a separate set of commands for each type of flash memory (nand for parallel NAND, erase/cp for parallel NOR, sf for SPI NOR), and it indeed seemed like adding yet another command was the way to go. Instead, we introduced a new mtd that can be used to access all flash memory devices, regardless of their specific type. We will discuss this mtd in more details in another blog post.

However, such a move to a generic mtd command forced us to do a lot more cleanup than expected, as we ended up reworking the MTD partition handling, and even making deep changes in the ubi command. This was more complicated than anticipated because of the SPI NOR support in U-Boot: it is not very well integrated with MTD subsystem, in the sense that there is a duplication of information between the SPI NOR and MTD subsystems, and when the duplicated information is no longer consistent, really bad things happen. As an example, any call to sf probe was doing a reset of the MTD device structure using memset, causing all other state information contained in this structure to be lost. Since the SPI NAND support relies on the MTD subsystem (much more than the current SPI NOR support), we had to mitigate those issues. Long term, a proper rework of the SPI NOR support in U-Boot is definitely needed.

Some of those issues are present in the 2018.11 release and were discovered by U-Boot users who started testing the new mtd command. We have contributed a patch series addressing them, which hopefully should be merged soon.

Now that those difficulties are hopefully behind us, the U-Boot SPI-NAND support looks pretty stable, and we have quite a few SPI-NAND manufacturer drivers in U-Boot mainline, with Gigadevice, Macronix, Micron and Winbond supported so far. We’re happy to have contributed this new significant feature, as it finally allows to use this popular type of flash memory in U-Boot.

Updated cross-compilation toolchains, RISC-V 64 bit toolchain added

RISC-V 64 toolchainWe have just published an updated version of the cross-compilation toolchains available at

The significant changes are:

  • A RISC-V 64 bit toolchain is now provided, following the addition of support for this architecture to the Buildroot project.
  • The stable toolchains are now using gcc 7.3.0 (instead of 6.4.0), gdb 7.12.1 (instead of 7.11.1), kernel headers 4.1.52 (instead of 4.1.49), glibc 2.27 (instead of 2.26), musl 1.1.19 (instead of 1.1.18) and uclibc 1.0.30 (instead of 1.0.28). We are still using a 7.x gdb version because the 8.x versions need C++11 support, which requires a recent enough host compiler, which in turn requires using a more modern distribution. Thus, those toolchains would be unusable with older distributions as they would require a recent glibc version on the host. Currently, our stable toolchains are still built within an old Debian Squeeze system, for maximum compatibility with old distributions.
  • The bleeding-edge toolchains are still using gcc 8.2.0, but gdb is now 8.1.1 (instead of 8.1), kernel headers 4.14.80 (instead of 4.14.57), glibc 2.28 (instead of 2.27), musl 1.1.20 (instead of 1.1.19).

We will continue to update those toolchains with more recent versions of gcc, binutils, gdb and the different C libraries, and add support for more architectures. Do not hesitate to ask for additional features or report any issue encountered when using those toolchains in our bug tracker.

Allwinner VPU support in mainline Linux November status update

Since our previous update back in September, we continued the work to reach the goals set by our crowdfunding campaign and made a number of steps forward. First, we are happy to announce that the core of the Cedrus driver was approved by the linux-media maintainers! It followed the final version of the media request API (the required piece of media framework plumbing necessary for our driver).

Both the API and our driver were merged in time for Linux 4.20, that is currently at the release candidate stage and will be released in a few weeks. The core of the Cedrus driver that is now in Linus’ tree supports hardware-accelerated video decoding for the MPEG-2 codec. We have even already seen contributions from the community, including minor fixes and improvements!

We have also been following-up on the other features covered by our crowdfunding campaign and made good progress on bringing them forward:

  • The series bringing H.264 decoding to our driver was updated for a second revision on November 15, rebased atop the upcoming Linux release and including a number of fixes as well as documentation;
  • H.265 decoding support followed with a second version sent on November 23, based on the updated H.264 series and bringing various minor improvements over the first iteration;
  • The patch series for the display engine DRM driver that adds support for the tiled YUV format used by the VPU was also updated, significantly reworked and submitted again on November 23;
  • Finally, we submitted a patch series adding support for the A64 and H5 Allwinner SoCs in the Cedrus VPU driver on November 15.
A64 and H5 boards provided by our sponsors Olimex and Libretech, now supported in Cedrus

With these patch series well on their way, we are closer than ever to delivering the remaining goals of the crowdfunding campaign!

Bootlin at Capitole du Libre in Toulouse, France

Capitole du LibreNext week-end, a local free and open-source software conference called Capitole du Libre will take place in Toulouse, France, where Bootlin has one of its offices. Bootlin will participate to this event in several ways:

We encourage free software developers and users from the south west of France to join this event, which has been organized for several years, and provides a very nice selection of talks and tutorials. And of course, this conference is entirely free, and no registration is required.

Back from ELCE 2018: our selection of talks

The Embedded Linux Conference Europe edition 2018 took place a few weeks ago in Edinburgh, Scotland, and no less than 9 engineers from Bootlin attended the conference. While our previous blog post shared the videos and slides of our talks, tutorials and demos, in this blog post we would like to highlight a selection of talks that Bootlin engineers found interesting. We asked each of the 9 engineers who attended the event to pick one talk they liked, and make a small write-up about it. Of course, many other talks were interesting and what makes a talk interesting is very subjective!

Getting Your Patches in Mainline Linux: What Not To Do (and a Few Things You Could Try Instead), by Marc Zyngier

Talk selected by Maxime Ripard

Marc gave a talk on a subject that is often debated, and still confusing to newcomers: how to contribute. He first started by presenting the various actors involved in a contribution: a contributor, a maintainer and a reviewer. He also took the time to explain the various objectives that everyone has which is something that is often overlooked by the other parties and the conferences on this subject. He then went on to explain and document the good practices that can be used in order to contribute to most subsystems. This was overall a great overview, and we definitely recommend it to people willing to start contributing.

Real Time is Coming to Linux; What Does that Mean to You? , by Steven Rostedt

Talk selected by Michael Opdenacker

In this talk about PREEMPT_RT, the speaker, who’s a long time contributor to this feature, was approaching the subject on a new angle, taking for granted that PREEMPT_RT is in mainline Linux. That’s not quite right yet, but this is possible before the next Embedded Linux Conference, in August next year. One proof that this is on the verge of being true is that its authors no longer call it a patch set, but just PREEMPT_RT. Rostedt also added that Linux can now be called a Deterministic Operating System (aka DOS!).

So, Rostedt first explains what PREEMPT_RT is about and how it addresses the challenges of users who are determined to be deterministic (that’s my pun here, not Steven’s).

Doing this, Steven recalled the “Priority inheritance” issue that is best known through the fact that it happened on Mars on the Pathfinder robot. A high priority and critical system process got starved by a lower priority one because an even lower priority process was holding the lock the high priority process was waiting for, causing some system services to be unavailable. This caused a watchdog to kick in and reboot the system endlessly. Such an issue is addressed by “Priority inheritance”, allowing a lock-holding process to inherit the priority of the highest priority process waiting for the lock. Priority inheritance is now supported in kernel locks thanks to PREEMPT_RT.

By the way, I learned that there are now 5 preemption models in the kernel, instead of four originally with PREEMPT_RT. There is now a “Basic RT” option in which you have all the PREEMPT_RT features except the sleeping spinlocks, which is useful for debugging such features.

So now that PREEMPT_RT is almost in mainline, what should kernel developers do? The main thing is to stop adding non determinism to Linux. For example, Rostedt strongly advised against rw_locks and semaphores on multiple CPUs. That’s horrible for cache lines, as they do not scale. You should use RCU mechanisms instead.

As a kernel developer, you shouldn’t use preempt_disable() either, unless you know it is done for a very short amount of time. Similarly, if you find code that uses local_irq_save(), that’s most likely a bug. Instead, people should use spin_lock_irqsave() and spin_lock_irq(), which disable interrupts only when PREEMPT_RT is not enabled.

Rostedt ended his talk by answering a question about what will remain of the PREEMPT_RT patch set. Even when the most important parts of PREEMPT_RT are in mainline, some changesets are likely to remain for some time, just to address cases that don’t have a solution yet. 99.9% of the users will be able to do without it. That’s what a mainline solution means: no patches to apply.

Uh-oh, It’s I/O Ordering! by Will Deacon

Talk selected by Miquèl Raynal

Will gave his second talk at an ELCE about I/O ordering, 6 years after the first talk on that subject. For this purpose, he started with an introduction to the memory consistency models (in 5 minutes!) to show the audience how a very simple program, ran on two CPUs, could produce very strange results due to store buffering. Because his assumption was a bit hard to believe for such a simple program, he proved us he was right by actually running it on his laptop. While such kind of tricky behavior applies to memory, the same odd situation may happen with I/Os! After a theoretical explanation, he gave a few examples (mostly taken from the mainline Linux kernel) of good and bad code sections and explained why. If you are a device driver writer, this talk should be of interest! The examples are real use cases that you might encounter someday (if not already) and knowing how to workaround the most generic caveats with the right memory barrier or even doing a dummy read to enforce ordering is something you will want to master to avoid strange random bugs.


The Power Supply Subsystem, by Sebastian Reichel

Talk selected by Quentin Schulz

Sebastian started the talk by presenting what this subsystem is used for and its history, which he knows in great length since he took over the maintainership of the power supply subsystem in the Linux kernel in 2014. While it’s not the subsystem with the hardest concepts to grasp, Sebastian explained that he aimed, with his talk, at providing an accessible approach to the subsystem for people who’re trying to get started in the Linux kernel or in this specific subsystem. Having contributed to this subsystem a few patches and drivers in my early days as a kernel developer, I can say that I wish I had seen his talk before to quicken my understanding of the power supply subsystem. Scrolling down the slides, he presented a very simple example of a dummy driver, Device Tree nodes and how to configure what’s exposed to sysfs. Sebastian also gave a few words on Open-Circuit Voltage in batteries which is interesting for getting more precise values of the battery capacity depending on its age and temperature, and the ongoing work on supporting this in the kernel. He concluded with the future plans for the subsystem, which are mainly related to batteries, their fuel gauges and chargers.


The End of Time, 19 Years to Go, by Arnd Bergmann

Talk selected by Alexandre Belloni

Arnd gave an update on the status of the effort to get a 32-bit kernel handle the 32-bit time_t overflow which will happen in January 2038. He first started to explain why this is necessary. This boils down to the huge number of 32-bit products that are still being introduced on the market with some of them having a very long service life. Arnd said this work has been on-going since 2014, when John Stultz switched the internal timekeeping code to a 64-bit second counter. The device drivers then needed fixing. This was done by addressing them individually by changing:

  • time* to ktime_t
  • time* to jiffies
  • time_t to time64_t
  • timespec/timeval to timespec64

The driver userspace interface also needed to be changed. Some IOCTLs were easy to change because they are already using different numbers depending on the size of the argument they take. The other IOCTLs had to be redefined. It gets worse Arnd said, explaining how the read, write and mmap callbacks are getting fixed.

While the VFS layer got fixed earlier this year, some filesystems are still work in progress and other ones are not fixable because they use a 32-bit time on disk. The only way is to move away from those.

Arnd then went over the biggest remaining part of the work, the system calls. The 32-bit compat syscalls mechanism is reused and a __kernel_timespec type has been introduced to handle time at the boundary. He then listed the affected system calls and their current status.

He ended by talking about userspace and the plan to handle the issue in glibc. Finally, he mentioned what distributions will have to do.


On this Rock I will Build my System – Why Open-Source Firmware Matters, by Lucas Stach

Talk selected by Grégory Clement

Lucas started to present what we used to have in embedded world: a minimalist firmware which acts only as a bootloader and with no interaction with the kernel.

Then he showed why with the virtualization there were some needs to have CPU power management in a single place. This was defined by the PSCI: the purpose of it was to have the bare-metal and the virtualized kernel seeing the same interface. What should have been a simple and delimited interface then became more and more complex due to the hardware constraints. Indeed, in many SoCs multiples devices or CPUs can share the same register. Besides, an interface such as the I2C used by a PMIC can also be shared. This lead to moving the entire register inside the firmware or to have lock mechanisms between the kernel and the firmware. In conclusion, the kernel implementation became easier but at the expense of a complex firmware.

The sad news, is that most of the firmwares are not copyleft which can lead to closed source binaries, making the debugging very difficult for the kernel. Even if the firmware remains open source, having the hardware management split in two parts, makes the debugging more complex. However, there is nothing we can do about it, because there are valid reasons to have a firmware. The only thing we should be vigilant about is the openness of the firmware source.


Handling Security Flaws in an Open Source Project, by Jeremy Allison

Talk selected by Antoine Ténart

Samba is a well known re-implementation of the SMB protocol and as such is used in several consumer devices — such as NAS. As open source software are more and more used in new products, correctly handling security flaws and their fixes is becoming an important topic.

Jeremy Allison, one of the core developers of Samba, gave a talk about how Samba is dealing with security issues and what questions other projects should ask themselves to handle those the right way. He talked about the process to put in place to take security seriously, how to respond to vulnerability reporters and to security issues, and how to notify downstream vendors so that products in the wild are patched before the CVE is made public.

Jeremy Allison also presented three examples of security flaws in Samba. He described how they were handled at the time, the difficulties the Samba developers encountered, and gave a postmortem.

Security is important and we found this talk to be a must-see for open source maintainers and developers, as it gave a good insight on how to properly handle security vulnerabilities in a project. One of the key points was how to coordinate the security responses to avoid having the users being at risk.


Improve Linux User-Space Core Libraries with Restartable Sequences, by Mathieu Desnoyers

Talk selected by Maxime Chevallier

Following-up on the good LWN coverage of the restartable sequences, Mathieu Desnoyers gave an interesting talk on the current userspace support, and some feedback regarding the shortcomings of the current implementation.

Restartable sequences allow to implement lockless per-cpu sections of code, that will be automatically aborted (or restarted) whenever migration, preemption or signal delivery occurs before the final “commit” operation is done.

This is useful to read some performance counters from userspace with a minimal overhead since there’s no lock involved to protect the critical section.

Mathieu explained that these critical sections need to be written in assembly code, but thanks to the librseq and its set of macros, users shouldn’t have to worry about this.

Mathieu then presented some of the shortcomings of rseqs, one of them being that they can’t be debugged in step-by-step (since a signal interrupts the sequence, causing it to abort). To solve these shortcomings, Mathieu gave a quick glimpse of a possible new system-call, cpu_opv(), that would allow users to execute a limited sequence of instructions with preemption and migration disabled.

Power Debugging with JTAG, by Patrick Titiano & Alexandre Bailon, Baylibre

Talk selected by Thomas Petazzoni

In this talk, BayLibre engineers Patrick Titiano and Alexandre Bailon introduced libSoCCA (SoC Continuous Analyzer), a Python library that allows to watch over JTAG what a SoC is doing.

This library allows remote access to the registers of a SoC through JTAG, and uses the SoC interconnect debug port rather than the CPU debug port. Non-intrusive observation of what the SoC is doing is thus possible, even when the CPU is idle or in a low-power state.

libSoCCA uses SVD (System View Description) files, which are XML files that describe all the registers of the SoC, their bitfields and possible values. This format is not specific to libSoCCA, since it is already used by Keil, and apparently some SoC vendors provide such SVD files for their SoCs. Unfortunately, not all vendors do this, and creating such SVD files from the SoC datasheet is a very long and boring process. In addition, the speakers pointed out that the SVD file format lacked an include directive, which would be very useful to share register definitions between SoC.

With the information provided by the SVD files and a connection to the target over JTAG that uses OpenOCD, libSoCCA is then used to implement a number of different

  • PMUGraph, which shows power management statistics of the device. Compared to solution such as perf or powertop, this solution has the advantage of being non-intrusive.
  • memtool, which provides a way of manipulating registers without having to manually fiddle with register offsets and bitfields. It could be summarized as a remote devmem that knows your SoC registers. This kind of feature can be found in proprietary JTAG tools, and was lacking in the open-source world.
  • clocktool (development not started yet), which shows the state of the SoC clocks remotely, a bit like clk_summary in debugfs, but which works even when the SoC is idle or in a low power state, which is precisely a moment where getting clock status may be useful for debugging.

Overall, we found libsocca very interesting as it opens up lots of possibilities. It would be useful to have a better file format than SVD to describe SoC registers though, and it would also be nice to have an on-target variant of memtool.


New embedded Linux engineer job opening in 2019 in Lyon, France

Penguin worksBootlin is going to move to a new and bigger office in Lyon, France, by the end of 2018. Our team in Lyon will therefore be able to welcome a new engineer in 2019.

Here are a few details about the job:

  • Job description: embedded Linux and kernel engineer
  • Profile: for this new position, meant to strengthen our small team in Lyon (currently two people), we are looking for someone with already valuable experience and autonomy in embedded Linux and kernel development. The positions that will follow should be open to junior engineers.
  • Lyon is a beautiful and vibrant city, the second largest urban area in France, which two rivers instead of one! Our office is within 5 minutes of a subway station, and is also easy to access from more residential areas in the south of Lyon.

If you are interested, please send a resume to, letting us know about your interests and ideas for the job.