Abstractions for a piece of software
Can be run in user space but is mostly run in the kernel space
On Linux file is the most common abstraction for communicating with devices
File (User space) → Device Driver (Kernel space) → Device (Hardware)
There are a few kernel APIs that expose the drivers to the user space (/sys, /dev)
Device drivers are exposed in the /dev directory
Device drivers are of two types : Char and Block
These drivers have three main info associated with them: Type, Minor & Major no.
We communicate with char devices by sharing bytes of data while with block devices a chunk of data is shared at a time
The major number represents the type of the device while the minor number is to distinguish devices that have the same major version
To implement a char driver we need to define the major and minor version of the device
Implement the file operations required by the driver (read, write, open, close)
Register the char driver in the kernel
Depending on the device architecture there are different ways to talk with the hardware device. Two of the most common approach is Port I/O and Memory-mapped I/O
In Port IO a dedicated bus is used to communicate with the hardware while in Mapped IO a memory space is shared by the hardware device
MIMO is the most common approach used by architectures like ARM
To write using MIMO we need to request the address of the MIMO registers
Map them an virtual address using kernel functions
And finally read and write data to these registers