The schedule for the upcoming Embedded Linux Conference Europe 2022 has been published recently.

Bootlin CEO Thomas Petazzoni is again a member of the program committee for this edition of ELCE, and has helped with other members of this committee in reviewing and selecting the numerous talk submissions that have been received.

Bootlin will obviously be present at this conference. With 13 engineers from Bootlin participating, almost the entire company will be in Dublin for this major event of the embedded Linux community. Also, 4 of the talks that we had submitted have been accepted:

• Luca Ceresoli on Basics of I2C on Linux
  This talk is an introduction to using I²C on embedded Linux devices. I²C (or I2C) is a simple but flexible electronic bus to allow low-speed communication between the CPU and all sorts of chips: PMICs, ADC/DACs, GPIO expanders, video sensors, audio codecs, EEPROMS, RTCs and many more. It is so popular that knowing it is a must for any embedded system engineer. Luca will first give an introduction to what I2C is at the electrical level. He will then describe how I2C is implemented in the Linux kernel driver model and how that appears in sysfs, how to describe I2C devices using device tree and how to write a driver for an I2C device. Finally he will present the tools to communicate with the chips from userspace and share some debugging techniques, including inspection of the physical bus and software-level debugging.
• Miquèl Raynal on Improving Wireless PAN Support
  Anybody eager to learn about IoT devices has at least once tried to play with Zigbee or 6lowpan sensors. These two protocols are built on top of a well common MAC/PHY specification: IEEE 802.15.4, also known as Wireless Personal Area Networks: WPAN, designed to be low-rate/low-range wireless networks. There is already substantial support for this protocol in the Linux kernel but when my journey started, several of the MAC-related operations well described in the specification were not implemented, making the subsystem mainly useful for very simple use cases: peer-to-peer transmissions. This is unfortunate as a significant part of the idea behind WPAN is to make these networks quite adaptive and resilient, which requires a minimal subset of the peer management procedure to be supported. Besides a number of preparation changes, the main idea behind the continuous flow of patches was to bring support for the scanning procedure which allows a PAN controller to detect all the compatible devices around it in different ways. Discovering these devices is the first step in order to associate them together and build up starred networks. This talk will be an opportunity to explain the new APIs allowing such discoveries and provide a state of the art of the support in the mainline kernel.
• Michael Opdenacker on Implementing A/B System Updates with U-Boot
  A popular way to implement system updates is through the A/B scheme, in which you have two copies of the root filesystem, one which is active, and one that is meant to contain the next update. When a new update is successfully applied, you need to make the corresponding partition become the new active one. That’s when a number of practical questions arise, such as how to identify the active partition, how to detect when the new system fails to boot properly, and how to fall back to the previous version? It was hard to find documentation about how U-Boot could address such needs to implement a functional and failsafe A/B system update mechanism. This presentation proposes to address this need by sharing the practical solutions we found, using lesser known commands and capabilities in U-Boot. We will also explain how the Linux side can cooperate with the U-Boot side. Fortunately, you won’t need to erase half of your brain to get updated on this topic.
• Paul Kocialkowski on Walking Through the Linux-Based Graphics Stack
  The graphics stack used with the Linux kernel is a notoriously complex beast. From userspace down to the kernel level, a number of components are involved and interact with eachother. It is also an area that is constantly evolving to meet new use cases, refresh legacy implementations and achieve better performance. This makes it difficult to have a clear idea of the big picture and what is actually happening when using graphics-related components. This presentation will detail a walk through the graphics stack, with actual examples of displaying a buffer and rendering using the GPU. Going from the application level through the system libraries, down to the kernel and ending with actual hardware configuration. State-of-the-art technologies such as Wayland and DRM will be highlighted with relevant excerpts from the source code of related free software projects that are widely used today.

We look forward to meeting again the embedded Linux community, its developers, users and maintainers, at Dublin during this conference!

The schedule for the next Embedded Linux Conference Europe has been recently published, and Bootlin will once again be strongly present at this (virtual) event by giving a number of presentations. The registration for ELC-E is open, and due to the virtual nature of the event, the registration cost is only $50, which makes is accessible to pretty much everybody.

• From the Camera Sensor to the User, the Journey of a Video Frame. In this talk, Bootlin engineer Maxime Chevallier will share his experience working in the Video4Linux subsystem, implementing support for the Rockchip camera interface controller, and a PAL/NTSC decoder used as the input source. Talk on Monday October 26 at 18:30 GMT.
• Yocto Project and OpenEmbedded: A Collection of Best Practices. In this talk, Bootlin engineer, and Yocto Project expert and trainer Alexandre Belloni will share his experience of using Yocto Project and OpenEmbedded through a collection of best practices. There are indeed numerous ways of using OpenEmbedded and the Yocto Project, but some ways and solutions are better than others! Talk on Tuesday October 27 at 13:00 GMT.
• Building Embedded Debian and Ubuntu Systems with ELBE. In this talk, Köry Maincent will share his experience using the ELBE build system, which can be used to automate the process of creating embedded Linux systems based on Debian or Ubuntu. Köry has contributed to ELBE the support for building Ubuntu systems, and has used ELBE on two different projects. This is an interesting alternative to the traditional cross-compilation approach taken by Yocto Project, OpenEmbedded or Buildroot. Talk on Tuesday October 27 at 15:15 GMT.
• Using Visual Studio Code for Embedded Development. In this talk, Michael Opdenacker will share his experiments of using VS Code for embedded Linux development, which extensions are available to help navigate in the code, build and debug code, from kernel-space and user-space. Talk on Tuesday October 27 at 17:15 GMT.
• Precision Time Protocol (PTP) and Packet Timestamping in Linux. Antoine Ténart has implemented PTP support in both an Ethernet switch driver, and an Ethernet PHY driver, both in the upstream Linux kernel. He will share his experience about PTP, its support in Linux, and its offloading at the MAC and PHY level. Talk on Tuesday October 27 at 19:30 GMT.
• Supporting Hardware-Accelerated Video Encoding with Mainline. After working on the HW-accelerated video decoding on Allwinner platforms as part of our crowdfunded effort, Paul Kocialkowski recently worked on HW-accelerated video encoding on Rockchip platforms. In this talk, he will share the issues encountered, and what needs to be resolved to create a useful kernel to userspace interface to properly support stateless video encoders. Talk on Wednesday October 28 at 16:15 GMT.
• Understand ECC Support for NAND Flash Devices in Linux. Miquèl Raynal, the Linux kernel NAND subsystem maintainer, has recently worked on improving support for various strategies to handle ECC for NAND flash devices. He will share some background information on ECC, why they are needed, how and where ECC are typically handled, and how the Linux kernel deals with the different possibilities. Talk on Wednesday October 28 at 18:30 GMT.

In addition to contributing talks, Bootlin CTO Thomas Petazzoni is also a member of the ELC-E program committee: he reviewed, ranked all talk submitted for the conference and participated with the rest of the committee to the selection of the talks that are now scheduled for the event.

Even though we once again won’t have the chance to meet our fellow members of the embedded Linux community in person, we look forward to attending a set of great talks, and have interesting discussions during the Q&A and through the instant messaging platform that will be available around the conference.

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.

[PDF]

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.

[PDF]

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
• CLOCK_REALTIME to CLOCK_MONOTONIC

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.

[PDF]

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.

[PDF]

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.

[PDF]

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
tools:

• 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.

[PDF]

The Embedded Linux Conference Europe edition 2018 took place last week in Edinburgh, Scotland, and no less than 9 engineers from Bootlin attended the conference. In this blog post, we would like to share the slides, materials and videos of the talks, tutorials and demos we gave during this conference.

Talk: Supporting Hardware Codecs in a Linux system – Maxime Ripard

This talk was given by Bootlin engineer Maxime Ripard, who has worked since spring 2018 with Paul Kocialkowski on adding support in the upstream Linux kernel for hardware-accelerated video decoding on Allwinner platforms. This project was the topic of the successful crowd-funding campaign we launched in February 2018, and for which we regularly posted updates on our blog.

[PDF] [Sources]

Talk: Networking: From the Ethernet MAC to the Link Partner – Maxime Chevallier & Antoine Ténart

This talk was given by Bootlin engineers Maxime Chevallier and Antoine Ténart, who shared their knowledge and experience working on enabling network hardware in Linux, trying to clarify how Ethernet MAC and PHYs interact, how PHYs communicate with their link partner, what are the protocols involved, etc.

[PDF] [Sources]

Talk: SPI Memory support in Linux and U-Boot – Miquèl Raynal

This talk was given by Miquèl Raynal, who has worked with Bootlin engineer Boris Brezillon on adding support for SPI NAND in U-Boot and Linux, as well as improving in general the support for SPI flash memory, see our previous blog post.

[PDF] [Sources]

Tutorial: Introduction to Linux kernel driver programming – Michael Opdenacker

This tutorial was given by Bootlin CEO and founder Michael Opdenacker, as part of the Embedded Apprentice Linux Engineer track.

[PDF] [Sources]

The video will be published later, as it was not recorded by the Linux Foundation, but by the E-ALE track organizers.

Tutorial: Getting started with Buildroot – Thomas Petazzoni

This tutorial was given by Bootlin CTO Thomas Petazzoni, as part of the Embedded Apprentice Linux Engineer track.

[PDF] [Sources]The video will be published later, as it was not recorded by the Linux Foundation, but by the E-ALE track organizers.

Demo: Hardware Video Codec Support on Allwinner SoCs – Maxime Ripard

In this demonstration, Maxime Ripard was showing the upstream Linux kernel support for the Allwinner VPU, which provides hardware-accelerated video decoding for MPEG2, H264 and H265 within the Kodi media player on Allwinner platforms.

[PDF] [Sources]

Demo: Upstream Linux kernel support for Microsemi switches – Alexandre Belloni

In this demonstration, Bootlin engineer Alexandre Belloni was showing the upstream Linux kernel support for the VSC5713 and VSC7514 Microsemi Ethernet switches, which we presented in a previous blog post. Thanks to this support in upstream Linux, the different ports of the switch are seen as regular Linux network interfaces, and standard Linux user-space tools can be used to bridge the ports, set up VLAN filtering, and more. This makes such switches a lot easier to use than vendor-specific SDKs.

[Poster] [Sources]

The Embedded Linux Conference is one of the most important events in the embedded Linux industry, and Bootlin has been participating to this event non-stop since its creation in 2007. So it should be no surprise that we will once again be participating to the 2018 edition of this conference, which will take place on October 22-24 in Edinburgh, Scotland.

In total, nine engineers from Bootlin (on a total headcount of 13!) will be attending: Alexandre Belloni, Antoine Ténart, Grégory Clement, Maxime Chevallier, Maxime Ripard, Michael Opdenacker, Miquèl Raynal, Quentin Schulz, Thomas Petazzoni.

Of course, we are not only attending, but also giving a number of talks and tutorials:

• Monday 22 October at 11:15, Maxime Ripard is giving a talk Supporting Hardware Codecs in a Linux system, in which he will explain how HW video decoders/encoders are supported in the Video4Linux kernel subsystem, and share his experience working on the Allwinner VPU support in Linux.
• Monday 22 October at 12:05, Antoine Ténart and Maxime Chevallier are giving a common talk Networking: From the Ethernet MAC to the Link Partner, in which they will demystify a number of acronyms and technologies used in networking hardware, and detail how Ethernet MAC and PHY are represented and managed in Linux.
• Tuesday 23 October at 12:20, Miquèl Raynal is giving a talk SPI Memory support in Linux and U-Boot, in which he will explain how SPI NAND and SPI NOR memories are supported.
• Thomas Petazzoni will be giving a tutorial as part of the Embedded Apprentice Linux Engineer track, titled Getting started with Buildroot. This tutorial has not been scheduled yet.
• Michael Opdenacker will also be giving a tutorial as part of the Embedded Apprentice Linux Engineer track, titled Introduction to Linux kernel driver programming. This tutorial has not been scheduled yet.
• We will have two demos at the Technical showcase: Alexandre Belloni will be demonstrating upstream Linux kernel support for Microsemi Ethernet switches, while Maxime Ripard will be demonstrating upstream Linux kernel support for HW accelerated video decoding on Allwinner platforms.

In addition, we will be participating to a number of co-located events:

• Thomas Petazzoni will participate to the Buildroot Developers Meeting, on October 20-21
• Maxime Ripard will participate to the Linux Media Summit, on October 25
• Maxime Chevallier will participate to the Real Time Summit, also on October 25

While Bootlin CEO Michael Opdenacker was in the Embedded Linux Conference Europe Program Commitee for a number of years, he’s been replaced this year by Bootlin CTO Thomas Petazzoni. Bootlin was thus involved in the daunting but very interesting task of reviewing and selecting the talks to compose the program of this year’s event.

This is going to be a very busy week for us, and we are looking forward to attending the great talks proposed by all other speakers, and meeting the embedded Linux community once again!