Lab 1 Using the Raspberry Pi 3 Solution

 { Kernel Module and Interrupt Service Routine






WEEK-1




Objective

In this part, students will learn how to access the General Purpose Input/Output (GPIO) ports on the RPi3 through a Kernel Module.


Prelab

Before coming to this lab, look up the use of the functions mmap and ioremap and their arguments.

Review the bit-wise operations in C/C++, and the concept of \Bit Masking".


Background

A kernel module is a piece of code that can be added to the kernel at run-time. It can then be removed, as well. Modules extend the functionality of the kernel while the system is up and running. That is, we do not need to rebuild an entire kernel to add new functions, and we do not need to build a large kernel that contains all functionality. There are di erent types of modules, including device drivers. You could nd a sample code on how a write a kernel module at this link.


Lab Procedure

1. Turn ON/OFF LEDs using Kernel Module

You will create a kernel module that simply turns on the LEDs of the auxiliary board when it is installed, and turns the LEDs o when it is removed.

In Kernel space, you do not have access to the wiringPi library. Therefore, you will need to access the GPIO ports directly through the GPSET, GPCLR and GPLEV registers. Before you can use the ports, you need to con gure them as inputs or outputs through the GPFSEL register (see the week 1 part).

Hint 1: You will need to use ioremap when accessing the I/O registers in a kernel module. You will need to include a speci c header le.

Hint 2: Consider the following function:


void    i o wr it e 32 ( u32  value ,  void  * addr )


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ECE 4220/7220 Real Time Embedded Computing, Fall 2020



Something to note about kernel modules is that they DO NOT contain a main(). Instead, they contain two functions:


int    i n i t _ m o d u l e ( void )

void    c l e a n u p _ m o d u l e ( void )

The init module function contains code that is run when the module is installed. This function should return 0 unless an error has occurred. cleanup module is the code that is run when the module is removed.


Alternatively, you can name your functions di erently, i.e.,


int    w h e n _ i n s t a l l e d ( void )

void    w h e n _ r e m o v e d ( void )

Your code should contain the lines:


    • function runs when module is installed m o d u l e _ i n i t ( w h e n _ i n s t a l l e d ) ;

    • function  runs  when  module  is  removed

m o d u l e _ e x i t ( w h e n _ r e m o v e d ) ;

To avoid warnings when installing the module, add the following line to your source code MODULE LICENSE(\GPL");


To compile the module, you may need to de ne the symbols: KERNEL and MODULE. Those macros indicate being part of the kernel, but not permanent, respectively. This can be done one of two ways. First, you can de ne the symbols at the beginning of your source code, which would look like #de ne MODULE. Another way is to de ne the symbols when you compile in Eclipse by going to the properties and clicking on symbols under the compiler settings.


Important: a Kernel module is compiled into object code, but it is not linked into a complete executable. That is, the Linker should not be invoked. The TA will show you how to compile a Kernel module.

There are three helpful shell commands to use when dealing with modules:

        1. lsmod: lists all of the modules currently installed in the kernel

        2. insmod NAME.ko: installs the module whose  lename is NAME.ko

        3. rmmod NAME: removes the module with name: NAME (notice the .ko is not included)

        4. modinfo NAME.ko: Can get the information about the loaded module

You need sudo to use 2 and 3.

In order to debug your module code, you cannot use the printf() function. Instead, you can use the printk() function and then check the printed lines using dmesg com-mand in the terminal. Modify your kernel module so that the message \MODULE INSTALLED" is printed when the module is installed, and the message \MOD-ULE REMOVED" is printed when the module is removed. Install and remove your module several times. What do you see when you run the dmesg command?


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WEEK-2




Objective

In the second part of the lab, students will learn how to trigger an interrupt through a push button. Also, write their own interrupt service routine (ISR) to handle the interrupt.


Prelab

Investigate and include a brief description for the following functions: request irq, free irq.


You should look at section 6 General Purpose I/O(GPIO) in the manual: BCM2837-ARM-Peripherals (on Canvas under Modules > Other Material). Pay attention to the registers associated to con guring and detecting level and edge events. You will need those for this lab.


Lab Procedure

Detect Push-Button using an Interrupt

You will add an Interrupt Service Routine (ISR) to your modules from Week-1. The purpose of the ISR is to handle one of ve events, which correspond to the ve push buttons on the auxiliary board. The corresponding GPIO ports and associated registers should be con gured so that pushing any of the buttons causes an interrupt. The handler will change state of the LEDs based on the button pressed. You will associate rst four buttons to four LEDs on the board, respectively to turn them on individually. And link the last button to turn OFF all the LEDs in one go. You should con gure these interrupts to be rising-edge-sensitive, so program the corresponding GPIO registers accordingly.

Check the le ece4220lab1 isr.c. It is a template that illustrates the basic steps needed to con gure an interrupt and attach a handler.


Note WiringPi provides a way to con gure GPIO interrupts in user space on the RPi.

You may explore that approach for your future work.


References

Wiring Pi

Raspberry Pi GPIO Pinout


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