Difference between revisions of "Embedded-Linux-Device-Driver/C3/Kernel-Memory-Allocation/English"

Welcome slide:

Learning objectives:

• Dynamically allocate the kernel memory using kmalloc() function and
• kfree() function to free the memory allocated by kmalloc().

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
• Basics of Linux kernel

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

Kernel Memory Allocation.

• Linux handles memory allocation by creating a set of memory objects of fixed sizes
• It will dynamically allocate portions of memory to programs at their request
• When the memory is no longer needed,we have to free it for reuse

Kernel Memory Allocation functions.

• kmalloc() and vmalloc() functions dynamically allocates the kernel memory.
• kmalloc() is similar to the malloc function in c programs.
• kmalloc() allocates contiguous physical memory and virtual memory.
• vmalloc() allocates contiguous virtual memory but not the physical memory.

Point to the files.

Here, I have created a directory named MemoryAllocation.

In this directory, I have saved simple_driver dot c driver file, user dot c and Makefile.

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/ MemoryAllocation

Go to the directory MemoryAllocation as shown here.

Press Enter key after every command.

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

In this file, I have used the kmalloc() function to allocate the memory.

Let me explain the code.

static char *ptr

The kmalloc() function is declared in the slab dot h header file.

To use the string functions in the driver, include the string dot h header file.

Here, the ptr character pointer is defined with NULL value.

We will use it to store the address returned by the kmalloc() function.

Highlight void *kmalloc(size_t size, int flags);

Highlight size

Use the dynamic allocation way when you don’t know how much memory will be needed.

Now let us see the prototype of the kmalloc() function.

size specifies the size of memory to be allocated.

Highlight GFP_ATOMIC.

Highlight void*

The two most widely used flags are GFP_KERNEL and GFP_ATOMIC.

It returns the virtual address of the first page allocated.

If there is any error, it returns NULL.

Highlight GFP_ATOMIC

We will use this allocated kernel memory space as a buffer.

It is also referred to as a virtual device.

GFP_ATOMIC flag will not put the current process in sleep state if memory is low.

This flag is mostly used in the interrupt handler to allocate memory.

Highlight ptr

Here, you can typecast it to store the data of respective data type.

I have typecast it as a character pointer to store a string in it.

The ptr contains the start address of the allocated kernel memory.

Highlight strcpy() and printk

We will use a strcpy function to copy the data to the buffer.

Here, the strcpy() function will copy a string IITB to the buffer.

This message will be printed in the kernel log level.

Highlight ptr

Highlight void kfree(const void *ptr)

We have to use the kfree() function in the exit function as shown here.

The kfree function is used to free the memory allocated by kmalloc().

ptr specifies the address returned by kmalloc().

It is mostly used in the cleanup function and avoids the leakage of memory.

It will avoid the wastage of the kernel memory as its memory or stack is limited.

Highlight NULL

If you try to access it again, the program could crash and may lead to memory fault.

To avoid such problems, assign the NULL value to it, which is a valid address.

Let’s create a Makefile.

Type gedit space Makefile.

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

Now let us see a simple user program which will open and close a device.

Highlight open() and close().

Save and close the file

Type >> clear

I have used the same user program user dot c which we used earlier.

This program opens and closes the new_device using respective system calls.

When the user program tries to open a device, the specified data will copy to the buffer.

Save and close the file.

Type >> system password.

Type >> make all

Type >> clear

Type >> insmod simple_driver.ko

Type >> clear

Type sudo space su to be a superuser.

And then type the system password.

Type make space all.

Clear the screen.

Now load the driver into the kernel.

Type insmod space simple_driver dot ko.

Clear the screen.

Highlight the output Highlight printk messages.

Type >> clear

Type dmesg space pipe space grep space simple_driver.

Here, this message shows that the kmalloc() function executed successfully.

Clear the screen.

Type >> gcc -o user user.o

Type >> ./user

Highlight output.

Type >> clear

Type gcc space hyphen o space user space user dot o

To execute the program type dot slash user.

The output shows that the device file opened and closed successfully.

Clear the screen.

Highlight the output

Highlight printk message.

Type >> clear

Type dmesg space pipe space grep space simple_driver.

This message shows the IITB string is stored in the allocated kernels memory.

Clear the screen.

.ko

Type dmesg | grep simple_driver

Highlight the message

Type >> clear

Type rmmod space simple_driver

dot ko

To see the unloaded printk messages type this dmesg command.

This message indicates that the kfree() function releases the memory.

Clear the screen.

Summary:

• Dynamically allocate the kernel memory using kmalloc() function and
• kfree() function to free the memory allocated by kmalloc().

Assignment :

1. Open the simple_driver.c file
2. Allocate the memory space using kmalloc in the driver as per your choice
3. Store a different data to the kernel buffer
4. Load the driver and then execute the user program
5. See the output using dmesg command and then unload the driver.

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 Mayuri Panchakshari signing off. Thanks for watching.