Gentoo Linux Debugging Guide

+dmesg Introduction + + +

+dmesg is a system program created with debugging kernel operation. It +basically reads the kernel messages and keeps them in buffer, letting the user +see them later on. Here's an example of what a dmesg output looks like: +

+ +

+SIS5513: IDE controller at PCI slot 0000:00:02.5
+SIS5513: chipset revision 208
+SIS5513: not 100% native mode: will probe irqs later
+SIS5513: SiS 961 MuTIOL IDE UDMA100 controller
+ide0: BM-DMA at 0x4000-0x4007, BIOS settings: hda:DMA, hdb:DMA
+ide1: BM-DMA at 0x4008-0x400f, BIOS settings: hdc:DMA, hdd:DMA
+Probing IDE interface ide0...
+input: ImPS/2 Generic Wheel Mouse on isa0060/serio1
+hda: WDC WD800BB-60CJA0, ATA DISK drive
+hdb: CD-RW 52X24, ATAPI CD/DVD-ROM drive
+ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
+Probing IDE interface ide1...
+hdc: SAMSUNG DVD-ROM SD-616T, ATAPI CD/DVD-ROM drive
+hdd: Maxtor 92049U6, ATA DISK drive
+ide1 at 0x170-0x177,0x376 on irq 15
+hda: max request size: 128KiB
+hda: 156301488 sectors (80026 MB) w/2048KiB Cache, CHS=65535/16/63,
+UDMA(100)
+hda: cache flushes not supported
+hda: hda1
+hdd: max request size: 128KiB
+hdd: 39882528 sectors (20419 MB) w/2048KiB Cache, CHS=39566/16/63,
+UDMA(66)
+hdd: cache flushes not supported
+hdd: unknown partition table
+hdb: ATAPI 52X CD-ROM CD-R/RW drive, 2048kB Cache, UDMA(33)
+Uniform CD-ROM driver Revision: 3.20
+hdc: ATAPI 48X DVD-ROM drive, 512kB Cache, UDMA(33)
+ide-floppy driver 0.99.newide
+libata version 1.11 loaded.
+usbmon: debugs is not available	  
+

+ +

+The dmesg displayed here is my machine's bootup. You can see harddrives and +input devices being initialized. While what you see here seems relatively +harmless, dmesg is also good at showing when things go wrong. Let's take +for example an IPAQ 1945 I have. After a couple of minutes of inactivity, the +device powers off. Now, I have the device connected into the USB port in the +front of my system. Now, I want to copy over some files using libsynCE, so I go +ahead and initiate a connection: +

+ +

+# synce-serial-start
+/usr/sbin/pppd: In file /etc/ppp/peers/synce-device: unrecognized option
+'/dev/tts/USB0'
+
+synce-serial-start was unable to start the PPP daemon!
+

+ +

+The connection fails, as we see here, and we assume that only the screen is in +powersave mode, and that maybe the connection is faulty. In order to see what +truly happened, we can use dmesg. Now, dmesg tends to give a +rather large ammount of output. One can use the tail command to help +keep the output down: +

+ +

+$ dmesg | tail -n 4
+usb 1-1.2: PocketPC PDA converter now attached to ttyUSB0
+usb 1-1.2: USB disconnect, address 11
+PocketPC PDA ttyUSB0: PocketPC PDA converter now disconnected from ttyUSB0
+ipaq 1-1.2:1.0: device disconnected
+

+ +

+This gives us the last 4 lines of the dmesg output. Now, this is enough to give +us some information on the situation. It seems that in the first 2 lines, the +pocketpc is recognized as connected. However, in the last 2 lines, it appears +to have been disconnected. With this information we check the pocketpc again, +and find out it is powered off, and now know about the powersave mode. We can +use this information to turn the feature off, or be aware of it next time. +While this is a somewhat simple example, it does go to show how well dmesg can +work. However, in more complex examples (such as kernel bugs), the entire dmesg +output may be required. To obtain that, simple redirect to a log file as such: +

+ +

+$ dmesg > dmesg.log
+

+ +

+You can then attach this to a bug report, or post it online somewhere for +collaborative debugging sessions. +

+ + +