Bootlin training courses for beginning of 2021

It’s the beginning of 2021, and Bootlin’s offering of online training courses continues. We have dates available for our 5 training courses, at an affordable cost, and with the same quality characteristics of all Bootlin courses: trainers with proven in-field experience, fully open-source training materials and worldwide recognized training contents.

Here are the dates of our upcoming sessions:

Embedded Linux system development course, taught by Bootlin CEO Michael Opdenacker, from January 11 to January 19, register for 829 EUR
Linux kernel driver development course, taught by Bootlin engineer Grégory Clement, who has 19 years of embedded development experience, is a Linux kernel maintainer, and has contributed over 400 patches to the official Linux kernel. This course takes place from January 11 to January 19, register for 829 EUR
Yocto and OpenEmbedded system development course, taught by Bootlin engineer Maxime Chevallier, one of Bootlin’s Yocto expert. This course takes place from January 18 to January 21, register for 519 EUR
Displaying and rendering graphics with Linux, taught by Bootlin engineer and multimedia expert Paul Kocialkowski. Paul has key experience in the Linux kernel DRM and Video4Linux subsystems. See for example his last two talks on graphics topics: An Overview of the Linux and Userspace Graphics Stack and Supporting Hardware-Accelerated Video Encoding with Mainline. This course will take place from January 19 to January 22, register for 519 EUR
Buildroot system development, taught by Bootlin CTO Thomas Petazzoni, who is one of the co-maintainers of Buildroot, and has contributed over 4000 patches to the project over the past 12 years. The course will take place from March 8 to March 11, register for 519 EUR

See our training page for more details about all our training courses!

Large Page Support for NAS systems on 32 bit ARM

The need for large page support on 32 bit ARM

Storage space has become more and more affordable to a point that it is now possible to have multiple hard drives of dozens of terabytes in a single consumer-grade device. With a few 10 TiB hard drives and thanks to RAID technology, storage capacities that exceed 16 or 32 TiB can easily be reached and at a relatively low cost.

However, a number of consumer NAS systems used in the field today are still based on 32 bit ARM processors. The problem is that, with Linux on a 32 bit system, it’s only possible to address up to 16 TiB of storage space. This is still true even with the ext4 filesystem, even though it uses 64 bit pointers.

We were lucky to have a customer contracting us to update older Large Page Support patches to a recent version of the Linux kernel. This set of patches are one way of overcoming this 16 TiB limitation for ARM 32-bit systems. Since updating this patch series was a non trivial task, we are happy to share the results of our efforts with the community, both through this blog post and through a patch series we posted to the Linux ARM kernel mailing list: ARM: Add support for large kernel page (from 8K to 64K).

How Large Page Support works

The 16 TiB limitation comes from the use of page->index which is a pgoff_t offset type corresponding to unsigned long. This limits us to a 32-bit page offsets, so with 4 KiB physical pages, we end up with a maximum of 16 TiB. A way to address this limitation is to use larger physical pages. We can reach 32 TiB with 8 KiB pages, 64 TiB with 16 KiB pages and up to 256 TiB with 64 KiB pages.

Before going further, the ARM32 Page Tables article from Linus Walleij is a good reference to understand how the Linux kernel deals with ARM32 page tables. In our case, we are only going to cover the non LPAE case. As explained there, the way the Linux kernel sees the page tables actually doesn’t match reality. First, the kernel deals with 4 levels of page tables while on hardware there are only 2 levels. In addition, while the ARM32 hardware stores only 256 PTEs in Page Tables, taking up only 1 KB, Linux optimizes things by storing in each 4 KB page two sets of 256 PTEs, and two sets of shadow PTEs that are used to store additional metadata needed by Linux about each page (such as the dirty and accessed/young bits). So, there is already some magic between what is presented to the Linux virtual memory management subsystem, and what is really programmed into the hardware page tables. To support large pages, the idea is to go further in this direction by emulating larger physical pages.

Our series (and especially patch 5: ARM: Add large kernel page support) proposes to pretend to have larger hardware pages. The ARM 32-bit architecture only supports 4 KiB or 64 KiB page sizes, but we would like to support intermediate values of 8 KiB, 16 KiB and 32 KiB as well. So what we do to support 8 KiB pages is that we tell Linux the hardware has 8 KiB pages, but in fact we simply use two consecutive 4 KiB pages at the hardware level that we manipulate and configure simultaneously. To support 16 KiB pages, we use 4 consecutive 4 KiB pages, for 32 KiB pages, we use 8 consecutive pages, etc. So really, we “emulate” having larger page sizes by grouping 2, 4 or 8 pages together. Adding this feature only required a few changes in the code, mainly dealing with ranges of pages every time we were dealing with a single page. Actually, most of the code in the series is about making it possible to modify the hard coded value of the hardware page size and fixing the assumptions associated to such a fixed value.

In addition to this emulated mechanism that we provide for 8 KiB, 16 KiB, 32 KiB and 64 KiB pages, we also added support for using real hardware 64 KiB pages as part of this patch series.

Overall the number of changes is very limited (271 lines added, 13 lines removed), and allows to use much larger storage devices. Here is the diffstat of the full patch series:

 arch/arm/include/asm/elf.h                  |  2 +-
 arch/arm/include/asm/fixmap.h               |  3 +-
 arch/arm/include/asm/page.h                 | 12 ++++
 arch/arm/include/asm/pgtable-2level-hwdef.h |  8 +++
 arch/arm/include/asm/pgtable-2level.h       |  6 +-
 arch/arm/include/asm/pgtable.h              |  4 ++
 arch/arm/include/asm/shmparam.h             |  4 ++
 arch/arm/include/asm/tlbflush.h             | 21 +++++-
 arch/arm/kernel/entry-common.S              | 13 ++++
 arch/arm/kernel/traps.c                     | 10 +++
 arch/arm/mm/Kconfig                         | 72 +++++++++++++++++++++
 arch/arm/mm/fault.c                         | 19 ++++++
 arch/arm/mm/mmu.c                           | 22 ++++++-
 arch/arm/mm/pgd.c                           |  2 +
 arch/arm/mm/proc-v7-2level.S                | 72 ++++++++++++++++++++-
 arch/arm/mm/tlb-v7.S                        | 14 +++-
 16 files changed, 271 insertions(+), 13 deletions(-)

This patch series is running in production now on some NAS devices from a very popular NAS brand.

Limitations and alternatives

The submission of our patch series is recent but this feature has actually been running for years on many NAS systems in the field. Our new series is based on the original patchset, with the purpose of submitting it to the mainline kernel community. However, there is little chance that it will ever be merged into the mainline kernel.

The main drawback of this approach are large pages themselves: as each file in the page cache uses at least one page, the memory wasted increases as the size of the pages increases. For this reason, Linus Torvalds was against similar series proposed in the past.

To show how much memory is wasted, Arnd Bergmann ran some numbers to measure the page cache overhead for a typical set of files (Linux 5.7 kernel sources) for 5 different page sizes:

Page size (KiB)	4	8	16	32	64
page cache usage (MiB)	1,023.26	1,209.54	1,628.39	2,557.31	4,550.88
factor over 4K pages	1.00x	1.18x	1.59x	2.50x	4.45x

We can see that while a factor of 1.18 is acceptable for 8 KiB pages, a 4.45 multiplier looks excessive with 64 KiB pages.

Actually, to make it possible to address large volumes on 32 bit ARM, another solution was pointed out during the review of our series. Instead of using larger pages which have an impact on the entire system, an alternative is to modify the way the filesystem addresses the memory by using 64 bits pgoff_t offsets. This has already been implemented in vendor kernels running in some NAS systems, but this has never been submitted to mainline developers.

Videos and slides of Bootlin’s talks at Live Embedded Event 2020

Yesterday, Bootlin co-organized and participated to the first edition of Live Embedded Event, a new online conference dedicated to embedded systems topics. In addition to co-organizing the event, we also gave four different talks at this conference, and we are happy to share the slides and videos of our talks.

Introduction video

The following video was presented during the introduction keynote of the event, where each sponsor and co-organizer could say a few words.

An overview of the Linux kernel and user-space graphics stack

Talk given by Bootlin engineer Paul Kocialkowski, slides available in PDF.

ASoC: Supporting Audio on an Embedded Board

Talk given by Bootlin engineer Alexandre Belloni, slides available in PDF.

Embedded Linux “from scratch” in 45 minutes…on RISC-V

Talk given by Bootlin founder and CEO Michael Opdenacker, slides available in PDF.

Device Tree: hardware description for everybody!

Talk given by Bootlin CTO Thomas Petazzoni, slides available in PDF.

4 talks from Bootlin at Live Embedded Event, December 3

As we announced back in October, Bootlin has participated to the organization of a new online conference around embedded systems: Live Embedded Event, which will take place on December 3. The registration is totally free, and the event will propose 4 tracks throughout the day, covering a wide range of topics. We encourage you to register and participate to the event!

Bootlin is not only co-organizing the event, but also has 4 talks that have been accepted:

An overview of the Linux and user-space graphics stack, by Paul Kocialkowski, Bootlin’s multimedia expert and author of our Linux Graphics training course. Talk abstract: Graphics with the Linux kernel is often perceived as a haystack, composed of many components that have complex interactions with eachother. While this is not always far from the truth, this talk will try to provide a clear overview of the different pieces in the stack and what their individual role is. Throughout the presentation, various APIs, protocols and software implementations will be demystified, covering both display and render aspects. This includes topics such as DRM, Wayland, Xorg, OpenGL, Mesa and more!
Embedded Linux from scratch in 45 minutes… on RISC-V, by Michael Opdenacker, Bootlin’s founder and CEO. Talk abstract: Join and discover how to build your own embedded Linux system completely from scratch. You will build your own toolchain, bootloader and kernel, this time on the Risc-V open Instruction Set Architecture, that you will run through a system emulated by QEMU. You will also build a minimal root filesystem by yourself thanks to the BusyBox project. You will finish by controlling the system through a tiny webserver. The approach will be to provide only the files that are strictly necessary. That’s all the interest of embedded Linux: you can really control and understand everything that runs on your system, and see how simple the system can be. That’s much easier than trying to understand how a GNU/Linux system works from a distribution as complex as Debian!
ASoC: audio on an embedded board, by Alexandre Belloni, Bootlin’s audio expert, Linux kernel and Yocto trainer, as well as kernel maintainer. Talk abstract: ASoC, which stands for ALSA System on Chip, is a Linux kernel subsystem created to provide better ALSA support for system-on-chip and portable audio codecs. It allows to reuse codec drivers across multiple architectures and provides an API to integrate them with the SoC audio interface. This talk will present the typical hardware architecture of audio devices on embedded platforms, how to integrate the audio codec and processor audio interface using device tree. Examples, common issues and debugging tips will also be discussed.
Device Tree: hardware description for everybody!, by Thomas Petazzoni, Bootlin’s CTO and co-maintainer of Buildroot. Talk abstract: The Device Tree has been adopted for the ARM 32-bit Linux kernel support almost a decade ago, and since then, its usage has expanded to many other CPU architectures in Linux, as well as bootloaders such as U-Boot and Barebox. Even though Device Tree is no longer a new thing, developers coming into the embedded Linux world often struggle to understand what Device Trees are, what is their syntax, how they interact with the Linux kernel device drivers, what Device Tree bindings are, and more. This talk will provide an introduction to the Device Tree, to jump start new developers in using this description language that is now ubiquitous in the vast majority of embedded Linux projects.

Note that we are aware of limitations of the platform that was chosen for the online conference: the details of the talks, and details about sponsors are only visible for people who registered. This is unfortunate, but we discovered this when it was too late to change the platform, together with a few other limitations. Let’s say this is a first edition, and regardless of those limitations, the selection of talks and contents is very promising!

Videos and slides from Bootlin talks at Embedded Linux Conference Europe 2020

The Embedded Linux Conference Europe took place online last week. While we definitely missed the experience of an in-person event, we strongly participated to this conference with no less than 7 talks on various topics showing Bootlin expertise in different fields: Linux kernel development in networking, multimedia and storage, but also build systems and tooling. We’re happy to be publishing now the slides and videos of our talks.

From the camera sensor to the user: the journey of a video frame, Maxime Chevallier