Tomorrow, on May 18, the third edition of Live Embedded Event will take place. Live Embedded Event is a free and fully online conference, dedicated to embedded topics at large. One can register directly online to receive a link to attend the conference.
Bootlin will be participating to this third edition, with 3 talks from 3 different Bootlin engineers:
Michael Opdenacker on LLVM tools for the Linux kernel, at 12:00 UTC+2 in Track 3. Details: Recent versions of Linux can be compiled with LLVM’s Clang C language compiler, in addition to Gcc, at least on today’s most popular CPU architectures. This presentation will show you how. Cross-compiling works differently with Clang: no architecture-specific cross-compiling toolchain is required. We will compare the Clang and Gcc compiled kernels, in terms of size and boot time. More generally, we will discuss the concrete benefits brought by being able to compile the kernel with this alternative compiler, in particular the LLVM specific kernel Makefile targets: clang-tidy and clang-analyzer.
Grégory Clement on AMP on Cortex A9 with Linux and OpenAMP, at 15:30 UTC+2 in Track 2. Details: While, usually, the Cortex A9 cores are used in SMP, one could want use one of the core to run an other OS. In this case the system becomes AMP. Typically, it allows running a dedicated real time OS on a core. This presentation will show the step that allow having this support using open sources stacks. First we will see what OpenAMP is, then how the Linux kernel can communicate with external OS using remote proc message, and finally what to adapt in the Linux kernel and OpenAMP in order to support the usage of a Cortex A9. This was experimented on an i.MX6 but the solution presented has the advantage to be easily adapted on any SoC using Cortex A9.
Thomas Perrot on PKCS#11 with OP-TEE at 15:00 UTC+2 in Track 2. Details:
PKCS#11 is a standard API that allows to manage cryptographic tokens, regardless of the platform such as Hardware Security Modules, Trusted Plaform Modules or smart cards. Moreover, modern processors offer a secure area, named Trusted Execution Environment (TEE) that allows the isolation of some operations, datas and devices to guarantee their integrity and confidentiality. OP-TEE is an open source implementation of Trusted Execution Environment that runs in parallel with the operating system, as a companion. In this talk, we will first introduce PKCS#11, then OP-TEE, and finally look at how PKCS#11 operations can be performed through OP-TEE, and what are the benefits. Our presentation will be illustrated with examples based on the NXP i.MX8QXP platform, but should be applicable to other platforms that have OP-TEE support.
Join us at Live Embedded Event, and discover our talks as well as the many other talks from other speakers!
In addition, both Thomas and Alexandre will be speaking at the event:
Thomas Petazzoni will give a talk Buildroot: what’s new?, providing an update on the improvements and new features in the Buildroot build system that have been integrated over the past two years
Alexandre Belloni will give a talk Yocto Project Autobuilders and the SWAT Team, during which he will explain what’s happening behind in the scenes in the Yocto Project to review and validate contributions before they are integrated.
Thomas and Alexandre will also naturally be available during the event to discuss business or career opportunities, so do not hesitate to get in touch if you’re interested.
Finally, prior to the event, Thomas Petazzoni will be in the Bay Area on June 13-15, also available for meetings or discussions.
As we mentioned in our last blog post about OP-TEE 3.16, Bootlin planned to and contributed some interesting features in the recently released OP-TEE 3.17 ! Here is a short presentation of our contributions to this release:
Summary
During this release cycle, Bootlin contributed the following features:
Watchdog support
Generic watchdog API
OP-TEE Watchdog service compatible with arm,smc-wdt Linux driver
As part of our work on Microchip SAMA5D2 support in OP-TEE, we wanted to have support for the SAMA5D2 watchdog. Doing so without exposing the watchdog to Linux would have been useless and thus, we implemented and contributed a new generic watchdog API to OP-TEE. This interface allows registering a watchdog against the system and exposing it to Linux through a specific SMC handler that interfaces with the Linux arm,smc-wdt compatible driver (see drivers/watchdog/arm_smc_wdt.c in the Linux kernel code). Our generic watchdog API is obviously used by the new watchdog driver for Microchip SAMA5D2, but was also quickly leveraged by ST who contributed a new watchdog driver for stm32mp1 based on this new watchdog API.
RTC
On Microchip SAMA5D2, the RTC is part of the system controller which needs to be secured since it contains critical features. Once in the secure world, the RTC is not available to the normal world. In order to expose this RTC device to the normal world (and particularly for Linux RTC subsystem), a new Pseudo Trusted Application (PTA) was added. This PTA communicates with a Linux OP-TEE compatible RTC driver and allows to get/set the date and time. This driver is generic and will allow any vendor which adds RTC support to OP-TEE to expose it transparently to Linux.
Contribution details
A total of 29 commits were contributed for OP-TEE 3.17:
Do not hesitate to contact us if you need help and support to integrate or deploy OP-TEE on your platform, either Microchip platforms, but also other ARM32 or ARM64 platforms.
Most of the BeagleBone boards from BeagleBoard.org share the same form factor, have the same headers and therefore can accept the same extension boards, also known as capes in the BeagleBoard world.
Of course, a careful PCB design was necessary to make this possible.
This must have been relatively easy with the early models (BeagleBone Black, Black Wireless, Green, Green Wireless, Black Industrial and Enhanced) which are based on the same Sitara AM3358 System on Chip (SoC) from Texas Instruments. However, the more recent creation of the BeagleBone AI board and keeping compatibility with existing capes must have been a little more complicated, as this board is based on a completely different SoC from Texas Instruments, the Sitara AM5729.
BeagleBone AI BeagleBone Green BeagleBone Black
Once the PCB design challenge was completed, the BeagleBoard.org crew set itself another challenge: implement software that supports each BeagleBone cape in the same way, whatever the board, in particular:
To have unique identifiers for devices in Linux, so that there is a stable name for Linux devices, even if at the hardware level they are connected differently, depending on whether the base board has a Sitara AM3358 and Sitara AM5729 SoC.
To have DT overlays for capes that are applicable to all base boards, even if peripherals are connected to different buses of the SoCs.
This article will explore the software solutions implemented by BeagleBoard.org. Their ideas can of course be reused by other projects with similar needs.
The need for a cape standard
A good summary can be found on Deepak Khatri’s Google Summer of Code 2020 page:
The idea of this project was to make the same user space examples work with both BeagleBone Black and BeagleBone AI, using the same references to drivers for peripherals assigned to the same pins between BeagleBone Black and BeagleBone AI. Also, Same DT overlays should work (whenever possible) for both BBB and BBAI, with updated U-Boot cape manager DT overlays will be automatically loaded during boot.
Software setup
The below instructions are for people owning the BeagleBone AI board and any other BeagleBone board, and interested in exploring the devices on their boards by themselves.
Uncompress each image, insert a micro-SD card in the card read in your PC, and then flash the corresponding card. Here are example commands, assuming that the micro-SD card is represented by the /dev/mmcblk0 device:
Connect the serial line of each board to your computer and then boot each board with it:
For BeagleBone AI: its sufficient to have the micro-SD card inserted.
On other BeagleBone boards, you may need to hold a button when you power up the board, to make it boot from the micro-SD card instead of from the internal eMMC. On the BeagleBone Black boards, for examples, that’s the USER button next to the USB host port. Note that you won’t need to do this again when you reset the board. The board will continue to boot from the external micro-SD card until it’s powered off.
You can connect with the default user, or connect as user root with password root.
Then, connect each board to the Internet, and get the latest package updates:
sudo apt update
sudo apt dist-upgrade
It’s then time to upgrade the kernel to the latest version supported by BeagleBoard. To do so, you’ll have to manually update the /opt/scripts/tools/update_kernel.sh file. In this file, go to the # parse commandline options section and add the below lines:
--lts-5_10-kernel|--lts-5_10)
kernel="LTS510"
;;
You can can then upgrade to the latest 5.10 kernel:
Note that on the BeagleBone Black, I2C1 and I2C2 are available at two different locations.
So, for both types of boards, we have at least I2C buses on P9_17/18, P9_19/20 and P9_24/26. However, that’s complicated because these pins don’t correspond to the same I2C buses. Therefore, users have to know that P9_19/20 correspond to I2C2 on BeagleBone Black and to I2C4 on BeagleBone AI.
The devices in /dev/ reflect such differences.
Here’s what we have on the BeagleBone AI:
root@beaglebone:~# ls -la /dev/i2c-*
crw------- 1 root root 89, 0 Mar 23 15:00 /dev/i2c-0
crw------- 1 root root 89, 3 Mar 23 15:00 /dev/i2c-3
Note that here with the AM5729 SoC, the first I2C bus is I2C1. Hence, /dev/i2c-0 corresponds to I2C1 (which is another I2C bus available on the SoC but not available through the cape headers) and /dev/i2c-3 corresponds to I2C4. Also note that I2C5 is not exposed in the default configuration that we have here, most probably because the corresponding header pins are used for other purposes.
And now let’s look at what we have on the BeagleBone Black:
root@beaglebone:~# ls -la /dev/i2c-*
crw------- 1 root root 89, 0 Mar 23 16:16 /dev/i2c-0
crw------- 1 root root 89, 1 Mar 23 16:17 /dev/i2c-1
crw------- 1 root root 89, 2 Mar 23 16:17 /dev/i2c-2
Here with the AM3358 SoC, /dev/i2c-0 corresponds to I2C0, /dev/ic2-1 to I2C1 and /dev/ic2-2 to I2C2.
So, on a running system, how to know which I2C bus device corresponds to the P9_19/20 header pins?
You can see that both devices, though they correspond to different devices, share the same symlink property, which is used to create a symbolic link in /dev/bone/i2c/ to the actual bus device file.
Let’s see such symbolic links on the BeagleBone AI:
root@beaglebone:~# ls -la /dev/bone/i2c/
total 0
drwxr-xr-x 2 root root 80 Jan 1 2000 .
drwxr-xr-x 4 root root 80 Jan 1 2000 ..
lrwxrwxrwx 1 root root 11 Mar 23 15:00 0 -> ../../i2c-0
lrwxrwxrwx 1 root root 11 Mar 23 15:00 2 -> ../../i2c-3
And on the BeagleBone Black:
root@beaglebone:~# ls -la /dev/bone/i2c/
total 0
drwxr-xr-x 2 root root 100 Mar 23 16:16 .
drwxr-xr-x 5 root root 100 Mar 23 16:16 ..
lrwxrwxrwx 1 root root 11 Mar 23 16:16 0 -> ../../i2c-0
lrwxrwxrwx 1 root root 11 Mar 23 16:17 1 -> ../../i2c-1
lrwxrwxrwx 1 root root 11 Mar 23 16:17 2 -> ../../i2c-2
You can see that at least /dev/bone/i2c/0 and /dev/bone/i2c/2 are shared between both types of boards. Userspace code examples can then support different boards by referring to such device file links, for example by using the I2C tools commands.
The symbolic links are created from the Device Tree sources not by the Linux kernel, but by the udev device manager, thanks to the following rule found in /etc/udev/rules.d/10-of-symlink.rules in the BeagleBoard Debian distribution:
# allow declaring a symlink for a device in DT
ATTR{device/of_node/symlink}!="", \
ENV{OF_SYMLINK}="%s{device/of_node/symlink}"
ENV{OF_SYMLINK}!="", ENV{DEVNAME}!="", \
SYMLINK+="%E{OF_SYMLINK}", \
TAG+="systemd", ENV{SYSTEMD_ALIAS}+="/dev/%E{OF_SYMLINK}"
Accessing other devices
Other devices are available in the same way through symbolic links in /dev/bone/, for example UART (serial port) devices.
Let’s check on the BeagleBone AI (run sudo apt install tree first):
Another challenge is with userspace software examples directly refer to header pins by their names. Here is a BoneScript push button demo, for example:
var b = require('bonescript');
b.pinMode('P8_19', b.INPUT);
b.pinMode('P8_13', b.OUTPUT);
setInterval(check,100);
function check(){
b.digitalRead('P8_19', checkButton);
}
function checkButton(x) {
if(x.value == 1){
b.digitalWrite('P8_13', b.HIGH);
}
else{
b.digitalWrite('P8_13', b.LOW);
}
}
For AM3358 BeagleBone boards, the am335x-bone-common-univ.dtsi file already associates the P8_19 name to a specific GPIO:
However, this driver is specific to the BeagleBoard.org kernel, and the Device Tree for Beagle Bone AI doesn’t use it yet, so this aspect is still work in progress. The main goal remains though: define generic names for header pins, which map to specific GPIOs on different boards.
The goal as stated in the beginning is to use the same Device Tree overlays on both types of SoCs. While as of today it doesn’t seem possible to generate compiled Device Tree Overlays (DTBO) which would support both SoCs at the same time, the BeagleBoard.org engineers have come up with a solution to achieve this at source level. This means that, for each cape to support, the Device Tree Overlay binaries for the supported SoCs can be produced from a unique source file.
While this hasn’t been deployed yet in the 5.10 BeagleBoard kernel, such source code is already available in Deepak Khatri’s own tree.
/*
* Copyright (C) 2020 Deepak Khatri
*
* Virtual cape for /dev/bone/can/1
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
/dts-v1/;
/plugin/;
/*
* Helper to show loaded overlays under: /proc/device-tree/chosen/overlays/
*/
&{/chosen} {
overlays {
BONE-CAN1 = __TIMESTAMP__;
};
};
/*
* Update the default pinmux of the pins.
* See these files for the phandles (&P9_* & &P8_*)
* https://github.com/lorforlinux/BeagleBoard-DeviceTrees/blob/compatibility/src/arm/am335x-bone-common-univ.dtsi
* https://github.com/lorforlinux/BeagleBoard-DeviceTrees/blob/compatibility/src/arm/am572x-bone-common-univ.dtsi
*/
&ocp {
P9_24_pinmux { pinctrl-0 = <&P9_24_can_pin>;}; /* can rx */
P9_26_pinmux { pinctrl-0 = <&P9_26_can_pin>;}; /* can tx */
};
/*
* See these files for the phandles (&bone_*) and other bone bus nodes
* https://github.com/lorforlinux/BeagleBoard-DeviceTrees/blob/compatibility/src/arm/bbai-bone-buses.dtsi
* https://github.com/lorforlinux/BeagleBoard-DeviceTrees/blob/compatibility/src/arm/bbb-bone-buses.dtsi
*/
&bone_can_1 {
status = "okay";
};
The &ocp code applies the pin muxing definitions for CAN on P9_24 (&P9_24_can_pin) and P9_26 (&P9_26_can_pin), which of course are different on AM3358 and AM5729.
By adding a symlink property to the Device Tree sources, BeagleBoard.org has made it possible to make userspace code, in particular its code examples, support all the BeagleBone boards at the same time, even though the devices they drive have are numbered differently on different SoCs.
Such a technique may be reused by other projects interested in running the same software on boards based on different SoCs.
As far as GPIOs are concerned, the drivers/gpio/gpio-of-helper.c driver is specific to the BeagleBoard.org kernel and is unlikely to be accepted in the mainline kernel in its current state. However, there are other solutions, supported by the mainline kernel, to associate names to GPIOs and then to look up such GPIOs by name through libgpiod.
Last but not least, it’s possible to use the same Device Tree Overlay source code to support an extension board on similar boards, just by using common definitions having different values on each different platform. Any project can reuse this idea, which just uses standard Device Tree syntax.
Bootlin thanks BeagleBoard.org for funding the creation of this blog post. Note that another post is coming in the next weeks, about the extension board manager we added to U-Boot thanks to funding from BeagleBoard.org.
Linux 5.17 has been released last Sunday. As usual, the best coverage of what is part of this release comes from LWN (part 1 and part 2), as well as KernelNewbies (unresponsive at the time of this writing) or CNX Software (for an ARM/RISC-V/MIPS focused description).
Bootlin contributed just 34 patches to this release, which isn’t a lot by the number of patches, but in fact includes a number of important new features. Also, we have many more contributions being discussed on the mailing lists or in preparation. For this 5.17 release here are the highlights of our contributions:
Alexandre Belloni, as the maintainer of the RTC subsystem, contributed one improvement to an RTC driver
Clément Léger improved the Microchip Ocelot Ethernet switch driver performance by implementing FDMA support. This allows network packets that are going from the switch to the CPU, or from the CPU to the switch to be received/sent in a much more efficient fashion than before. The Microchip Ocelot Ethernet switch driver was developed and upstreamed several years ago by Bootlin, see our previous blog post.
Clément Léger also contributed smaller fixes: a bug fix in the core software node code, and one PHY driver fix.
Hervé Codina implemented support for GPIO interrupts on the old ST Spear320 platform.
Miquèl Raynal contributed a brand new NAND controller driver, for the NAND controller found in the Renesas RZ/N1 SoC. We expect to contribute to many more aspects of the Renesas RZ/N1 Linux kernel support in the next few months.
Miquèl Raynal contributed a few Device Tree changes enabling the ADC on the Texas Instruments AM473x platform, after contributing the driver changes a few releases ago.
Miquèl Raynal started contributing some improvements to the 802.15.4 Linux kernel stack, and we also have many more changes in the pipe for this Linux kernel subsystem.
Thomas Perrot added support for the Sierra EM919X modem to the existing MHI PCI driver.
In the past months, the Linux kernel driver for the Ethernet MAC found in a number of Marvell SoCs, mvneta, has seen quite a few improvements. Lorenzo Bianconi brought support for XDP operations on non-linear buffers, a follow-up work on the already-great XDP support that offers very nice performances on that controller. Russell King contributed an improved, more generic and easier to maintain phylink support, to deal with the variety of embedded use-cases.
At that point, it’s getting difficult to squeeze more performances out of this controller. However, we still have some tricks we can use to improve some use-cases so in the past months, we’ve worked on implementing QoS features on mvneta, through the use of mqprio.
Multi-queue support
A simple Ethernet NIC (Network Interface Controller) could be described as a controller that handles some protocol-level aspect of the Ethernet standard, where the incoming and outgoing packets would be enqueued in a dedicated ring-buffer that serves as an interface between the controller and the kernel.
The buffer containing packets that needs to be sent is called the Transmit Queue, often written as txq. It’s fed by the kernel, where the NIC driver converts some struct sk_buff objects called skb, that represent packets trough their journey in the kernel, into buffers that are enqueued in the txq.
On the ingress side, the Receive Queue, written rxq, is fed by the MAC, and dequeued by the NIC driver, who creates skb containing the payload and attached metadata.
In the end, every incoming or outgoing packet is enqueued and dequeued in a typical first-in first-out fashion. When the queue is full, packets are dropped, everything is neat, simple and deterministic.
However, typical use-cases are never simple. Most modern NICs have several queues in TX and RX. On the receive side, it’s useful to have multiple queues for performance reasons. When receiving packets, we can spread the traffic across multiple queues (with RSS for example), and have one CPU core dedicated to each queue. More complex use-cases can be expressed, such as flow steering, that you can learn about in the kernel documentation.
On the transmit side, it’s a bit less obvious why we want to have multiple queues. If the CPU is the bottleneck in terms of performances, having multiple TX queues won’t help much. Still, there are ways to benefit from having multiple TX queues on a multi-cpu system with XPS.
However, when the line-rate is the bottleneck, having multiple queues gets useful. By sorting the outgoing traffic by priority and assign each priority to a TX queue, the NIC can then pick the next packet to send from the highest priority queue until it’s empty, and then move on to the next priority. That way, we implement what’s called QoS (Quality of Service), where we care about high-priority traffic making it through the controller.
QoS itself is useful for various reasons. Besides the obvious prioritization of high-value streams for not-so-neutral networking, we can also imagine tagging management traffic as high-priority, to guarantee the ability to remotely access a machine under heavy traffic.
Other applications can be in the realm of Time Sensitive Networking, where it’s important that time-sensitive traffic gets sent right-away, using the high-priority queues as shortcuts to bypass the low-priority over-crowded queues.
NICs can use more advanced algorithms to pick the queue to de-queue the packet from, in a process called arbitration, to give low-priority traffic a chance to get sent even when high-priority traffic takes most of the bandwidth.
Typical algorithms range from strict priority-based FIFO to more flexible Weighted Round-Robin arbitration, to allow at least a few low-priority packets to leave the NIC.
In the case of mvneta, we’re limited in terms of what we can do in the receive path. Most of the work we’ve done focused on the transmit side.
Traffic Priorisation and Arbitration
Multi-queue support on the TX path is a three-fold process.
First, we have to know which packets are high-priority and which aren’t, by assigning a value to the skb->priority field.
There are several ways of doing so, using iptables, nftables, tc, socket parameters through SO_PRIORITY, or switching parameters. This is outside of the scope of this article, we’ll assume that incoming packets are already prioritized through one of the above mechanisms.
Once the packet comes into the network stack as a skb, we need to assign it to a TX queue. That’s where tc-mqprio comes in play.
In that second step, we build a mapping between priorities and queues. This mapping is done through the use of an intermediate mapping, the Traffic Classes. A Traffic Class is a mapping between N priorities and M transmit queues. We therefore have 2 levels of mappings to configure :
The priority to Traffic Class mapping (N to 1 mapping)
The Traffic Class to TX queue mapping (1 to M mapping)
All of this is done in one single tc command. We’re not going to dive too much into the tc tool itself, but we’ll still see a bit how tc-mqprio works.
Queuing Disciplines (QDiscs) are algorithms you use to select how and when you enqueue and dequeue traffic on a NIC. It benefits from great software support, but can also be offloaded to hardware, as we’ll see.
So the first part of the command is about attaching the mqprio QDisc to the network interface :
tc qdisc add dev eth0 parent root handle 100 mqprio
After that, we configure the traffic classes. Here, we create 8 traffic classes :
num_tc 8
And then we map each class to a priority. In this list, the n-th element corresponds to the class you want to assign to priority n. Here, we map the 8 first priorities to a dedicated class. Priorities that aren’t assigned a class are mapped to the class 0.
map 0 1 2 3 4 5 6 7
Finally, we map each class to a queue. Classes can only be assigned a contiguous set of queues, so the only parameters needed for the mapping are the number of queues assigned to the class (the number before the @), and the offset in the queue list (after the @). We specify one mapping per class, the m-th element in the list being the mapping for class number m. In the following example, we simply assign one queue per traffic class :
queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7
Under the hood, tc-mqprio will actually assign one QDisc per queue and map the Traffic Classes to these QDiscs, so that we can still hook other tc configurations on a per-queue basis.
Finally, we enable hardware offloading of the prioritization :
hw 1
The last part of the work consists in configuring the hardware itself to correctly prioritize each queue. That’s done by the NIC driver, which gets the configuration from the tc hooks.
If we take a look at the Hardware, we see that the queues are exposed to the kernel, which will enqueue packets into them. Each queue then has a Bandwidth Limiter, followed by the arbitration layer. Finally, we have one last global Rate limiter, and the path then continues to the egress blocks of the controller.
The arbiter configuration is easy, it’s just a matter of enabling the strict priority arbitration in a few registers.
Let’s summarize what the stack looks like :
Traffic shaping
Being able to prioritize outgoing traffic is good, but we can step-it up by allowing to limit the rate on each queue. That way, we can neatly organize and control how we want to use our bandwidth, not on a per-packet level but really on a bits/s basis.
Controlling the rate of individual flows or queues is called Traffic Shaping. When configuring a shaper, not only do we focus on the desired max rate, but also how we deal with traffic bursts. We can smooth them by tightly controlling how and when we send packets, or we can allow some burstiness by being more lenient on how often we enforce the rate limitation.
In the mvneta hardware, we have 2 levels of shaping : one level of per-queue shapers, and one per-port shaper. They can all be controlled semi-independently, some parameters being shared between all shapers.
With mqprio, the shapers are configured on a per-Traffic-Class basis. Since the hardware only allows per-queue shaping, we enforce in the driver that one traffic class is assigned to only one queue.
A final configuration with mqprio looks like this :
Most shapers use a variant of the Token Bucket Filter algorithm. The principle is the following :
Each queue has a Token Bucket, with a limited capacity. When a packet needs to be sent, one token per bit in the packet gets taken from the bucket. If the bucket is empty, transmission stops. The bucket then gets refilled at a configurable rate, with a configurable amount of tokens.
Configuring the TBF for each queue boils down to setting a few parameters :
– The Bucket Size, sometimes called burst, buffer or maxburst.
It should be big enough to accommodate for the shaping rate required, but not too big. If the bucket is too big and you have a very slow traffic going out, the bucket is going to fill up to its full size. When you suddenly have a lot of traffic to send, you’ll first have a huge burst, the time for the bucket to empty, before the traffic starts to actually get limited. Unlike the tc-tbf interface, tc-mqprio doesn’t allow to change the burst size, so it’s up to the driver to set it.
– The Refill Rate, sometimes called tick, defining how often you refill the bucket.
– The Refill Value, defining how many tokens you put back into the bucket at each refill.
These two, combined together, define the actual rate limitation. The approach taken to select the value is to select a fixed value for one of the parameters, and vary the other one to select the desired rate. In the case of mvneta, we chose to use a fixed refill rate, and change the refill value. This means that we have a limited resolution in the rates we can express. In our case, we have a 10kbps resolution, allowing us to cover any rate limitation between 10kbps and 5Gbps, with 10k increments.
– One final parameter is the minburst or MTU parameter, and controls the minimum amount of tokens required to allow packet transmission.
Since transmission stops when the bucket is empty, we can end-up with an empty bucket in the middle of a packet being transmitted.The link-partner may not be too happy about that, so it’s common to set the Maximum Transmission Unit as the minimum amount of tokens required to trigger transmission.
That way, we are sure that when we start sending a packet, we’ll always have enough tokens in the bucket to send the full packet. We can play a bit with this parameter if we want to buffer small packets and send them in a short burst when we have enough. In our case, we simply configured it as the MTU, which is good enough for most cases.
In the end, the gains are not necessarily measurable in terms of raw performances, but thanks to mqprio and traffic shaping, we can now have better control on the traffic that comes out of our interface.
There are tons of other use-cases, for example limiting per-vlan speeds, or in contrary making sure that traffic on a specific vlan has the highest priority, and all of that mostly offloaded to the hardware itself !
The entire team at Bootlin is extremely happy to welcome Luca Ceresoli, who started working with us on March 1, 2022. Based in Italy, Luca is the first employee of Bootlin based outside of France, and we plan to continue to expand our hiring in a similar way in the future.
Luca brings a vast amount of embedded and embedded Linux expertise to the Bootlin team. Luca has been working on embedded products since 2002, and embedded Linux products since 2008. He has helped design, develop and ship products based on a large number of embedded processors, developing complete BSPs, developing Linux kernel drivers, integrating complete systems using Buildroot or Yocto. Luca has also contributed to the Linux kernel, Buildroot, and several open-source projects, and has spoken several times at FOSDEM, the Embedded Linux Conference, and other events.
Luca’s arrival at Bootlin not only means that we have more capacity to help our customers with their embedded Linux projects, but also that we have more capacity to deliver training courses, and a new ability to deliver some of our training courses in Italian.
Last year, Bootlin started contributing to the OP-TEE project, which is an open source Trusted Execution Environment (TEE) implemented using the Arm TrustZone technology. We published a blog post about our contribution of a generic clock framework to OP-TEE, and also presented a talk OP-TEE: When Linux Loses Control (slides, video).
As part of this work, Bootlin engineer Clément Léger contributed to the OP-TEE project, and many of his contributions have already been merged, and released as part of the 3.15 and 3.16 OP-TEE releases. In this blog post, we present some details of our contributions to OP-TEE so far, and our next steps.
Summary
Since then, we contributed a number of features and improvements to OP-TEE. First, a number of generic, HW-agnostic contributions:
We contributed 48 commits to OP-TEE 3.16.0. This level of contribution makes Bootlin engineer Clément Léger the second most active contributor by number of commits for this OP-TEE release.
We will continue our effort on sam5d2 support on OP-TEE and as part of this, there will be contributions on several generic subsystems as well as SAMA5D2 support:
Watchdog support
Generic watchdog API
OP-TEE Watchdog service compatible with arm,smc-wdt Linux driver
Sama5d2 watchdog driver
RTC support
Generic RTC API
OP-TEE RTC PTA to expose RTC to Linux
sama5d2 RTC driver
Linux driver for OP-TEE RTC
SAMA5D2 suspend support
Support forULP0, ULP1, ULP0 Fast and backup modes
PSCI support
SAMA5D2 interrupt controller support
Do not hesitate to contact us if you need help and support to integrate or deploy OP-TEE on your platform, either Microchip platforms, but also other ARM32 or ARM64 platforms.
The Device Tree language is a way to describe hardware that is present in a system and cannot be automatically detected. That’s the case of devices directly implemented on a System on a Chip, such as serial ports, Ethernet or Nand flash controllers. That’s also the case of devices connected to a number of buses, such as I2C and SPI, that do not provide mechanisms for dynamic enumeration and identification of devices.
For a given CPU architecture (ARM, PowerPC, etc), such a description allows to have a unique kernel supporting many different systems with distinct Systems on a Chip. The compiled Device Tree (DTB: Device Tree Binary), passed to the kernel by the bootloader at boot time, lets the kernel know which SoC and devices to initialize. Therefore, when you create a new board, and want to use a standard GNU/Linux distribution on it, all you have to do is create a new Device Tree describing your new hardware, compile it, and boot the distribution’s kernel with it. You don’t need to recompile that kernel, at least when it supports your SoC and the devices on your board.
Kernel developers and board maintainers are not the only ones who may need to make changes to the board device trees, though. Normal users also have legitimate reasons for tweaking the device tree of their board, such as:
Declaring external devices connected to board connectors. The simplest example is an I2C temperature sensor, but this can also include entire expansion boards plugged into the main board.
Changing the hardware blocks that are exposed on the external pins of their System on Chip. This is called Pin Multiplexing, and the Device Tree is the place where such multiplexing is configured.
Modifying operating system related properties, such as the Linux kernel command line or flash partitions.
In you have such a need, a first way to do this would be to directly modify the Device Tree for your hardware in the kernel sources, and recompile it. However, this is a rather bad idea because it will make it more difficult to upgrade to a later version of the kernel, possibly featuring changes to the Device Tree file you modified too. It’s always better not to tweak files maintained by others, and find a way to override them instead.
A second possibility to tweak the Device Tree for a board is to include it from a custom Device Tree for your own use case, using the /include/ statement from the Device Tree language, or #include and the C preprocessor. You can then use references to nodes in the original tree, using labels, and then modify the properties of such nodes or add new subnodes to them. It’s even possible to remove existing nodes and properties by using the /delete-node/ and /delete-property/ statements, as shown in the Device Tree Source Undocumented page.
However, this is still an inconvenient solution. Any time you plug in a new device or an expansion board, or want to tweak other settings, you have to rebuild and update the full Device Tree for your board. What if you could prepare Device Tree fragments for each change to make, compile them once for all, and then, when you boot your device, load the main Device Tree and only the fragments you need, without having anything else to recompile?
This is exactly what Device Tree Overlays are about. For example, in /lib/firmware, the Debian image shipped by BeagleBoard.org contains compiled Device Tree Overlays (DTBO) for the extension boards (also known as capes in the BeagleBoard world) they support, such as:
Then, at boot time, all you have to do is load the main DTB for your board, and then override it using the DTBOs corresponding to the capes which are currently mounted. Kernel contributors have also worked on solutions to load the DTBOs dynamically in the live system, but this solution is not mature yet. Therefore, the bootloader based solution at boot time remains so far the best one.
Writing Device Tree overlays
When the Device Tree overlays were first proposed, their syntax was very special and hard to remember. Fortunately, since version 1.5, the Device Tree compiler (dtc) supports a much more natural syntax for writing such overlays. For example, here are the sources for the overlay for the Relay cape, taken from the BBORG_RELAY-00A2.dts file in BeagleBoard.org’s Linux kernel tree:
BeagleBone Relay Cape
This example shows us what is specific to Device Tree Overlay source code, compared to the normal Device Tree syntax:
Before any definition, the code must include the /plugin/; statement.
As in normal Device Tree code, you can refer to labels (such as &am33xx_pinmux in the above example), modify their properties, add or remove nodes and properties.
However, you cannot modify nodes which do not have a label. In this case, you have to recall such nodes by describing their absolute path from the root device between curly braces. There are two such instances in the above code: &{/chosen} and &{/}.
Compiling Device Tree overlays
Once you have the source code of the your overlay, it is supposed to be easy to compile it with the Device Tree Compiler. This time, let’s take the example of the BBORG_FAN-A000.dts overlay for the Fan cape, slightly modified:
The only change we made to this file was to replace __TIMESTAMP__ by a real timestamp. This statement is actually a macro meant to be substituted by the C Preprocesor before compiling with dtc.
This is where real life Device Tree overlay examples are getting more difficult to compile. Very simple overlays can be compiled with the above command, but as soon as the code contains #include statements or macros to be substituted by the C preprocessor, you have to call this preprocessor and give it the path to where the corresponding .h files are found in the kernel sources.
Fortunately, the kernel Makefile knows a suitable series of commands to compile such overlay sources.
Let’s clone BeagleBoard.org’s Linux kernel source tree. It contains Device Tree Overlays for most capes that are compatible with BeagleBoarg.org’s boards, and if I understood correctly, BeagleBoard.org’s kernel developers haven’t solved all the conflicts between their definitions and recent kernels yet. So, let’s checkout their 5.10 branch:
git clone https://github.com/beagleboard/linux.git
cd linux
git branch -a
git checkout 5.10
First, let’s install a toolchain, for example the one on Ubuntu:
sudo apt install gcc-arm-linux-gnueabihf
And then let’s prepare the environment for compiling the kernel:
Then, we can configure the kernel for the OMAP SoCs:
make omap2plus_defconfig
We also need to tweak the configuration for compiling the Device Trees and Overlays properly. So, run make menuconfig and set CONFIG_OF_OVERLAY=y. For the practical manipulations, you will also need to set CONFIG_LEDS_GPIO=y and CONFIG_LEDS_CLASS=y.
Device Tree Overlays are found in arch/arm/boot/dts/overlays/. You can compile them along with all normal DTBs with:
make dtbs
In case you want to recompile a specific DT overlay, for example the BBORG_RELAY-00A2.dts file:
$ touch arch/arm/boot/dts/overlays/BBORG_RELAY-00A2.dts
$ make dtbs
DTCO arch/arm/boot/dts/overlays/BBORG_RELAY-00A2.dtbo
Of course, for any of this to be useful at the end, you will also need to compile the kernel:
make -j8 zImage
Applying Device Tree overlays
As we already explained, U-Boot is the place where the Device Tree Overlays should be applied. To do this, here are the commands to run in U-Boot. As an example, we’re using the BeagleBone Black and its Relay Cape as example, assuming all the files where copied to the first partition of a micro-SD card, containing a FAT filesystem:
Allocate extra space for the DTB for future overlays, here adding 8192 bytes for example:
fdt resize 8192
Apply the overlay that we just loaded to the main DTB:
fdt apply 0x83000000
We are then ready to load the Linux kernel and boot it. Let’s see a more complete example…
Example usage on BeagleBone Black
You can of course follow this example, but you can also test it by yourself if you own the BeagleBone Black (or similar BeagleBone boards), and its Relay Cape.
Board setup
First, connect the Relay Cape to your board. In the below example, more capes are actually stacked, the Relay Cape being on top, as its volume doesn’t allow for further capes:
Then, take a microSD card and format its first partition with the FAT32 filesystem:
sudo mkfs.vfat -F 32 -n boot /dev/mmcblk0p1
Remove the microSD card and insert it again. It should now be mounted on /media/$USER/boot. Download an archive containing all the files you should need for the BeagleBone Black and its Relay Cape, and extract this archive in this mount point.
This archive contains U-Boot binaries for the BeagleBone Black, a DTB for the same board, Device Tree Overlays for BeagleBone capes under the overlays/ directory, and a kernel including its own root filesystem as an initramfs. This way, you don’t have to prepare another partition to hold the root filesystem. Note that the src/ subdirectory contains the Buildroot 2021.02 configuration to generate the root filesystem, as well as the Linux kernel configuration that we used.
To boot on U-Boot on the microSD card, press the USER button next to the USB host connector, when you power-up the board. Note that this is needed only once, until the board is completely switched off again. The board will continue to boot from the microSD card across reboots and resets. Make sure that the U-Boot prompt shows a 2021.07 U-Boot version.
Once in U-Boot, you can load the kernel, DTB and DTBO, apply the overlay as shown in the previous session:
You are now ready to boot your kernel with its updated Device Tree:
bootz 0x81000000 - 0x82000000
Then, log in with the root user and an empty password.
Checking from Linux that the overlay was applied
The first thing you can check is that the device tree overlay was indeed applied. You can do this by checking properties which are specific to the overlay, such as the one that was added to the chosen node (see the BBORG_RELAY-00A2.dts sources once again.
$ cd /proc/device-tree/chosen/overlays/
$ ls
BBORG_RELAY-00A2.kernel name
Our overlay specific property is there, but in a more general case, look for properties under /proc/device-tree which are specific to each overlay.
Testing the Relay cape
Have a look again at BBORG_RELAY-00A2.dts file. In particular, for each relay, it declares an LED which is controlled by the same GPIO:
Each LED is given a label that will correspond to a directory under /sys/class/leds:
# ls /sys/class/leds/relay-*
/sys/class/leds/relay-jp1:
brightness max_brightness subsystem uevent
device power trigger
/sys/class/leds/relay-jp2:
brightness max_brightness subsystem uevent
device power trigger
/sys/class/leds/relay-jp3:
brightness max_brightness subsystem uevent
device power trigger
/sys/class/leds/relay-jp4:
brightness max_brightness subsystem uevent
device power trigger
Since the each LED and associated relay are controlled by the same GPIO, this gives us a convenient interface to control the each relay. We just have to control the corresponding LED by using its interface in /sys/class/leds.
This may not be a generic solution to control relays, but it will be easier than having to use the libgpiod library and its associated tools.
Testing a relay for real
Now, let’s try to control Relay 3.
You are ready to connect a real circuit now. When the relay is inactive, the COM (Common) and NO (Normally Open) ports should be disconnected, while COM and NC (Normally Connected) should be connected. When you write 1 to the GPIO, the relay should then connect COM and NO:
The high voltage and low voltage parts of a relay are supposed to be separate and insulated, but in case their was a defect on my cape, I made my tests with a simple low-voltage circuit. However, relays are meant to be used for controlling normal AC voltages (110V / 230V).
Initial state – Relay off
Now, turn on Relay 3 by setting a non zero brightness on the corresponding LED:
echo 1 > /sys/class/leds/relay-jp3/brightness
Relay on, after echo 1 > /sys/class/leds/relay-jp3/brightness
You can turn the relay back off as follows:
echo 0 > /sys/class/leds/relay-jp3/brightness
Don’t hesitate to use this cape in your home automation projects!
What to remember
Device Tree Overlays are Device Tree fragments used to customize the Device Tree of a given board, typically to describe components added externally, including entire expansion boards, or tweak pin multiplexing. That’s the mechanism used by BeagleBoard.org’s kernel developers to support the numerous expansion boards (also known as capes) compatible with their boards. All you have to do is load the main Device Tree Binary for a board (DTB), and then load and apply Device Tree Overlay Binaries (DTBO).
Bootlin thanks BeagleBoard.org for funding the creation of this blog post. Note that more posts are coming in the next weeks, one about the BeagleBone Cape Interface Specification and one about the extension board manager we added to U-Boot.
Linux 5.16 has been released on January 9. As usual, our recommended reading to learn more about this release is the corresponding Kernelnewbies.org page and the two articles from LWN covering the 5.16 merge window: part 1 and part 2.
As usual, Bootlin contributed a number of patches to this release, with a total of 117 commits, making Bootlin the 22th contributing company according to statistics (Unknown and Hobbyists not counting as companies).
Here are the main highlights of our contributions to 5.16:
Alexandre Belloni, as the maintainer of the RTC subsystem, continued to improve the core subsystem and RTC drivers. He added a new user-space interface to RTC to allow getting/settings RTC parameters. It is used to get the features of an RTC from user-space, to get/set the RTC correction or to configure Backup Switch Mode. In addition, Alexandre made various improvements to several RTC drivers, such as adding Backup Switch Mode, and general refactoring.
Clément Léger did a small fix in the clock drivers for Microchip ARM platforms, fixing an issue discovered as part of his work porting OP-TEE to Microchip ARM platforms.
Hervé Codina made some fixes to the fsmc NAND controller driver, which is used on a number of old platforms from ST. They fix support of certain NAND chips on those platforms. These issues were discovered as part of the development of a Linux BSP for an old ST Spear320 platform.
Maxime Chevallier fixed a deadlock in the network stack, that was causing the kernel to stop booting when using a root filesystem over NFS combined with the network interface using a SFP module.
Miquèl Raynal contributed many improvements to the max1027 ADC driver in the IIO subsystem, supporting hardware triggers.
Miquèl Raynal contributed support for the ADC found on Texas Instruments AM437x platforms. This required significant changes in the MFD driver that is used to support the multiple features of this ADC, as well as improvements in the existing IIO driver for TI ADCs.
Paul Kocialkowski contributed a small addition enabling the Rockchip VPU on Rockchip PX30 platforms, and merged the Device Tree bindings for the logiCVC display controller (but not yet the driver itself).
Tomorrow, on May 18, the third edition of Live Embedded Event will take place. Live Embedded Event is a free and fully online conference, dedicated to embedded topics at large. One can register directly online to receive a link to attend the conference.
The entire team at Bootlin is extremely happy to welcome 
