pseudo_char_drv

Pseudo Char Driver

This is a pseudo character driver implementation.
The driver manages a memory region in the hardware allowing to the user to access this memory for performing write and read operations. This region is defined as char device_buffer[DEV_MEM_SIZE]; where DEV_MEM_SIZE is set to 512 bytes.
This driver implements:

• llseek as pcd_llseek.
• read as pcd_read.
• write as pcd_write.
• open as pcd_open.
• release as pcd_release.

When the driver is registered in the virtual file system (VFS) the user can access to the driver in /dev/pcd and /sys/class/pcd_class/pcd.
A diagram showing an overview is here:

  flowchart BT
    subgraph Hardware
      A(Memory Buffer)
    end
    subgraph Kernel Space
      subgraph pcdev [pcdev driver pcd.c]
        B1(pcd_lseek)
        B2(pcd_read)
        B3(pcd_write)
        B4(pcd_open)
        B5(pcd_release)
      end
      A --- pcdev
      pcdev --- C(Virtual File System)
    end
    subgraph User Space
      C --- D(/dev/pcd)
      C --- E(/sys/class/pcd_class/pcd)
    end

Loading

Compile

For compiling you can use:

make host

For loading the kernel object:

sudo insmod pcd.ko 

You can watch the printed information using (this command shows the last 5 lines of dmesg output):

dmesg | tail -5

Test

For testing the write function:

echo "Hello, I am a linux user" > /dev/pcd 
dmesg | tail -8

You will get an output like this:

[  474.722033] pcd_driver_init : Device number <major>:<minor> = 234:0
[  474.722377] pcd_driver_init : Module init was successful
[  976.007133] pcd_open : open was successful
[  976.007214] pcd_write : write requested for 25 bytes
[  976.007223] pcd_write : current file position = 0
[  976.007232] pcd_write : number of bytes successfully written = 25
[  976.007239] pcd_write : updated file position = 25
[  976.007293] pcd_release : release was successful

You can check the read function:

cat /dev/pcd 
dmesg | tail -10

You will get an output like this (the number of read bytes is 512 because it is the size of the memory driver):

[ 1193.243847] pcd_open : open was successful
[ 1193.243862] pcd_read : read requested for 131072 bytes
[ 1193.243864] pcd_read : current file position = 0
[ 1193.243866] pcd_read : number of bytes successfully read = 512
[ 1193.243867] pcd_read : updated file position = 512
[ 1193.243879] pcd_read : read requested for 131072 bytes
[ 1193.243880] pcd_read : current file position = 512
[ 1193.243881] pcd_read : number of bytes successfully read = 0
[ 1193.243882] pcd_read : updated file position = 512
[ 1193.243890] pcd_release : release was successful

You can also test an error for no memory space in the driver:

cp test_file /dev/pcd
dmesg | tail -9

You will get an output like this:

[ 2064.957977] pcd_open : open was successful
[ 2064.958066] pcd_write : write requested for 1024 bytes
[ 2064.958073] pcd_write : current file position = 0
[ 2064.958078] pcd_write : number of bytes successfully written = 512
[ 2064.958082] pcd_write : updated file position = 512
[ 2064.958089] pcd_write : write requested for 512 bytes
[ 2064.958092] pcd_write : current file position = 512
[ 2064.958096] pcd_write : No space left on the device
[ 2064.958280] pcd_release : release was successful