Embedded Linux device drivers: Reading driver state at runtime
Editor's Note: Embedded Linux has consistently ranked among the top operating systems used in embedded system design. With the rapid growth in interest in the Internet of Things (IoT), the ability of embedded Linux to serve multiple roles will prove vital in supporting diverse needs at each layer of the IoT application hierarchy. In turn, the ability of engineers to master embedded Linux systems will become critical for achieving rapid, reliable development of more sophisticated systems. In Mastering Embedded Linux Programming - Second Edition, author Chris Simmonds takes the reader on a detailed tour across the breadth and depth of this important operating system, using detailed examples to illustrate each key point.
In this excerpt, Chapter 9, from the book, the author describes how kernel device drivers interact with system hardware and how developers can write device drivers and use them in their applications. This article continues this excerpt. Be sure to read the earlier entry: Part 1.
Adapted from Mastering Embedded Linux Programming - Second Edition, by Chris Simmonds.
Chapter 9. Interfacing with Device Drivers (Continued)
By Chris Simmonds
Finding out about drivers at runtime
Once you have a running Linux system, it is useful to know which device drivers are loaded and what state they are in. You can find out a lot by reading the files in /proc and /sys.
First of all, you can list the character and block device drivers currently loaded and active by reading /proc/devices:
# cat /proc/devices Character devices: 1 mem 2 pty 3 ttyp 4 /dev/vc/0 4 tty 4 ttyS 5 /dev/tty 5 /dev/console 5 /dev/ptmx 7 vcs 10 misc 13 input 29 fb 81 video4linux 89 i2c 90 mtd 116 alsa 128 ptm 136 pts 153 spi 180 usb 189 usb_device 204 ttySC 204 ttyAMA 207 ttymxc 226 drm 239 ttyLP 240 ttyTHS 241 ttySiRF 242 ttyPS 243 ttyWMT 244 ttyAS 245 ttyO 246 ttyMSM 247 ttyAML 248 bsg 249 iio 250 watchdog 251 ptp 252 pps 253 media 254 rtc Block devices: 259 blkext 7 loop 8 sd 11 sr 31 mtdblock 65 sd 66 sd 67 sd 68 sd 69 sd 70 sd 71 sd 128 sd 129 sd 130 sd 131 sd 132 sd 133 sd 134 sd 135 sd 179 mmc
For each driver, you can see the major number and the base name. However, this does not tell you how many devices each driver is attached to. It only shows ttyAMA but gives you no clue that it is attached to four real serial ports. I will come back to that later when I look at sysfs.
Of course, network devices do not appear in this list, because they do not have device nodes. Instead, you can use tools such as ifconfig or ip to get a list of network devices:
# ip link show 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT qlen 1000 link/ether 54:4a:16:bb:b7:03 brd ff:ff:ff:ff:ff:ff 3: usb0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000 link/ether aa:fb:7f:5e:a8:d5 brd ff:ff:ff:ff:ff:ff
You can also find out about devices attached to USB or PCI buses using the well-known commands: lsusb and lspci. There is information about them in the respective manual pages and plenty of online guides, so I will not describe them any further here.
The really interesting information is in sysfs, which is the next topic.
Getting information from sysfs
You can define sysfs in a pedantic way as a representation of kernel objects, attributes, and relationships. A kernel object is a directory, an attribute is a file, and a relationship is a symbolic link from one object to another. From a more practical point of view, since the Linux device driver model represents all devices and drivers as kernel objects, you can see the kernel's view of the system laid out before you by looking in /sys, as shown here:
# ls /sys block class devices fs module bus dev firmware kernel power
In the context of discovering information about devices and drivers, I will look at three of these directories: devices, class, and block.
The devices: /sys/devices
This is the kernel's view of the devices discovered since boot and how they are connected to each other. It is organized at the top level by the system bus, so what you see varies from one system to another. This is the QEMU emulation of the ARM Versatile:
# ls /sys/devices platform software system tracepoint virtual
There are three directories that are present on all systems:
system/: This contains devices at the heart of the system, including CPUs and clocks.
virtual/: This contains devices that are memory-based. You will find the memory devices that appear as /dev/null, /dev/random, and /dev/zero in virtual/mem. You will find the loopback device, lo, in virtual/net.
platform/: This is a catch-all for devices that are not connected via a conventional hardware bus. This maybe almost everything on an embedded device.
The other devices appear in directories that correspond to actual system buses. For example, the PCI root bus, if there is one, appears as pci0000:00.
Navigating this hierarchy is quite hard, because it requires some knowledge of the topology of your system, and the path-names become quite long and hard to remember. To make life easier, /sys/class and /sys/block offer two different views of the devices.