Marcus Folkesson

Embedded Linux Artist

I2C Bus Recovery

I2C Bus Recovery Brief I was working on a project where we had a problem with an I2C bus that was sporadically hanging during communication with a certain device. I2C works with open-drain lines, which means that devices can only pull the lines low, and a pull-up resistor is used to pull the line high. If a device misbehaves and holds one of the lines low, the bus will be stuck, and no further communication can take place.

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Posted by Marcus Folkesson on Saturday, October 4, 2025

Mutex guards in the Linux kernel

Mutex guards in the Linux kernel I found an interresting thread [1] while searching my inbox for something completely unrelated. Peter Zijistra has written a few cleanup functions that where introduced in v6.4 with this commit: commit 54da6a0924311c7cf5015533991e44fb8eb12773 Author: Peter Zijlstra <peterz@infradead.org> Date: Fri May 26 12:23:48 2023 +0200 locking: Introduce __cleanup() based infrastructure Use __attribute__((__cleanup__(func))) to build: - simple auto-release pointers using __free() - 'classes' with constructor and destructor semantics for scope-based resource management. - lock guards based on the above classes. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> It adds functionality to "guard" locks. The guard wraps the lock, takes ownership of the given mutex and release it as soon the guard leaves the scope. In other words - no more forgotten locks due to early exits.

Posted by Marcus Folkesson on Friday, December 1, 2023

Use b4 for kernel contributions

Use b4 for kernel contributions There is a little tool called b4 [1] that has been part of my workflow with the Linux kernel for a while. It's developed to be a tool used to simplify the work of the maintainers, but my main use of the tool has been to fetch patch series from the mailing list and apply them to my local git repository during reviews. I recently noticed that it got a lot of handy features (experimental though) for the contributors as well, which I now want to test!

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Posted by Marcus Folkesson on Wednesday, August 23, 2023

Linux wireless regulatory domains

Linux wireless regulatory domains I had a case where I had an embedded system that should act as a WiFi Access Point on the 5GHz band. The HW was capable and the system managed to act as a client to 5GHz networks, so everything looked good. However, the system could not create an access point on some frequencies. How is it that? It's all about regulatory domains! Regulatory domains Radio regulations is something that applies to all devices that make transmissions in the radio spectrum. The Linux kernel makes itself compliant to these regulations by make the regulatory restrictions as a directly part of the cfg80211 configuration API that all (new) wireless device drivers use.

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Posted by Marcus Folkesson on Monday, August 14, 2023

Add support for MCP39XX in Linux kernel

Add support for MCP39XX in Linux kernel I've maintained the MCP3911 driver in the Linux kernel for some time and continuously add support for new features [1] upon requests from people and companies. Microchip has several IC:s in this series of ADC:s that works similar to MCP3911. Actually, all other IC:s are register compatible but MCP3911. The IC:s I've extended support for is MCP3910, MCP3912, MCP3913, MCP3914, MCP3918 and MCP3919. The main difference between these IC:s from the driver perspective is the number of channels ranging from 1 to 8 channels and that the register map is not the same for all devices.

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Posted by Marcus Folkesson on Friday, August 4, 2023

Contiguous Memory Allocator

Contiguous Memory Allocator Introduction I do find memory management as one of the most fascinating subsystem in the Linux kernel, and I take every chance I see to talk about it. This post is inspired by a project I'm currently working on; an embedded Linux platform with a camera connected to the CSI-2 bus. Before we dig into which problems we could trip over, lets talk briefly about how the kernel handles memory.

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Posted by Marcus Folkesson on Monday, February 6, 2023

Audio and Embedded Linux

Audio and Embedded Linux Brief Last time I used wrote kernel drivers for the ASoC (ALSA System on Chip) subsystem, the functionality was split up into these parts: Platform class driver that defines the SoC audio interface for the actual CPU itself. This includes both the DAI (Digital Audio Interface) and any potential audio muxes (e.g. i.MX6 has its AUDMUX). CODEC class driver that controls the actual CODEC. Machine drivers that is the magic glue between the SoC and the CODEC which connect the both interfaces. Such driver had to be written for each SoC-CODEC combination, and that does not scale very well. Nowadays, most of the CODEC class drivers is now adapted to be described with the simple-audio-card [1] in a device tree, which will completely replace the machine drivers.

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Posted by Marcus Folkesson on Friday, December 23, 2022

Debug kernel with KGDB

Debug kernel with KGDB What is KGDB? KGDB intend to be used as a source code level debugger on a running Linux kernel. It works with GDB and allows the user to inspect memory, variables, setup breakpoints, step lines and instructions. Pretty much the same that all application developers are used to, but for the kernel itself. Almost every embedded Linux system does have a serial port available, and that is all that you need to connect GDB to your kernel.

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Posted by Marcus Folkesson on Saturday, December 17, 2022

What is libcamera and why should you use it?

What is libcamera and why should you use it Read out a picture from camera Once in a time, video devices was not that complex. To use a camera back then, your application software could iterated through /dev/video* devices and pick the camera that you want and then immediately start using it. You could query which pixel formats, frame rates, resolutions and all other properties that are supported by the camera. You could even easily change it if you want.

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Posted by Marcus Folkesson on Friday, November 25, 2022

HID report descriptors and Linux

HID report descriptors and Linux HID Devices USB HID (Human Interface Device) device class is the type of computer peripherals that human interacts with, such as keyboards, mice, game controllers and touchscreens. The protocol is probably one of the most simple protocols in the USB specification. Even if HID was originally written for USB in mind, it works with several other transport layers. Your mouse and keyboard do probably use HID over USB, the touchscreen in your smartphone could use HID over I2C. Even Bluetooth/BLE make use of the same protocol for this type of devices.

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Posted by Marcus Folkesson on Saturday, October 15, 2022