Difference between revisions of "Embedded-Linux-Device-Driver/C3/Data-Transfer-between-User-and-Kernel-Space/English"

Welcome slide:

Learning objectives:

• kernel space to the user space and
• User space to the kernel space.

System Requirements:

• VirtualBox 5.2.
• Ubuntu Linux 18.04 LTS Operating System.
• Linux kernel version 5.0.0-31 generic.
• gedit text editor.

Prerequisites:

• C programming language and
• Basics of Linux kernel

If not, then go through the C/C++ and Linux spoken tutorials on this website.

Point to the files.

In this directory, I have created a directory named DataTransfer.

Here, I have saved a simple_driver dot c driver file, Makefile and user-read-write dot c file.

I’ll use these files for demonstration.

Code files:

• The files used in this tutorial are available in the Code Files link on this tutorial page.
• Please download and extract them
• Make a copy and then use them while practising

Type >> cd Desktop/DeviceDriver/ DataTransfer

Go to the directory where DataTransfer is saved in your system.

Press Enter key after every command.'

I have used the same file simple_driver dot c which we used earlier.

Highlight mydevice_read() and mydevice_write()

It contains the read() and write() functions declarations.

These are used to transfer the data between user space and kernel space.

We have to add the protocols of read() and write() functions as shown here.

mydevice_write (struct file *filp, const char __user *buff, size_t len, loff_t *loff)

Highlight *buff

Highlight len

Highlight *loff

Highlight mydevice_write

buff represents the user space data buffer.

len specifies the size of the requested data transfer.

loff indicates the start position from which data should be written in the file.

The return value is the number of bytes written.

I have defined the write function as shown here.

copy_from_user (void * to, const void __user * from, unsigned long n)

Highlight to

Highlight from

Highlight n

Highlight unsigned long

Highlight unsigned long

to specify the destination address in the kernel space.

from specifies the source address in the user space.

n specifies the number of bytes to copy.

On failure, it returns a number of bytes that could not be copied.

It will return zero on success.

If it is more, then this error message will print with the termination of a program.

Highlight copy_from_user(ptr,buff,len)

Highlight EFAULT

The copy_from_user also checks whether the user pointer buff is valid or not.

If it is invalid, it will not copy the data and returns the EFAULT error.

Depending upon its success or failure, the corresponding messages will be printed.

mydevice_read (struct file *filp, char __user *buff, size_t len, loff_t *loff)

Highlight *buff

Highlight len

Highlight *loff

Highlight mydevice_read()

buff specifies the pointer to buffer from the user space.

len specifies the size of the requested transfer.

loff indicates the start position from which data should be read.

The return value is the number of bytes read.

I have defined the read function as shown here.

unsigned long copy_to_user(void __user *to, const void *from, unsigned long count)

Highlight to

Highlight from

Highlight n

Highlight unsigned long

Highlight unsigned long

to specifies the destination address in the user space.

from specifies the source address in the kernel space.

n' specifies the number of bytes to copy.

On failure, it returns a number of bytes that could not be copied.

It will return zero on success.

Highlight copy_to_user(buff,ptr,len)

Highlight printk

Highlight unsigned long copy_to_user()

The copy_to_user also checks the validity of the user pointer.

Depending upon its success or failure, the corresponding messages will be printed.

Using these functions, the driver can access the user space buffer in a safe way.

Switch back to the terminal.

Clear the screen.

Type >> password.

Now, type the system password.

Open the terminal

Type gedit space Makefile.

Type the code as shown here or you can use the downloaded Makefile.

Save and close the file.

Highlight *userData

Highlight string

I have already created a user program.

It will open, close, read and write data to the new_device by its driver.

Here, I have defined the userData pointer which points to the string “Linux Device Driver”.

Highlight DataSize

It will write the string of length DataSize to a device as shown here.

When it executes, the kernel internally calls a write function from the driver.

Highlight DataSize

Open the terminal

It will read the data from a device and store it in the kernelData as shown here.

When it executes, the kernel internally calls read function from the driver

Save and close the file.

Clear the screen.

Type make space all.

Clear the screen.

.ko

Type insmod space simple_driver

dot ko.

Highlight the output

Type dmesg space pipe space grep space simple_ driver.

Here, you can see the device, its class were created successfully.

Clear the screen.

Type >> gcc -o user-read-write user_read_write.o

Type gcc space hyphen c space user-read-write dot c

Then type the command as shown here.

Highlight the output

Type dot slash user.

We can see the data write and data read message from the device successfully.

Clear the screen.

Highlight the respective messages

Type >> clear

This shows that the kernel called the write and read functions from the driver.

Clear the screen.

Type dmesg | grep simple_driver

Highlight the respective messages

Type rmmod space simple_driver

dot ko

To see the unloaded printk messages type this dmesg command.

It shows that the struct cdev, device file, it’s class are removed from the system.

The device is also unregistered here.

Summary:

• Kernel space to the user space
• User space to the kernel space.

Assignment :

1. Change the userData string in the user hyphen read hyphen write.c program.
2. Load the driver into the kernel and execute the user program
3. See the output using dmesg command.
4. Unload the driver from the kernel.

About Spoken Tutorial Project:

• The video at the following link summarizes the Spoken Tutorial project.

• Please download and watch it.

Spoken Tutorial workshops :

For more details, please write to us.

Forum questions :

Forum for specific questions :

• Do you have any general or technical questions on Embedded Linux Device Driver?
• Please visit the FOSSEE forum and post your question.

Acknowledgment:

Thank you slide:

This is Usha signing off.

Thanks for watching.