v  ARTICLE ON CHARACTER DRIVER:

THIS  ARTICLE   IS ALL ABOUT  TO  THE CONCEPTS  THAT  I HAVE  LEARNED  AND  MAINLY  CONCERNED TO THE WORK  WHATEVER  I HAVE PERFORMED  IN THE PROJECT  REGARDING ASSIGNMENT IN ORDER  TO  SUCCESSFUL  COMPLETION OF TRAINING.

v  INTRODUCTION OF CHARACTER DRIVER:

AS THE  NAME  SUGGESTS  ,THE DEVICE -DRIVER  PROVIDES THE MECHANISM  TO  DRIVE THE  HARDWARE(DEVICE)  ACCORDING  TO  THE  APPLICTION.THE TYPE & CATEGORY   OF THE DRIVER  IS DEPENDENT  ON  THE  TYPE &  REQUIREMENT OF APPLICATION  WHICH  WEAREUSING. WHICH MEANS TOSAYTHAT  TO ACCESS  THE  PENDRIVE WE NEED USB  DRIVER  ,FOR BTLUETOOTH WE WE NEED BT DRIVER & FOR  MEDIA  PLAYER  WE  NEED  A  SEPRATE ONE.  DIFFERENT TYPES OF APPLICATION  REQUIRES   DIFFERENT  TYPE  OF  DRIVERS.

CHARACTER DRIVER WORKS FOR CHARACTER DEVICES   WHICH  CAN BE ACCESSED AS STREAM OF BYTES. SUCH DRIVER USUALLY IMPLEMENTS OPEN, WRITE,  READ  &  CLOSE  SYSTEM  CALLS.

I think One of the good features of Linux is the ability to extend at runtime the set of features offered by the kernel. This means that you can add modules to the kernel (and remove modules as well) while the system is up and running. The need of using device drivers is to increase the functionality of kernel . Each piece of code that can be added to the kernel at runtime is called a module. Each module is made up of piece of codes that can be dynamically linked to the running kernel by the insmod program and can be unlinked by the rmmod program.

Currently I am working on character driver & its practical implementation. In this article I am going to discuss what i know about character driver briefly.

Now as I have to make my driver capable of directly handshake-able with the kernel so it was essential to work in kernel space instead of user space ,so i have to use the kernel libraries(like kernel.h, init.h ,fs.h etc..) instead of glibc libraries(like stdio.h,stdlib.h,fcntl.h etc…).

v  HOW   TO   IMPLEMENT?

• I AM GOING TO DESCRIBE THE BASIC  STEPS WHICH  I  HAVE  FOLLOWED WHILE  IMPLEMENTING CHARACTER DRIVER:

–     I started to write the program  using two predefined macros named as module_init (entry_function) and module_exit(exit_function) as entrance point as main() in user space and exit point respectively.In which I have written my own start &  end function. So after executing module_init execution will start from my function where I have to specify the major number and name of our driver, capacity to handle maximum number of device at a time and maximum memory of the driver. The major number identifies the driver associated with the device. I am firstly allocating major number to my driver which is basically a highest possible integer value between 0 to 255 by which our driver will be known in the kernel like other device drivers as stored in /proc/devices file .We can select major number for our driver from previously mentioned path but generally we privilege kernel to select major number because kernel will allocate it dynamically .This can be done with the register_chrdev_region() function call defined in fs.h  which is in kernel library .In current scenario register_allok_chardev_region is in trend to do the errlier mentioned task .This will give the result of 32 bit(12 + 20 bit) at the address as give in first argument of call. After that I have used the first 12 bit i.e major number but i specify my own minor number generally 0 to upward in MKDEV() macro defined in kdev_t.h. After that I have allocated memory to sculldev structure(user defined structure that contain all information of the driver)) & separately initialized all its members as sculldev initialization function .Now used the cdev_init() and cdev_add() to initialize and add the cdev structure for our device which will contains the device specifications. Now I have written separate functions for each ( to create scullQsets structures, create q-sets structures, create quantums)& called these functions from write & read functions of driver modules to perform the desired operations in device.

In order to perform various operations like open, write, read, lseek and close file I have written an application in  user space & mapped it similar kind if file operations (open, write read, llseek, release)that is in my driver program .So i have mapped these system calls in struct file_operations structure with my respective open,write,read,lseek and close functions according to prototype defined in <fs.h> kernel header file that will now handles the application calls instead of drivers of the OS.

But Before doing anything we have to map our allocated memory with the device memory this is done in the open function of driver with

container_of(ptr, type, member) in which first argument is pointer to i_cdev member of struct inode,2nd argument is user defined structure which contains pointer to struct cdev and third argument is pointer to struct cdev.Actually struct cdev is actual representation of the device. Now return of container_of will be starting address of mapped memory that we will use to read and write on device and i store this address in the private_data of the struct file structure defined in fs.h library.

After doing this now i have made my read, write and lseek function compatible with the almost all type of requests from application.In this write function will store the written data in own quantums when write request comes and give back that data according to the read request of application.I have also handled the single multithreaded application.

Now to test this driver we have to compile and insert our module in running kernel dynamically. This is done with command insmod ./modules/program name(without .c).ko  & Compilation of the file should be by Makefile. After completion of work (successful insertion & execution of task) we have to remove our module this is done by command rmmod  program name(without .c).

If every step is correct & written in proper manner then  program will run & it will give desired result otherwise kernel will be crashed.

REMARKS: THERE MAY BE  POSSIBILITY OF ERROR IN SYNTEX, SENTENCES,ARGUMENTS (& SOME-WHAT IN CONCEPTS ALSO ). SO ANY COMMENT, CORRECTION, SUGGESTION  IS CORDIONLY INVITED.

REGARD’S

RANA BRIJENDRA SINGH