Nancyvarkey at 05:00, 30 October 2020

2020-10-30T05:00:33Z

Nirmala Venkat at 10:31, 23 October 2020

2020-10-23T10:31:39Z

Nirmala Venkat at 09:35, 23 October 2020

2020-10-23T09:35:36Z

Nancyvarkey at 02:17, 28 September 2020

2020-09-28T02:17:34Z

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Nirmala Venkat: Created page with " {| style="border-spacing:0;" | style="border:1pt solid #000000;padding:0.176cm;"| '''Visual cue :''' | style="border:1pt solid #000000;padding:0.176cm;"| '''Narration :''' |..."

2020-09-23T09:58:55Z

Created page with " {| style="border-spacing:0;" | style="border:1pt solid #000000;padding:0.176cm;"| '''Visual cue :''' | style="border:1pt solid #000000;padding:0.176cm;"| '''Narration :''' |..."

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{| style="border-spacing:0;"
| style="border:1pt solid #000000;padding:0.176cm;"| '''Visual cue :'''
| style="border:1pt solid #000000;padding:0.176cm;"| '''Narration :'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 1:

Welcome slide:
| style="border:1pt solid #000000;padding:0.176cm;"| Welcome to the spoken tutorial on''' Kernel Memory Allocation.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 2:

Learning objectives:
| style="border:1pt solid #000000;padding:0.176cm;"| In this tutorial, we will learn how to

* Dynamically allocate the''' kernel memory''' using '''kmalloc'''() '''function '''and
* '''kfree() function''' to free the memory allocated by '''kmalloc().'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 3:

System Requirements:
| style="border:1pt solid #000000;padding:0.176cm;"| To record this tutorial, I am using,

* '''VirtualBox 5.2.'''
* '''Ubuntu Linux 18.04 LTS''' operating system.
* '''Linux kernel version 5.0.0-31 generic.'''
* '''gedit text editor.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 4:

Prerequisites:
| style="border:1pt solid #000000;padding:0.176cm;"| To follow this tutorial, you should be familiar with:

* '''C programming language '''
* Basics of''' Linux kernel'''

If not, then go through the '''C/C++''' and''' Linux spoken tutorials''' on this website.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 5:

'''Kernel Memory Allocation.'''

| style="border:1pt solid #000000;padding:0.176cm;"| * 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
|-
| style="border:1pt solid #000000;padding:0.176cm;"|
| style="border:1pt solid #000000;padding:0.176cm;"| Let us see the commonly used '''kernels functions''' to allocate its memory.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 6:

'''Kernel Memory Allocation functions.'''
| style="border:1pt solid #000000;padding:0.176cm;"| * '''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. '''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Point to the '''folder''' and''' file''' in '''desktop'''

Point to the '''files.'''
| style="border:1pt solid #000000;padding:0.176cm;"| Go to the '''DeviceDriver folder''' in the '''desktop''' which we have created earlier.

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.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide 7:

'''Code files''':
| style="border:1pt solid #000000;padding:0.176cm;"| * 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
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Open the '''terminal'''

Type >> '''cd Desktop/DeviceDriver/ MemoryAllocation'''
| style="border:1pt solid #000000;padding:0.176cm;"| Open the '''terminal''' by pressing '''ALT+Ctrl+T '''keys simultaneously.

Go to the directory''' MemoryAllocation '''as shown here.

Press '''Enter key''' after every '''command.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''gedit simple_driver.c'''

| style="border:1pt solid #000000;padding:0.176cm;"| Type''' gedit space simple driver dot c.'''

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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| '''Highlight <nowiki><linux/slab.h> </nowiki>'''

'''static char <nowiki>*ptr</nowiki>'''
| style="border:1pt solid #000000;padding:0.176cm;"| Here, we have to include the '''slab dot h''' '''kernel header file'''.

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 return by the '''kmalloc() function.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''init_function()'''

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

Highlight '''size'''
| style="border:1pt solid #000000;padding:0.176cm;"| In the''' initialisation function''', we have used the '''kmalloc() function.'''

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.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''flag'''

Highlight '''GFP_ATOMIC'''.

Highlight''' void* '''
| style="border:1pt solid #000000;padding:0.176cm;"| '''flag''' denotes the behavior of '''kmalloc''' call.

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.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' kmalloc()'''

Highlight '''GFP_ATOMIC'''

| style="border:1pt solid #000000;padding:0.176cm;"| Here, '''kmalloc()''' dynamically allocates 8 bytes of the '''kernel memory'''.

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.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' char *'''

Highlight''' ptr'''
| style="border:1pt solid #000000;padding:0.176cm;"| The '''kmalloc()''' returns an address which can be used to store any type of data.

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'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''printk messages'''
| style="border:1pt solid #000000;padding:0.176cm;"| Depending upon its success or failure, the corresponding messages will be printed.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''open()'''

Highlight''' strcpy()''' and '''printk'''

| style="border:1pt solid #000000;padding:0.176cm;"| '''open function''' will execute when a user program opens the '''device'''.

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.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"|
| style="border:1pt solid #000000;padding:0.176cm;"| Let us see the''' kernel function''' to release the dynamically allocated''' memory.'''

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''exit_function.'''

Highlight '''ptr'''

Highlight''' void kfree(const void *ptr)'''
| style="border:1pt solid #000000;padding:0.176cm;"| As we allocated the''' kernel '''memory dynamically, it's our responsibility to release it.

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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight''' kfree(ptr)'''

| style="border:1pt solid #000000;padding:0.176cm;"| Here''', the kfree() function''' will release the memory whose '''address''' is stored in the '''pointer.'''

It will avoid the wastage of the '''kernel memory''' as its '''memory''' or '''stack''' is limited.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Highlight '''ptr'''

Highlight''' NULL'''
| style="border:1pt solid #000000;padding:0.176cm;"| Now, '''ptr''' is a '''dangling pointer''' as it points to '''freed memory.'''

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'''.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Save and close the''' file.'''
| style="border:1pt solid #000000;padding:0.176cm;"| Save and close the '''file.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >>''' gedit Makefile'''
| style="border:1pt solid #000000;padding:0.176cm;"| To compile the '''program''', we have to create a''' Makefile'''.

Let’s create a '''Makefile.'''

Type '''gedit space Makefile.'''

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

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Switchback to the '''terminal'''
| style="border:1pt solid #000000;padding:0.176cm;"| Save and close the file

Now let us see a simple '''user program''' which will open and close a '''device.'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''gedit user.c'''

Highlight '''open()''' and '''close().'''

Save and close the file

Type >>''' clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Type '''gedit space user dot c.'''

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.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''sudo su'''

Type >>''' system password.'''

Type >> '''make all'''

Type >>''' clear'''

Type >>''' insmod simple_driver.ko'''

Type >> '''clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Let us compile the '''driver.'''

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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''dmesg | grep simple_driver'''

Highlight the''' output'''
Highlight '''printk '''messages.

Type >> '''clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Let’s see the loaded '''printk '''messages.

Type '''dmesg space pipe space grep space simple_driver.'''

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

Clear the screen.

|-
| style="border:1pt solid #000000;padding:0.176cm;"|
| style="border:1pt solid #000000;padding:0.176cm;"| Now let us compile the user program to open the device.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >>''' gcc -c user.c'''
Type >> '''gcc -o user user.o'''

Type >> '''./user'''

Highlight output.

Type >> clear
| style="border:1pt solid #000000;padding:0.176cm;"| Type''' gcc space hyphen c space user dot c'''

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.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type >> '''dmesg | grep simple_driver'''

Highlight the''' output'''

Highlight '''printk '''message.

Type >> '''clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Let’s see the '''printk '''messages from the''' open function''' of a '''driver.'''

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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Type''' rmmod simple_driver'''

'''.ko'''

Type '''dmesg | grep simple_driver'''

Highlight the '''message'''

Type >>''' clear'''
| style="border:1pt solid #000000;padding:0.176cm;"| Now lets unload the '''driver.'''

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.

|-
| style="border:1pt solid #000000;padding:0.176cm;"|
| style="border:1pt solid #000000;padding:0.176cm;"| With this, we come to the end of this tutorial. Let us summarise.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Summary:
| style="border:1pt solid #000000;padding:0.176cm;"| In this tutorial, we learnt how to

* Dynamically allocate the kernel memory using '''kmalloc() function '''and
* '''kfree() function''' to free the memory allocated by '''kmalloc().'''
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Assignment :
| style="border:1pt solid #000000;padding:0.176cm;"| As an assignment:

# Open the '''simple_driver.c file'''
# Allocate the memory space using '''kmalloc''' in the '''driver''' as per your choice
# Store a different data to the '''kernel buffer'''
# Load the '''driver''' and then execute the '''user program'''
# See the output using '''dmesg''' command and then unload the '''driver'''.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

About Spoken Tutorial Project:
| style="border:1pt solid #000000;padding:0.176cm;"| * The video at the following link summarizes the Spoken Tutorial project.

* Please download and watch it.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Spoken Tutorial workshops :
| style="border:1pt solid #000000;padding:0.176cm;"| The''' Spoken Tutorial Project''' Team:

* conducts workshops and
* gives certificates.

For more details, please write to us.

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Forum questions :
| style="border:1pt solid #000000;padding:0.176cm;"| * Please post your timed queries in this forum

|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Forum for specific questions :

| style="border:1pt solid #000000;padding:0.176cm;"| * Do you have any general or technical questions on Embedded Linux Device Driver?
* Please visit the FOSSEE forum and post your question.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Acknowledgment:
| style="border:1pt solid #000000;padding:0.176cm;"| The Spoken Tutorial Project is funded by MHRD, Government of India.
|-
| style="border:1pt solid #000000;padding:0.176cm;"| Slide :

Thank you slide:

| style="border:1pt solid #000000;padding:0.176cm;"| This tutorial has been contributed by FOSSEE and Spoken Tutorial Project, IIT Bombay.

This is Mayuri Panchakshari signing off.
Thanks for watching.

|}

@@ Line 68: / Line 68: @@
 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.'''
+In this directory, I have saved '''simple_driver dot c file, user dot c''' and '''Makefile.'''
 I’ll use these files for demonstration.

@@ Line 85: / Line 85: @@
 | style="border:1pt solid #000000;padding:0.176cm;"| Open the '''terminal''' by pressing '''ALT+Ctrl+T '''keys simultaneously.
-Go to the directory''' MemoryAllocation '''as shown here.
+Go to the directory where '''MemoryAllocation''' is saved on your system.
 Press '''Enter''' key after every '''command.'''
@@ Line 223: / Line 223: @@
 | style="border:1pt solid #000000;padding:0.176cm;"| Type >>''' gedit Makefile'''
 | style="border:1pt solid #000000;padding:0.176cm;"| To '''compile''' the '''program''', we have to create a''' Makefile'''.
 Type '''gedit space Makefile.'''

@@ Line 418: / Line 418: @@
 | style="border:1pt solid #000000;padding:0.176cm;"| This tutorial has been contributed by FOSSEE and Spoken Tutorial Project, IIT Bombay.
-This is Mayuri Panchakshari signing off.
+This is Usha signing off.
 Thanks for watching.
 |}

Embedded-Linux-Device-Driver/C3/Kernel-Memory-Allocation/English - Revision history

Nancyvarkey at 05:00, 30 October 2020

Nirmala Venkat at 10:31, 23 October 2020

Nirmala Venkat at 09:35, 23 October 2020

Nancyvarkey at 02:17, 28 September 2020

Nirmala Venkat: Created page with " {| style="border-spacing:0;" | style="border:1pt solid #000000;padding:0.176cm;"| '''Visual cue :''' | style="border:1pt solid #000000;padding:0.176cm;"| '''Narration :''' |..."