Plaintext
How to avoid writing device drivers
for embedded Linux
How to avoid writing device drivers for embedded Linux 1 Copyright © 2011-2016, 2net Ltd
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How to avoid writing device drivers for embedded Linux 2 Copyright © 2011-2016, 2net Ltd
About Chris Simmonds
• Consultant and trainer
• Author of Mastering Embedded Linux
Programming
• Working with embedded Linux since 1999
• Android since 2009
• Speaker at many conferences and
workshops
"Looking after the Inner Penguin" blog at http://2net.co.uk/
https://uk.linkedin.com/in/chrisdsimmonds/
https://google.com/+chrissimmonds
How to avoid writing device drivers for embedded Linux 3 Copyright © 2011-2016, 2net Ltd
Conventional device driver model
Application
User
space
C library
System call handler
Linux
Generic services kernel
Device drivers
interrupts
Hardware
How to avoid writing device drivers for embedded Linux 4 Copyright © 2011-2016, 2net Ltd
How applications call device drivers
• In Linux, everything is a file 1
• Applications interact with drivers via POSIX functions
open(2), read(2), write(2), ioctl(2), etc
• There are two types of interface
• 1. Device nodes in /dev
• The serial driver, ttyS is an example
• Device nodes are named /dev/ttyS0, /dev/ttyS1 ...
• 2. Driver attributes, exported via sysfs
• For example /sys/class/gpio
1 Except network interfaces, which are sockets
How to avoid writing device drivers for embedded Linux 5 Copyright © 2011-2016, 2net Ltd
Userspace drivers
• Writing kernel device drivers can be difficult
• Luckily, there are generic drivers that that allow you to
write most of the code in userspace
• We will look at three
• GPIO
• PWM
• I2C
• Note: applications will need read/write permissions
for the files. Consequently, they usually have to run
as user root
How to avoid writing device drivers for embedded Linux 6 Copyright © 2011-2016, 2net Ltd
/sys/class/gpio
# ls /sys/class/gpio/
export gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
This device has 4 gpio chips
each with 32 pins
Write to this Write to this
file to export file to unexport
a GPIO pin a GPIO pin
to user space to user space
How to avoid writing device drivers for embedded Linux 7 Copyright © 2011-2016, 2net Ltd
gpiochip
# /sys/class/gpio/gpiochip0
base device label ngpio power subsystem uevent
The number of GPIO pins (32)
A lable to identify the chip
(gpiochip0)
The starting GPIO number (0)
How to avoid writing device drivers for embedded Linux 8 Copyright © 2011-2016, 2net Ltd
Exporting a GPIO pin
# echo 42 > /sys/class/gpio/export
# ls /sys/class/gpio
export gpio42 gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
If the export is successful, a new
directory is created
How to avoid writing device drivers for embedded Linux 9 Copyright © 2011-2016, 2net Ltd
Inputs and outputs
# ls /sys/class/gpio/gpio42
active_low device direction edge power subsystem uevent value
Set to 1 to invert
input and ouput
Set direction by
writing "out" or
"in". Default "in"
The logic level of the
pin. Change the level
of outputs by writing
"0" or "1"
How to avoid writing device drivers for embedded Linux 10 Copyright © 2011-2016, 2net Ltd
Interrupts
• If the GPIO can generate interrupts, the file edge can
be used to control interrupt handling
• edge = ["none", "rising", "falling","both]
• For example, to make GPIO60 interrupt on falling
edge:
• echo falling > /sys/class/gpio/gpio60/edge
• To wait for an interrupt, use the poll(2) function
• Example on next slide
How to avoid writing device drivers for embedded Linux 11 Copyright © 2011-2016, 2net Ltd
GPIO interrupt code example
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <poll.h>
int main (int argc, char *argv[])
{
int f;
struct pollfd poll_fds [1];
int ret;
char value[4];
f = open("/sys/class/gpio/gpio60/value", O_RDONLY);
poll_fds[0].fd = f;
poll_fds[0].events = POLLPRI | POLLERR;
while (1) {
if (poll(poll_fds, 1, -1) > 0) {
read(f, &value, sizeof(value));
printf("Interrupt! value=%c\n", value[0]);
}
}
}
How to avoid writing device drivers for embedded Linux 12 Copyright © 2011-2016, 2net Ltd
PWM
# echo 6 > /sys/class/pwm/export
# ls /sys/class/pwm
export pwm6 pwmchip0 pwmchip2 pwmchip3 pwmchip5 pwmchip7 unexport
If the export is successful, a new
directory is created
# ls /sys/class/pwm/pwm6/
device duty_ns period_ns polarity power run subsystem uevent
duty_ns
period_ns
How to avoid writing device drivers for embedded Linux 13 Copyright © 2011-2016, 2net Ltd
I2C
• Device nodes, one per I2C bus controller:
# ls -l /dev/i2c*
crw-rw---T 1 root i2c 89, 0 Jan 1 2000 /dev/i2c-0
crw-rw---T 1 root i2c 89, 1 Jan 1 2000 /dev/i2c-1
• Some functions are implemented using ioctl(2), using
commands and structures defined in
usr/include/linux/i2c-dev.h
How to avoid writing device drivers for embedded Linux 14 Copyright © 2011-2016, 2net Ltd
i2c-utils
• Command-line tools for interacting with I2C devices
• i2cdetect - list I2C adapters and probe bus
• i2cget - read data from an I2C device
• i2cset - write data to an I2C device
How to avoid writing device drivers for embedded Linux 15 Copyright © 2011-2016, 2net Ltd
i2cdetect
• i2cdetect - list i2c adapters and probe bus
• Example: detect devices on bus 1 (/dev/i2c-1)
# i2cdetect -y -r 1
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- 39 -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
UU = device already handled by kernel driver
0x39 = device discovered at address 0x39
How to avoid writing device drivers for embedded Linux 16 Copyright © 2011-2016, 2net Ltd
i2cget/i2cset
• i2cget <bus> <chip> <register>: read data from an I2C
device
• Example: read register 0x8a from device at 0x39
# i2cget -y 1 0x39 0x8a
0x50
• i2cset <bus> <chip> <register>: writedata to an I2C
device
• Example: Write 0x03 to register 0x80:
# i2cset -y 1 0x39 0x80 3
How to avoid writing device drivers for embedded Linux 17 Copyright © 2011-2016, 2net Ltd
I2C code example
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/i2c-dev.h>
int main(int argc, char **argv)
int f;
char buf[4];
f = open("/dev/i2c-1", O_RDWR);
ioctl(f, I2C_SLAVE, 0x39) < 0) {
buf[0] = 0x8a; /* Chip ID register */
write(f, buf, 1);
read(f, buf, 1);
printf("ID 0x%x\n", buf [0]);
}
How to avoid writing device drivers for embedded Linux 18 Copyright © 2011-2016, 2net Ltd
Other examples
• SPI: access SPI devices via device nodes
/dev/spidev*
• USB: access USB devices via libusb
• User defined I/O: UIO
• Generic kernel driver that allows you to write
userspace drivers
• access device registers and handle interrupts from
userspace
How to avoid writing device drivers for embedded Linux 19 Copyright © 2011-2016, 2net Ltd