Introduction to Linux Device Driver Programming

Author: Gaurav Dhiman
Email: gauravd_linux@yahoo.com, gaurav4lkg@gmail.com


Introduction to Linux Device Drivers:

Linux Device Driver is actually the peace of code which very well knows the device it is controlling. It knows the behavior and has knowledge of device internals. Device Drivers in Linux can be a part of core kernel it self or it can even be developed as a separate module, which can be attached/detached from running kernel anytime, providing a flexibility in kernel to support multiple devices in dynamic environment.

In this article we will talk about writing a device driver as a kernel module. Before talking about device drivers, we should have some basic knowledge of kernel module programming in Linux. In next few sections we will discuss the basics of kernel module programming before jumping to the driver intricacies.


Introduction to Kernel Module Programming:

Kernel module is a piece of code written, compiled and loaded separately from core kernel but linked to the core kernel at load time. Kernel modules have some specific structure to follow. There are two standard functions which need to be implemented in any Linux Kernel Module; we will talk about them bit later. As earlier also mentioned kernel module is a code which attaches to the core kernel at load time and is being executed in kernel or privileged mode. Its not a user program which runs in restricted mode and have limited access to memory and other system resources. Module being a part of the kernel can access any system resource. Due to this fact, kernel module programmers need to take care that their module does not create the security loopholes in kernel and must be well-disciplined. Kernel modules should only be doing what they are supposed to do. One loosely written kernel module loaded to the core kernel can make the whole system venerable.

Module structure:
Module need to have two standard functions which take care of initializing and cleaning up of resources used by module. The standard format of module is as follows:

#include
#include

/*standard function for initializing the resources used by module*/

int init_module(void){
}

/*other function, which mplements the functionality of module*/

func_1(){
……..
……..
}

func_2(){
………
………
}

func_3(){
………
……..
}

……..
…….
………

/*standard function for initializing the resources used by module*/

void cleanup_module(void){
}

In init_module() function, we request and acquire the resources required by our module. For e.g. resources can be an IRQ line (interrupt line on which our device is going to interrupt), I/O memory region, DMA channel etc. We also register few other things like major number used by our driver or the interrupt handlers for the interrupts generated by our device. Init_module() function is called at the time of loading a module to core kernel, using “insmod” or “modprobe” command.

In cleanup_module() function, we release the resources acquired in init_module() function. This function is called at the time uloading a module from core kernel, using “rmmod” command.


Presenting real time Kernel Module:
Let’s follow the tradition and write the first kernel module “hello_world.c”

#define __KERNEL__
#define MODULE

#include
#include

int init_module(void){
printk(<1> “My Module: Hello World !! \n”);
}

void cleanup_module(void ){
printk(<1> “My Module: Bye World ….. I am going !! \n”);
}


We need to define above mentioned two macros (__KERNEL and MODULE) for any module we write. printk() function is a brother of printf() function. The major difference between them is that printf() is a standard C library function which resides in user space and printk() is kernel function written from scratch.


Now coming the real thing – Device Drivers
Before going to the real code of device driver, let’s first understand the some basic fundamentals of what devices are in Linux. We will also discuss about how they are accessed by user processes.


Device Files and Major Numbers:
In Linux or and other Unix variant, devices re represented by files in file system. These files are known as device files or even nodes. Device files are special files through with the user process (program) can communicate (open, read, write, close etc) with the underlining device. Process uses the standard system calls, like open(), read(), write(), ioctl() etc to interact with device. With every device file a two special number are associated, which are known as a major number and minor number of a device. Major number is used by kernel to direct the user process request to right kernel driver and minor number is useless for kernel. Minor number is only used by the driver to identify the exact device which needs to be manipulated. The reason of existence of minor number is that, in practical scenario one driver can handle or control more than one device of same type or even of different types as well, so driver needs some mechanism through which it can identify which device it need to manipulate. Lets take an example, if there is a driver ”A”, which actually controls three physical devices “B”, “C” and “D”, then there need to be three device files in file system with same major number but different minor numbers.

Device files can be created with “mknod” command. It has the following syntax.

mknod {device name} {device type} {major number} {minor number}

For help on this command, refer to man pages of it.
It is a convention that device files reside in “/dev” directory of root file system, so it’s always better to create our own device files there rather than creating them somewhere else in file system.


Device Types:
Devices can mainly be categorized in three groups: character devices, block devices and the network devices.

Character Devices: These are the devices to which a user process can write or read a single character at a time. That means the interaction between device driver and actual physical device is in terms of single character. Example: keyboard, serial ports, parallel ports.

Block Devices: These are the devices to which a user process can write or read a single block of data at a time. Reading and writing on these devices are done in terms of block data. A block can be of 512 bytes, 1024 bytes or so. Example: Hard Disk, Floppy Disk.

Network Devices: These are asynchronous devices and are responsible for establishing a network connection to outside world. Best example of this type of device can be NIC card.

For not complicating things, this article will only talk about the character device drivers.


Opening a Device:
As mentioned earlier also that user process can communicate with the underlining device through device file, so for interacting with device, user process should first open the related device file, using open() system call. Once the device file is opened, user process receives the file descriptor, which it can refer in further file manipulation file system calls.

Now we will see how things work in kernel when a device file is opened by a user process. Before discussing it, let’s discuss about some related data structures.

task_struct: This data structure represents the process or task in kernel. Kernel uses this data structure to keep track of process in system and resources they are using. It is one of the main data structures in kernel and contains number of elements to track process specific information.

files_struct: This structure contains the information related to the open files per process. It keeps information to track the open files of a process. One of the elements of this structure is an array of pointers to “file” structure. This array points to different “file” structures and the index of this array is returned to process as a file descriptor when open system call is made. It also keep a count of total number of open files for a process.

file: This structure represents an open file for a process. Do not confuse it with physical file, which is represented by “inode” structure in kernel. This structure only remains in kernel memory till the file is open for a process. As soon as the process closes, exits or aborts, all the “file” structures (representing open files for that process) are destroyed, if those are not anymore pointed by any other process.

Some of the elements of “file” structure:

- f_mode: This element tells in what mode the file has been opened by the process. Process can open a file in either read (FMODE_READ) or write (FMODE_WRITE) mode. This element is normally used by device drivers to check in what mode the device file has been opened.

- f_pos: This element tells the offset (in bytes) from where to read and write in file.
- f_flags: This element also tells us in which mode the file has been opened (read or write), but it’s always recommendable to use “f_mode” element to check the mode of file. This element remembers one important thing, which might be helpful to driver writers and that is if the file has been opened in non-blocking mode or not. By default (if O_NONBLOCK flag is not mentioned at opening time) the file is opened in blocking mode. Driver checks this flag at the time of reading or writing a device. If the device file is opened in blocking mode (default mode) and at read time there is no data to be read from device or at write time driver specific buffer is full, driver puts the process to sleep on one of its local wait queues (we will soon see what wait queue are). But on other hand if the device file has been opened in non-blocking mode then the driver does not put the process to sleep, rather control returns back to user process with error.
- f_count: This element keeps track of how many processes are referring to this instance of file. As we know that all files of parent process are inherited by child process if file does not have close_on_exec element set. If the child process inherits the files from parent process, the “f_count” element of all inherited files is incremented, so that kernel can keep track of number of process this file structure. “file” structure (representing an open file) does not get destroyed on all close system calls, as it might be shared by other process also. During close system call kernel checks the “f_count” element of “file” structure and if it is zero then only “file” structure is released and its memory Is released.
- f_owner: This element tells which process is the owner of this open file. It contains the pid of the owner process. This element is used for sending the SIGIO signal to the right process in case of asynchronous notification. User process can change this element by using fcntl() system call.
- f_op: This is an important element from the perspective of device driver. This element actually points to the structure of pointers to file operations. For a device file (represented by “file” structure), this element points to the structure, which further contains pointers to driver specific functions. We will discuss in detail, the structure (file_operations) to which this element points.


- file_operations: This is an important data structure for device driver, as this is the structure through which driver registers its functions with kernel, so that kernel can call them on different events, like opening a device file, reading/writing a device file or sending ioctl commands to device. In case of device file, this structure contains pointer to different functions of driver, through which kernel invokes the driver. Now we will briefly discuss the elements of this data structure.


Some of the elements of “file_operations” structure

- llseek: This is a pointer to driver function, which actually moves or sets the “f_pos” element of device file (discussed earlier).
- read: This is a pointer to driver function, which actually physically reads data from device.
- write: This is a pointer to driver function, which actually physically writes data to device.
- poll: This function is called by either “poll” or “select” system calls.
- ioctl: This function is called by ioctl system call. This function in drier is used to pass on the special commands to device, format the device or setting the read/write head of device, which are different from normal read / write commands.
- mmap: This function of driver is used to map the device memory area to process virtual address space.
- open: This function is used to open a file. Incase of device file, this function of driver initialize the device or initializes other book keeping data structures.
- flush: This function flushes the driver buffer to physical file. This should be implemented in driver if driver wants to provide a facility to application to make sure that all the data is physically put on device.
- release: This function is called by close system call, but it is not called for every close system call. As described earlier also that one “file” data structure (which represents an open file in kernel) can be referred by more than one process, if processes are sharing the file (best example is FIFO files), in that case close system file does not release the “file” data structure and only decrement the “f_count” element of “file” data structure. If after decrementing “f_count” element turns to be zero, close system call, calls the associated “release” function, which is a driver function in case of device files. So “release” function of driver should clear and release all the memory acquired. “release” is just an opposite of “open” function.


Few Important Kernel Mechanisms used in Drivers

Wait Queues:
Wait queue is a mechanism in kernel through which the kernel code can put the process to sleep. This is used in different parts of kernel where the kernel decides to put the process to sleep. Kernel puts the process to sleep in case the required event has not yet occurred (for e.g. some process wants to read from device and there is no data to be read), in this case kernel puts the process to sleep and gets back the processor from it by calling the schedule() function, which is a scheduler in Linux Kernel. schedule() function schedules and dispatches the other process.

Before discussing the function related to sleeping a process, we should look what data structures are used for implementing a wait queue in kernel.

Wait queue is actually a linked list of “wait_queue_t” type of structures. The head of wait queue is represented by “wait_queue_head_t” structure, which contains the spin lock to synchronize the access to wait queue. “wait_queue_head_t” structure also contains the pointer to the first element in wait queue. Each element in the wait queue is represented by “wait_queue_t” structure, which contains the pointer to the “task_struct” type of structure. It also contains the pointer to next element in the wait queue. “task_struct” represents the alive process in kernel. So with this mechanism of wait queue driver or any kernel part can keep track of process waiting for a specific event to occur.


Putting process to sleep:
Process can be put to sleep by using any of the following kernel functions. You can call these functions from anywhere in the kernel (drivers, modules or the core kernel) in case you want to put your process to sleep. Whenever a kernel code is executed (when system call is made by the user process), kernel code executes in the context of process which has made a system call. But there is exception to this rule, whenever the interrupt occurs the kernel code (interrupt handler) does not execute in process context, it’s a anonymous context. This is the reason that we should be careful to not to call any function in interrupt handler which can put the execution thread to sleep. If we do so the kernel will hang, that means the system will hang.

Functions which can put a process to sleep:
- sleep_on(wait_queue_head_t * wait_queue)
- interruptible_sleep_on(wait_queue_head_t * wait_queue)
- sleep_on_timeout(wait_queue_head_t * wait_queue, long timeout)
- interruptible_sleep_on_timeout(wait_queue_head_t * wait_queue, long timeout)

In above functions, “wait_queue” is the wait_queue_head and “timeout” is the value mentioned in terms of jiffies. We will talk about jiffies very soon. Now we will see the difference between above mentioned functions.

- sleep_on: This function puts the process to sleep in TASK_UNINTERRPTIBLE mode, which means the process will not be waked up in case process receives any signal while it was in sleep. The process will only be waked up any other part of kernel code wakes it up (normally on the occurrence of some event) deliberately by calling any of the waking function (we will be discussing the waking up functions very soon). Process put to sleep with this function can sometimes cause some problem. For e.g. if a process is put to sleep with this function and the event on which it need to be waked up does not occur then your process will not come back to the execution stage. That process can not even be killed by sending a KILL signal, as process in sleep in TASK_UNINTERRUPTIBLE mode ignores all signals. Process put to sleep with this function can be waked if any of the following conditions occur:

o Process is deliberately waked up by some part of the kernel code on the occurrence of event for which it was waiting

- interruptible_sleep_on: This function in kernel is written to avoid the problem caused by “sleep_on” function. This function puts the process to sleep in TASK_INTERRUPTIBLE mode. When a process sleeps in this mode, it can be waked up if any of the following condition occurs:

o Process receives the signal either from any other process or kernel itself.
o Process is deliberately waked up by some part of the kernel code on the occurrence of event for which it was waiting.

- sleep_on_timeout: This function is similar to “sleep_on” function but is not that much dangerous as “sleep_on”. Process put to sleep with this function can be waked if any of the following conditions occurs:

o Time mentioned in the timeout parameter has expired
o Process is deliberately waked up by some part of the kernel code on the occurrence of event for which it was waiting.

- interruptible_sleep_on_timeout: I hope by now you can easily guess what this function does. Well the process put to sleep with this function is waked up when any of the following conditions occurs:

o Process receives the signal either from any other process or kernel itself.
o Time mentioned in the timeout parameter has expired
o Process is deliberately waked up by some part of the kernel code on the occurrence of event for which it was waiting.

Waking up a process:
Process put to sleep should also be waked up by some kernel code else the process will never return to the execution state. If your driver is putting the process to sleep, it’s the responsibility of that driver itself to wake up the sleeping processes when the required event occurs for which those processes are waiting. For e.g. if your driver put the reading process to sleep on its internal waiting queue, if there is nothing to read from driver buffer (driver buffer empty) then the process put to sleep should also be waked up whoever new data arrives in driver buffer (this will occur when device interrupts, so interrupt handler will be responsible for waking up the process sleeping on the driver’s waiting queue).

Functions which can be explicitly called to wake up the process:
- wake_up(wait_queue_head_t * wait_queue)
- wake_up_interruptible(wait_queue_head_t * wait_queue)
- wake_up_sync(wait_queue_head_t * wait_queue)
- wake_up_interruptible_sync(wait_queue_head_t * wait_queue)

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That’s it for this time. In next article I will cover ‘ioctl’ interface of devices and different timing mechanisms available in Linux Kernel.