Slides and videos of Bootlin talks at Live Embedded Event #2

The second edition of Live Embedded Event took place on June 3rd, exactly 6 months after the first edition. Even though there were a few issues with the online platform, it was once again great to learn new things about embedded, and share some of the work we’ve been doing at Bootlin on various topics. For the next edition, we plan to switch to a different online platform, hopefully providing a better experience.

But in the mean time, all videos of the event have been posted on the Youtube Channel of the event. The talks from Bootlin have been posted on Bootlin’s Youtube Channel.

Indeed, in addition to being part of the organization committee, Bootlin prepared and delivered 5 talks as part of Live Embedded Event, covering different topics we have worked on in the recent months for our customers.

Understanding U-Boot Falcon Mode and adding support for new boards, Michael Opdenacker

Slides [PDF]

Introduction to RAUC, Kamel Bouhara

Slides [PDF]

Security vulnerability tracking tools in Buildroot, Thomas Petazzoni

Slides [PDF]

Secure boot in embedded Linux systems, Thomas Perrot

Slides [PDF]

Device Tree overlays and U-boot extension board management, Köry Maincent

Slides [PDF]

Upcoming online training courses in 2021

Online training courseThroughout this first half of 2021, our online training courses available for individual registration have been very popular. We have added some new dates for this summer and early fall for all our courses:

You can register directly online through Eventbrite and pay by credit card, or request an invoice. Note that our sessions are regularly full: our embedded Linux training course next month is full one month before the session, so make sure to book your seat early enough. We offer a 100 EUR early bird discount for registrations taking place at least one month before the course.

These courses are delivered entirely online: you don’t need any hardware to participate, as the hands-on labs are replaced by live demonstrations made by the trainer. All you need is a web browser, a good Internet connection and an audio headset!

We can also organize private courses upon request, if you have a larger group of engineers to train on these topics. Contact us for details!

Bootlin welcomes Clément Léger in its team

Welcome on board!Since June 1st, we’re happy to have an additional engineer in our team, Clément Léger!

After graduating from ENSIMAG in 2012, Clément spent 9 years working for Kalray, a silicon vendor company based in France, designing and producing an innovative and advanced new multi-core CPU architecture. At Kalray, Clément was in charge of porting a bootloader and the Linux kernel to this new CPU architecture, working on all aspects needed to support the CPU in the arch/ of Linux (syscalls, interrupts, exceptions, MMU, etc.) as well as developing a number of core kernel drivers such as pinctrl, irqchip, remoteproc and spimem.

In our engineering team at Bootlin, Clément will help our customer with bootloader porting, Linux kernel porting, device driver development, integration of complete Linux BSPs and more. He brings an additional significant experience in low-level kernel development and debugging to our team of experts. Clément will be working remotely from Grenoble, in tight connection with our team in Lyon.

See Clément Léger’s page on our site, as well as the rest of our team.

Bringing NV-DDR support to parallel NAND flashes in Linux

We have recently contributed support for NV-DDR interfaces to parallel NAND flashes in the Linux kernel, which brings performance improvements for a number of NAND flash devices. In this article, we will detail what are the ONFI specifications, the historical SDR interface, then the introduction of faster interfaces in the ONFI specification, and finally our work to support such interfaces in the Linux kernel.

ONFI specifications

Even though specifications came after the introduction of NAND devices on the market, the Open NAND Flash Interface (ONFI) specification is nowadays a de-facto specification which many NAND chip support (even non-ONFI ones). For instance, in the Linux kernel, we assume that any NAND flash device will by default, after a reset command, at least support the slowest set of ONFI timings. Other specifications exist, like the Joint Electron Device Engineering Council (JEDEC), but as it is a bit less common in the parallel NAND flashes world, we will focus on the ONFI details in this blog post.

The early days of the SDR interface

At the time of the first ONFI specification back in 2006, there was only a single interface detailed: the asynchronous data interface. Also known as Single Data Rate or SDR interface in modern language, it defines the timings sequence that should be respected in order for any NAND controller to be able to deal with almost any kind of NAND device. As an asynchronous interface, in this interface, the data bus has no clock signal. Instead, it features a specific set of signals which are asserted by the controller to signal read data latch and write data latch: Read Enable (RE#) and Write Enable (WE#).

The data interface can work in 6 different timing modes, from 0 to 5. 0 is the slowest mode and the default one at boot time with a theoretical data rate of about 10MiB/s (assuming an 8-bit bus). Mode 4 and 5 are the fastest, they leverage the ability of Extended Data Output (EDO) to latch data on both RE#/WE# edges and may reach a theoretical data rate of 50MiB/s.

The introduction of faster interfaces

Shortly after, at the beginning of 2008, the ONFI consortium released the second version of the ONFI specification and included a new interface: the source synchronous data interface. This interface is backward compatible with the asynchronous interface and allows the host to switch from one interface to the other if this is needed. In the particular case of the source synchronous interface, a clock (CLK) signal is replacing the legacy WE# signal and indicates when the commands and address should be latched. The direction of the transfers is handled by the Write/Read signal (W/R#) in place of RE# signal. Finally, a data strobe (DQS) signal is being introduced and indicates when the data should be latched. As both edges of the DQS signal advertise for a data latch, the source synchronous interface is also called Double Data Rate (DDR) interface even though this naming was only introduced in the version 3.0 of the specification, in 2011.

The exact terms that are used in more recent specifications are NV-DDR (Non-Volatile DDR), NV-DDR2 and NV-DDR3 which are backward compatible improvements of the NV-DDR interface. For instance, the first NV-DDR specification has a range of theoretical rates from 40MiB/s to 200MiB/s.

ONFI datasheet on data interfaces

Support in the Linux kernel

While the addition of the MTD/NAND subsystem in the Linux kernel predates the Git era and is now over 20 years old, Linux users have always been limited to use the asynchronous interface (SDR modes). At Bootlin, we recently started an effort to bring support for the NV-DDR interface to the Linux kernel MTD/NAND subsystem, and this involved the following changes:

  • Introducing an API to propose timings to the host controller driver, so that it might either accept or refuse them (only SDR mode 0 cannot be refused) and be aware of all timings that this choice involves so that the host controller registers will be configured properly.
  • Adding the possibility for NAND chip drivers to tweak the timings if the parameter page is not present or inaccurate.
  • Adding the core logic to ask the NAND chip to change its data interface through the use of GET_FEATURE and SET_FEATURE calls, as well as verifying that this operation worked correctly and handling the fallback in case of error.

We recently reached a final step in this effort as the last missing parts will be part of the next Linux kernel release (v5.14). This final series aiming at bringing NV-DDR support to Linux carries the following changes:

  • Adding the necessary bits to parse the parameter page of the NAND device in order to know which NV-DDR modes the chips support.
  • Providing the reference implementation of all NV-DDR timing modes and various helpers to manage them.
  • Adding the necessary infrastructure and helpers to the host controller drivers in order to allow them to distinguish between SDR and NV-DDR, as well as advertise which mode they are willing to support based on the controller’s constraints.
  • Updating the existing logic to take into account the existence of NV-DDR timings and select them when appropriate. This part is a bit trickier as the core must gracefully fallback to SDR modes under certain conditions.

Overall, thanks to the major cleanups which happened in the NAND subsystem in the last three years, it was pretty straightforward to add support for these new timings.

Future work

It is worth mentioning that accelerating the overall throughput on the data bus without a deeper rework of the MTD core than just enabling faster timings is very limiting: data reads must respect a tR delay before starting and writes are considered effective only after a tPROG delay. Both are significantly high in practice: respectively about 25-45us and 200-600us, compared to the time needed to store/fetch the data through the I/O bus: a few dozens of micro-seconds.

To fully leverage the power of NV-DDR timings the NAND and MTD cores should be partially rewritten to bring parallel multi-die support and cached operations. Such features would allow to optimize the use of the I/O bus in order to mitigate the performances impact of tR and tPROG during massive I/O operations. This is precisely one of the tricks used by SSD drives to exhibit very fast I/Os while using multiple NAND chips behind. There is therefore interesting additional work to do in the Linux kernel MTD subsystem to fully benefit from NV-DDR interfaces.

CFP for the Embedded Linux Conference 2021 ends June 13th

Initially planned to take place in Dublin, Ireland, the unique edition this year of the Embedded Linux Conference will take place in Seattle, US and virtually from September 27 to September 30, 2021. See also the conference website. Bootlin CEO Thomas Petazzoni is again a member of the program committee for this edition of ELC.

Embedded Linux Conference 2021

This kind of event is only possible thanks to the talks proposed by its participants! As detailed on the Call For Papers, the last date to submit your proposals is June 13, 2021. There is really a wide range of suggested topics, and ELC is an excellent place to talk about advancements in the Linux kernel for embedded platforms, in user-space libraries and stacks relevant to embedded, about practical experiences in using Linux in embedded devices, about real-time, boot time, power management, build systems, open hardware, and more.

We look forward to seeing your proposals for ELC!