pseudo_char_drv_multiple

Pseudo Char Driver for Multiple Devices

This is a multi device pseudo character driver implementation.
The driver manages one memory region in the hardware for each device (4 devices are implemented), allowing to the user to access this memory for performing write and read operations. This region is defined as char device_buffer_pcdevx[MEM_SIZE_MAX_PCDEVX]; for x from 1 to 4. Each device has some particular data in addition of the memory size, these are a serial number and an access permission. These data are listed below:

	pcdev-1	pcdev-2	pcdev-3	pcdev-4
Size (bytes)	1024	512	1024	512
Serial Number	PCDEV1XYZ123	PCDEV2XYZ123	PCDEV3XYZ123	PCDEV4XYZ123
Access Permissions	RDONLY	WRONLY	RDWR	RDWR

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 devices in /dev/pcdev-1, /dev/pcdev-2, /dev/pcdev-3, /dev/pcdev-4.
A diagram showing an overview is here:

  flowchart BT
    subgraph Hardware
      A1(Memory Buffer 1)
      A2(Memory Buffer 2)
      A3(Memory Buffer 3)
      A4(Memory Buffer 4)
    end
    subgraph Kernel Space
      subgraph pcdev [pcdev driver pcd_n.c]
        B1(pcd_lseek)
        B2(pcd_read)
        B3(pcd_write)
        B4(pcd_open)
        B5(pcd_release)
      end
      A1 --- pcdev
      A2 --- pcdev
      A3 --- pcdev
      A4 --- pcdev
      pcdev --- C(Virtual File System)
    end
    subgraph User Space
      subgraph pcdev-1
        direction TB
        D(/dev/pcdev-1) ~~~ E(/sys/class/pcd_class/pcdev-1)
      end
      subgraph pcdev-2
        direction TB
        F(/dev/pcdev-2) ~~~ G(/sys/class/pcd_class/pcdev-2)
      end
      subgraph pcdev-3
        direction TB
        H(/dev/pcdev-3) ~~~ I(/sys/class/pcd_class/pcdev-3)
      end
      subgraph pcdev-4
        direction TB
        J(/dev/pcdev-4) ~~~ K(/sys/class/pcd_class/pcdev-4)
      end
    end
    C --- pcdev-1
    C --- pcdev-2
    C --- pcdev-3
    C --- pcdev-4

Loading

Compile

For compiling the driver:

make host

Test

For loading the kernel object:

sudo insmod pcd_n.ko
dmesg | tail -5

You will get an output like this:

[ 5930.575731] pcd_driver_init : Device number <major>:<minor> = 234:0
[ 5930.576019] pcd_driver_init : Device number <major>:<minor> = 234:1
[ 5930.576069] pcd_driver_init : Device number <major>:<minor> = 234:2
[ 5930.576105] pcd_driver_init : Device number <major>:<minor> = 234:3
[ 5930.576140] pcd_driver_init : Module init was successful

You can list the created devices (it should be 4 devices):

ls -l /dev/pcdev-*

Trying to write in device 1:

echo hello > /dev/pcdev-1
dmesg | tail -2

You will get an error due to this device has only read permission:

[ 5960.275978] pcd_open : minor access = 0
[ 5960.275985] pcd_open : open was unsuccessful

Other way to test the write permission in device 1:

strace dd if=pcd_n.c of=/dev/pcdev-1

In the output of this command you will find this line:

openat(AT_FDCWD, "/dev/pcdev-1", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 EPERM (Operation not permitted)

You can try writing in device 2 (I use count=1 for avoiding to request more than once write process and generate an no memory error):

dd if=pcd_n.c of=/dev/pcdev-2 count=1

You will get an output like this:

1+0 records in
1+0 records out
512 bytes copied, 0.000130635 s, 3.9 MB/s

And using dmesg:

[ 6920.575835] pcd_open : minor access = 1
[ 6920.575842] pcd_open : open was successful
[ 6920.575867] pcd_write : write requested for 512 bytes
[ 6920.575869] pcd_write : current file position = 0
[ 6920.575871] pcd_write : number of bytes successfully written = 512
[ 6920.575873] pcd_write : updated file position = 512
[ 6920.575888] pcd_release : release was successful

You can test the reading of device 2:

dd if=/dev/pcdev-2 of=file.txt count=1

You will get an error like this:

dd: failed to open '/dev/pcdev-2': Operation not permitted

If you try the write and read process in device 3 (with bs=100 you order to write only 100 bytes):

dd if=pcd_n.c of=/dev/pcdev-3 count=1 bs=100

The output:

1+0 records in
1+0 records out
100 bytes copied, 0.000223722 s, 447 kB/s

The dmesg output:

[ 7331.087137] pcd_open : minor access = 2
[ 7331.087154] pcd_open : open was successful
[ 7331.087204] pcd_write : write requested for 100 bytes
[ 7331.087206] pcd_write : current file position = 0
[ 7331.087208] pcd_write : number of bytes successfully written = 100
[ 7331.087210] pcd_write : updated file position = 100
[ 7331.087237] pcd_release : release was successful

For reading the device 3:

cat /dev/pcdev-3

The output:

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/device.h>
#i

The dmesg output (observe the number of read bytes is 1024, the size of the memory for this device):

[ 7852.730835] pcd_open : open was successful
[ 7852.730847] pcd_read : read requested for 131072 bytes
[ 7852.730849] pcd_read : current file position = 0
[ 7852.730853] pcd_read : number of bytes successfully read = 1024
[ 7852.730854] pcd_read : updated file position = 1024
[ 7852.730869] pcd_read : read requested for 131072 bytes
[ 7852.730870] pcd_read : current file position = 1024
[ 7852.730871] pcd_read : number of bytes successfully read = 0
[ 7852.730873] pcd_read : updated file position = 1024
[ 7852.730882] pcd_release : release was successful

Using test_read Application:

You can compile an application for testing the reading. This application read data from device 3. For compiling:

make test

You need to write some data into the device 3 (write, at least, 100 or 200 bytes):

echo "bla bla bla ... bla bla" > /dev/pcdev-3

Then you can read, for example, the first 100 bytes executing:

./test_read 100