Ftrace: The Linux kernel hacker’s swiss knife

Introduction I ran into ftrace completely by chance, while trying to figure out why the call to usb_submit_urb() took so long time. In fact, it wasn’t. It was pr_info() that delayed the output. And it was ftrace that got me to realize that. Whether you’re into dissecting existing kernel code, and want to know which [...]

Linux driver: Creating device files for a hotpluggable device

Introduction Most device drivers hook onto an already existing subsystem: Audio, networking, I2C, whatever. If it’s a specialized piece of hardware, a single device file is typically enough, and you get away with a misc device or, as shown in the kernel’s USB example (drivers/usb/usb-skeleton.c), with the USB framework’s usb_register_dev() call. However in some cases, [...]

Linux kernel programming: Do I need a lock?

Introduction Writing a device driver for Linux (or other kernel programming) always requires keeping parallel execution in mind. It’s often enough to follow common programming patterns, using spinlocks and mutexes in the same way that everyone else does. But somehow, I always find myself looking at my own code, and ask myself: Am I absolutely [...]

Jots on named pipes (FIFOs in Linuxish)

Major disclaimer These are pretty random jots that I made while evaluating named pipes as a solution for project. I eventually went for a driver in the kernel for various reasons, so I never got to verify that anything written below is actually correct. I’ve also written a small post on epoll with named pipes [...]

usbpiper: A single-threaded /dev/cuse and libusb-based endpoint to device file translator

Introduction Based upon CUSE, libusb and the kernel’s epoll capability, this is a single-threaded utility which generates one /dev/usbpiper_* device file for each bulk / interrupt endpoint on a USB device. For example, /dev/usbpiper_bulk_in_01 and /dev/usbpiper_bulk_out_03. It’s an unfinished project, that was stopped before a lot of obvious tasks in the TODO list were done. [...]

FUSE / CUSE kernel driver dissection notes

What this post is about Before anything: If you’re planning on using FUSE / CUSE for an application, be sure to read this first. It also explains why I bothered looking at the kernel code instead of using libfuse. So these are some quite random notes I took while trying to figure out how to [...]

FUSE / CUSE signal handling: The very gory details

First: If you’re planning on using FUSE / CUSE for an application, be sure to read this first. It also explains why I didn’t just take what libfuse offered. Overview This is a detour from another post of mine, which dissects the FUSE / CUSE kernel driver. I wrote this separate post on signal handling [...]

Linux CUSE (and FUSE): Why I ditched two months of work with it

Introduction If you’re planning to use CUSE (or FUSE) for an application you care about, this post is for you. That includes future self. I’m summarizing my not-so-pleasant journey with this framework here, with focus on how I gradually realized that I should start from the scratch with an old-school kernel module instead. Most important, [...]

xhci_hcd WARN Event TRB for slot x ep y with no TDs queued

What’s this? There’s a chance that you’re reading this because the message in the title appeared (or flooded) your kernel log. This post attempts to clarify what to do about it, depending on how much you want to get involved in the matter. So the short answer: The said warning message is a bug related [...]

Linux: Atheros QCA6174′s Bluetooth disappearing after reboot

When Bluetooth goes poof Having rebooted my computer after a few months of continuous operation, I suddenly failed to use my Bluetooth headphones. It took some time to figure out that the problem wasn’t with the Cinnamon 3.8.9 fancy stuff, nor the DBus interface, which produced error messages. There was simply no Bluetooth device in [...]