Embedded Android : System Development - Part II (Linux device drivers)

Team Emertxe
Linux Device Drivers
An Introduction

Familiarity Check
●
Good C & Programming Skills
●
Linux & the Filesytem
– Root, User Space Headers & Libraries
●
Files
– Regular, Special, Device
●
Toolchain
– gcc & friends
●
Make & Makefiles
●
Kernel Sources (Location & Building)

The Flow
● Introduction
– Linux kernel Ecosystem
– Kernel Souce Organization
– Command set and Files
– Writing the first driver (Module)
● Character Drivers
– Device files
– Device access from user space (End to End flow)
– Registering the driver
– File Operations and registration
– Data transfer between User and Kernel space
– ioctl
● Memory & Hardware
● Time & Timings
● USB Drivers
● Interrupt Handling
● Block Drivers
● PCI Drivers
● Debugging

The Flow...
●
Introduction
– Linux kernel Ecosystem
– Kernel Souce Organization
– Command set and Files
– Writing the first driver (Module)
●
Character Drivers
– Device files
– Device access from user space (End to End flow)
● Memory & Hardware
● Time & Timings
● USB Drivers
●
Interrupt Handling
●
Block Drivers
● PCI Drivers
● Debugging

Hands-On
●
Your First Driver
●
Character Drivers
– Null Driver
– Memory Driver
– UART Driver for Customized Hardware
●
USB Drivers
– USB Device Hot-plug-ability
– USB to Serial Hardware Driver
●
Filesystem Modules
– VFS Interfacing
– “Pseudo” File System with Memory Files

Linux Driver
Ecosystem
bash gvim X Server gcc firefox
`
`
Process
Management
ssh
Memory
Management
File Systems
Device
Control
Networking
Architecture
Dependent
Code
Character
Devices
Memory
Manager
Filesystem
Types
Block
Devices
Network
Subsystem
IF
Devices
Concurrency
MultiTasking
Virtual
Memory
Files & Dirs:
The VFS
Ttys &
Device Access
Connectivity
CPU Memory
Disks &
CDs
Consoles,
etc
Network
Interfaces

Kernel Source
Organization
Kernel Source
include
net
drivers
block
fs
mm
kernel
arch
char mtd/ide net pci ...usbserial
asm-<arch>linux
arm powerpc sparc x86 ...

The Locations &
Config Files
● Kernel Source Path: /usr/src/linux
● Std Modules Path:
– /lib/modules/<kernel version>/kernel/...
●
Module Configuration: /etc/modprobe.conf
● Kernel Windows:
– /proc
– /sys
●
System Logs: /var/log/messages

The Commands
●
lsmod
●
insmod
●
modprobe
●
rmmod
●
dmesg
●
objdump
●
nm
●
cat /proc/<file>

The Kernel's C
●
ctor & dtor
– init_module, cleanup_module
●
printf
– printk
● Libraries
– <kernel src>/kernel
●
Headers
– <kernel src>/include

The Init Code
static int __init mfd_init(void)
{
printk(KERN_INFO "mfd registered");
...
return 0;
}
module_init(mfd_init);

The Cleanup Code
static void __exit mfd_exit(void)
{
printk(KERN_INFO "mfd deregistered");
...
}
module_exit(mfd_exit);

Usage of printk
● <linux/kernel.h>
● Constant String for Log Level
– KERN_EMERG "<0>" /* system is unusable */
– KERN_ALERT "<1>" /* action must be taken immediately */
– KERN_CRIT "<2>" /* critical conditions */
– KERN_ERR "<3>" /* error conditions */
– KERN_WARNING "<4>" /* warning conditions */
– KERN_NOTICE "<5>" /* normal but significant condition */
– KERN_INFO "<6>" /* informational */
– KERN_DEBUG "<7>" /* debug-level messages */
●
printf like arguments

The Other Basics &
Ornaments
● Headers
– #include <linux/module.h>
– #include <linux/version.h>
– #include <linux/kernel.h>
● MODULE_LICENSE("GPL");
● MODULE_AUTHOR("Emertxe");
● MODULE_DESCRIPTION("First Device Driver");

Building the Module
● Our driver needs
– The Kernel Headers for Prototypes
– The Kernel Functions for Functionality
– The Kernel Build System & the Makefile for Building
●
Two options
– Building under Kernel Source Tree
● Put our driver under drivers folder
● Edit Kconfig(s) & Makefile to include our driver
– Create our own Makefile to do the right invocation

Our Makefile
ifneq (${KERNELRELEASE},)
obj-m += <module>.o
else
KERNEL_SOURCE := <kernel source directory path>
PWD := $(shell pwd)
default:
$(MAKE) -C ${KERNEL_SOURCE} SUBDIRS=$(PWD) modules
clean:
$(MAKE) -C ${KERNEL_SOURCE} SUBDIRS=$(PWD) clean
endif

Major &
Minor Number
● ls -l /dev
● Major is to Driver; Minor is to Device
● <linux/types.h> (>= 2.6.0)
– dev_t: 12 & 20 bits for major & minor
● <linux/kdev_t.h>
– MAJOR(dev_t dev)
– MINOR(dev_t dev)
– MKDEV(int major, int minor)

Registering &
Unregistering
●
Registering the Device Driver
– int register_chrdev_region(dev_t first, unsigned int count,
char *name);
– int alloc_chrdev_region(dev_t *dev, unsigned int firstminor,
unsigned int cnt, char *name);
●
Unregistering the Device Driver
– void unregister_chrdev_region(dev_t first, unsigned int
count);
●
Header: <linux/fs.h>

The file operations
● #include <linux/fs.h>
● struct file_operations
– int (*open)(struct inode *, struct file *);
– int (*release)(struct inode *, struct file *);
– ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
– ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
– struct module owner = THIS_MODULE; / linux/module.h> */
– loff_t (*llseek)(struct file *, loff_t, int);
– int (*ioctl)(struct inode *, struct file *, unsigned int, unsigned long);

User level I/O
●
int open(const char *path, int oflag, ... )
●
int close(int fd);
●
ssize_t write(int fd, const void *buf, size_t nbyte)
●
ssize_t read(int fd, void *buf, size_t nbyte)
●
int ioctl(int d, int request, ...)
– The ioctl() function manipulates the underlying device
parameters of special files.
– The argument d must be an open file descriptor.
– The second argument is a device-dependent request code.

The file &
inode structures
●
struct file
– mode_t f_mode
– loff_t f_pos
– unsigned int f_flags
– struct file_operations *f_op
– void * private_data
●
struct inode
– unsigned int iminor(struct inode *);
– unsigned int imajor(struct inode *);

Registering the
file operations
●
#include <linux/cdev.h>
●
1st way initialization:
– struct cdev *my_cdev = cdev_alloc();
– my_cdev->owner = THIS_MODULE;
– my_cdev->ops = &my_fops;
● 2nd way initialization:
– struct cdev my_cdev;
– cdev_init(&my_cdev, &my_fops);
– my_cdev.owner = THIS_MODULE;
– my_cdev.ops = &my_fops;

Registering the
file operations...
● The Registration
– int cdev_add(struct cdev *cdev, dev_t num,
unsigned int count);
●
The Unregistration
– void cdev_del(struct cdev *cdev);

Registering/Unregistering
Old Way
● Registering the Device Driver
– int register_chrdev(undigned int major, const char *name,
struct file_operations *fops);
● Unregistering the Device Driver
– int unregister_chrdev(undigned int major, const char
*name);

The read flow
struct file
-------------------------
f_count
f_flags
f_mode
-------------------------
f_pos
-------------------------
...
...
ssize_t my_read(struct file *f, char __user *buf, size_t cnt, loff_t *off)
Buffer
(in the driver)
Buffer
(in the
application
or libc)
Kernel Space (Non-swappable) User Space (Swappable)
copy_to_user

The /dev/null
read & write
ssize_t my_read(struct file *f, char __user *buf, size_t cnt, loff_t
*off)
{
...
return read_cnt;
}
ssize_t my_write(struct file *f, char __user *buf, size_t cnt, loff_t
*off)
{
...
return wrote_cnt;
}

The mem device read
ssize_t my_read(struct file *f, char __user *buf, size_t cnt, loff_t
*off)
{
...
if (copy_to_user(buf, from, cnt) != 0)
{
return -EFAULT;
}
...
return read_cnt;
}

The mem device write
ssize_t my_write(struct file *f, char __user *buf, size_t cnt, loff_t
*off)
{
...
if (copy_from_user(to, buf, cnt) != 0)
{
return -EFAULT;
}
...
return wrote_cnt;
}

Dynamic Device Node
& Classes
● Class Operations
– struct class *class_create(struct module *owner, char
*name);
– void class_destroy(struct class *cl);
●
Device into & Out of Class
– struct class_device *device_create(struct class *cl, NULL,
dev_t devnum, NULL, const char *fmt, ...);
– void device_destroy(struct class *cl, dev_t devnum);

The I/O Control API
●
int (*ioctl)(struct inode *, struct file *, unsigned int cmd, unsigned long arg)
●
int (*unlocked_ioctl)(struct file *, unsigned int cmd, unsigned long arg)
●
Command
– <linux/ioctl.h> -> ... -> <asm-generic/ioctl.h>
– Macros
●
_IO, _IOR, _IOW, _IOWR
– Parameters
●
type (Magic character) [15:8]
●
number (index) [7:0]
●
size (param type) [29:16]

The I/O Control API
● Macro Usage
_IO(type, index)
[_IOR | _IOW | _IOWR](type, index,
datatype/size)

Module Parameters
●
<linux/moduleparam.h>
– Macros
● module_param(name, type, perm)
● module_param_array(name, type, num, perm)
● Perm (is a bitmask)
– 0
– S_IRUGO
– S_IWUSR | S_IRUGO
– Loading
● insmod driver.ko name=10

Physical Vs
Virtual Memory
●
The kernel Organizes Physical memory in to pages
– Page size Depends on Arch
● X86-based 4096 bytes
●
On 32-bit X86 system Kernel total Virtual address space
– Total 4GB (pointer size)
– Kernel Configuration Splits 4GB in to
● 3BG Virtual Sp for US
● 1GB Virtual Sp for Kernel
– 128MB KDS
– Virtual Address also called “Logical Address”

Memory Access from
Kernel Space
●
Virtual Address on Physical Address
– #include <linux/gfp.h>
●
unsigned long __get_free_pages(flags, order); etc
●
void free_pages(addr, order); etc
– #include <linux/slab.h>
●
void *kmalloc(size_t size, gfp_t flags);
– GFP_ATOMIC, GFP_KERNEL, GFP_DMA
● void kfree(void *obj);
– #include <linux/vmalloc.h>
● void *vmalloc(unsigned long size);
●
void vfree(void *addr);

Memory Access from
Kernel Space...
●
Virtual Address for Bus/IO Address
– #include <asm/io.h>
● void *ioremap(unsigned long offset, unsigned long size);
● void iounmap(void *addr);
● I/O Memory Access
● unsigned int ioread[8|16|32](void *addr);
● unsigned int iowrite[8|16|32](u[8|16|32] value, void *addr);
●
Barriers
– #include <linux/kernel.h>: void barrier(void);
– #include <asm/system.h>: void [r|w|]mb(void);

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