Principle of device driver

Linux And Unix

Linux yes Unix A variant of the operating system , stay Linux The principle and idea of writing driver under is completely similar to other Unix System . stay Linux Environment design driver , Concise thought , It is easy to operate , It's also powerful , But there are few support functions , Can only rely on kernel The function in , Some common operations should be written by yourself , And debugging is not convenient .

Device driver

system call yes Operating system kernels and applications Between Interface , Device driver yes Operating system kernel and machine hardware Between Interface . The device driver is an application Shielding the hardware details , So in the eyes of the application , A hardware device is just a device file , Applications can operate on hardware devices as if they were ordinary files . Device drivers are part of the kernel , Running in a nuclear mindset , It performs the following functions ：

1. Initialize and release the device .
2. Transferring data from the kernel to the hardware and reading data from the hardware .
3. Read the data sent by the application to the device file and send back the data requested by the application .
4. Detect and handle errors in the device .

Device file type

stay Linux There are three main types of device files under the operating system ︰ Character device 、 Block device and Network interface .

Character device It means in I/O A device that transmits characters during transmission , For example, keyboard , Printers, etc . stay UNIX In the system , The character device occupies a position in the file directory tree in the form of a special file and has corresponding nodes . The character device can use the same file operation command as the normal file to operate the character device file , For example, open 、 close 、 read 、 Writing, etc .

Block device yes I/O One of the devices , Is to store information in fixed size blocks , Each block has its own address , You can also read a certain length of data at any position of the device , For example, hard disk ,U disc ,SD Card, etc .

Network interface Refers to various interfaces of network equipment , The network interfaces we are using today are all Ethernet interfaces . Common Ethernet interface types are RJ-45 Interface ,RJ-11 Interface ,SC Fiber optic interface ,FDDI Interface ,AUI Interface ,BNC Interface ,Console Interface .

Character device and block device The main difference yes ： Sending a read to a character device / When writing a request , Actual hardware I/O It usually happens immediately , Block devices are not . It uses a block of system memory as a buffer , When the user process requests the device to meet the user's requirements , Return the requested data . If not , Just call the request function to do the actual I/O operation . Block devices are mainly designed for slow devices such as disks , So as not to consume too much CPU Time to wait .

As mentioned above , User processes interact with actual hardware through device files . Each device file has its file attributes (c/b), Indicates whether it is a character device or a block device . In addition, each file has two device numbers , The first is the main equipment number , Identify the driver . The second is the slave device number , Identify different hardware devices that use the same device driver , For example, there are two floppy disks , They can be distinguished by the slave device number . The master device number of the device file must be consistent with the master device number applied by the device driver at the time of registration , Otherwise, the user process will not be able to access the driver .

Finally, it must be mentioned that , When the user process calls the driver , The system goes into a nuclear mindset , It is no longer preemptive scheduling . in other words , The system must wait until your driver's child functions return .