Marcus Folkesson

Embedded Linux Artist

Memory management in the kernel

Memory management in the kernel Memory management is among the most complex parts in the Linux kernel. There is so many critical parts such as page allocator, slab allocator, virtual memory handling, memory mapping, MMU, IOMMU and so on. All these parts has to work perfect (or at least allmost perfect :-) ) because all system do use them either they want to or not. If there is a bug or performance issue you will be noticed quite soon.

Posted by Marcus Folkesson on Wednesday, September 27, 2017

MMAP memory between kernel and userspace

MMAP memory between kernel and userspace Allocate memory in kernel space and then let the userspace map it to their virtual address space sounds like an easy task, and sure it's. There are just a few things that is good to know about page mapping. The MMU (Memory Management Unit) contains page tables with entries for mapping between virtual and physical addresses. These pages is the smallest units that the MMU deals with.

Posted by Marcus Folkesson on Wednesday, September 27, 2017

PID1 in containers

PID1 in containers What is PID 1 The top-most process in a UNIX system has PID (Process ID) 1 and is usually the init process. The Init process is the first userspace application started on a system and is started by the kernel at boottime. The kernel is looking in a few predefined paths (and the init kernel parameter). If no such application is found, the system will panic().

Posted by Marcus Folkesson on Wednesday, September 27, 2017

2.2" TFT display on Beaglebone

2.2" TFT display on Beaglebone I recently bought a 2.2" TFT display on Ebay (come on, 7 bucks...) and was up to use it with my BeagleBone. Luckily for me there was no Linux driver for the ILI9341 controller so it's just to roll up my sleeves and get to work. Boot up the BeagleBone I haven't booted up my bone for a while and support for the board seems

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Posted by Marcus Folkesson on Sunday, August 27, 2017

Magic SysRq

Magic SysRq Every kernel-hacker should knows about the magic SysRQ. Period. To enable the magic, make sure CONFIG_MAGIC_SYSRQ is set in your Kernel hacking tab. I use this feature a lot, mostly for set loglevel and reboot the system. But it is also very useful to get information out of the system even if it is completely frozen. As everybody is using GNU Screen (what else?) as their TTY terminal, the keyboard combination is: ctrl+A B.

Posted by Marcus Folkesson on Sunday, August 27, 2017

Take control of your Buffalo Linkstation NAS

Take control of your Buffalo Linkstation NAS I finally bought a NAS for all of my super-important stuff. It became a Buffalo Linkstation LS200, most because of the price ($300 for 4TB). It supports all of the standard protocols such as FTP, SAMBA, ATP and so on. However, it would be really useful to use some sane protocols like sftp so you could use rsync for your backup scripts.

Posted by Marcus Folkesson on Sunday, August 27, 2017

High resolution timers

High resolution timers Nearly all systems has some kind of Programmable Interrupt Timer (PIT) or High Precision Event Timer (HPET) that is programmed to periodically interrupt the operating system (if not configured with CONFIG_NO_HZ). The kernel performs several tasks in these ticks such as timekeeping, calculate statistics for the currently running process, schedule a new process and so on. The interrupt occurs at regular intervals - exactly HZ times per second.

Posted by Marcus Folkesson on Thursday, July 27, 2017

NAT with Linux

NAT with Linux To share an internet connection may sometimes be very practical when working with embedded devices. The network may have restrictions/authentications that stops you from plug in your device into the network of the big company you are working for. But what about creating your own network and use your computer as NAT (Network Address Translation)? It's not that hard to setup, it's actually just a few command lines away.

Posted by Marcus Folkesson on Thursday, July 27, 2017

-ENOENT, but believe me, it's there

-ENOENT, but believe me, it's there Almost every ELF-file in a Linux environment is dynamically linked, and the operating system has to locate all dynamic libraries in order to execute the file. To its help, it has the runtime dynamic linker, whose only job is to interpret the ELF file format, load the shared objects with unresolved references, and, at last, execute and pass over the control to the ELF file.

Posted by Marcus Folkesson on Thursday, July 13, 2017

Modules with parameters

Modules with parameters Everybody knows that modules can take parameters, either via /sys/modules/<module>/parameters, sysctl or via cmdline to the kernel, but how are these parameters created? Parameters without callbacks The Linux kernel provides the module_param() macro. The syntax is: 1module_param(name, type, perm) Which will simply create the module parameter and expose it as an entry in /sys/modules/<module>/parameters. Code example 1int debug_flag; 2module_param(debug_flag, bool, S_IRUSR | S_IWUSR | S_IRGRP) 3MODULE_PARM_DESC(debug_flag, "Set to 1 if debug should be enabled, 0 otherwise"); MODULE_PARM_DESC() is a short description of the parameter.

Posted by Marcus Folkesson on Tuesday, June 27, 2017