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Linux driver example for Per CPU Variable

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Per CPU Variables is an interesting feature available from Linux Kernel Version 2.6 When you define a Per CPU Variable, each processor in the system will have its own copy of the variable, hence no locking is required and better performance is achieved. Header File: <linux/percpu.h> To create a Per CPU Variable at compile time, use the macro: DEFINE_PER_CPU(type, name); E.g. DEFINE_PER_CPU(int, counter); As linux kernel is preemptible, you must use get_cpu_var macro to access the current processor's copy of a given variable And finally call put_cpu_var macro after you have completed using it. You can access another processor's copy of the variable with: per_cpu(variable, int cpu_id); Example: Output:
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Installing Raspbian Lite on Raspberry pi3

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Raspbian Lite is the operating system you want to choose, when you want to use sensors or web based applications on RaspberryPi. Raspbian vs Raspbian Lite: Raspbian lite is a minimal version of Raspbian. Following software/packages are removed from raspbian lite: Wolfram Libre Office Xserver It’s ideal if you intend to run the Raspberry Pi as a headless server without a GUI. Download Raspbian Lite from the Raspberry Website: http://downloads.raspberrypi.org/raspbian_lite/images/ To install, you can use various softwares. I am using etcher. Connect the SD Card, and flash the image  After flashing on the SD Card, connect it to the Raspberry Pi 3, connect Display over HDMI and power the device. You will get a login screen. Login Details: User name: pi, Password: raspberrypi Complete Initial Setup without connecting monitor and keyboard 1. Mount the SD Card and change directory to 'boot' volume. 2. Create wpa-supplicant.conf file wit...
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Linux driver example to print the current running CPU - smp_processor_id()

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When you have multiple processors present in the system, and want to find out on which the processor your driver code is running, use smp_processor_id(). Header file: #include <linux/smp.h> Example: Output: You can see from the above screenshot, the init function of the driver was running on Processor 1 and Kernel thread was running on Processor 4
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Single Linux Device Driver code for multiple Linux versions using LINUX_VERSION

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The arguments of the kernel functions changes with new releases. For example, Kernel versions > 3.10 use the below function to create a proc entry proc_file_entry = proc_create("proc_file_name", 0, NULL, &proc_file_fops); For kernel versions < 3.10, this API was proc_file_entry = create_proc_entry("proc_file_name", 0, NULL); If you want to have a Linux Device Driver which supports multiple Linux Versions, use the macro LINUX_VERSION_CODE This macro expands to the binary representation of the kernel version For example for Linux version 2.6.10, the value is 132618(0x02060a) There is another macro KERNEL_VERSION(major, minor, release). This builds up the version number from the individual numbers. For example KERNEL_VERSION(2,6,10) will return 132618 Sample Code: Output:
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Linux module to convert virtual to physical and vice versa

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virt_to_phys: Function converts kernel virtual address to physical address. phys_to_virt: Function converts physical address to kernel virtual address. Code: Output:
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Linux Device Driver code to load another module

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Linux Kernel code can load module whenever needed. int request_module(const char *module_name); Header File: <linux/kmod.h> Source Code: kernel/kmod.c request_module is synchronous, it will sleep until the attemp to load the module has been completed. When the kernel code calls request_mode(), a new kernel thread is created, which runs modprobe program in the user context. Code: Output:
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Linux Module to print number of CPU's

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We can look at /proc/cpuinfo to find out the number of processors present in Linux from user space. What if we want to find out the number of cpus in kernel module. num_online_cpus() function can give the number of CPU's which are online. Code: Output: References: https://stackoverflow.com/questions/43171805/find-number-of-cpus-in-linux-kernel
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