Making static devices in UDEV (Shallow Thoughts)

Akkana's Musings on Open Source Computing, Science, and Nature.

Wed, 20 Apr 2011

Making static devices in UDEV

(or: Fixing a multi flash card reader on Ubuntu Natty)

For the first time after installing Ubuntu Natty, I needed to upload some photos from my camera -- and realized with a sinking feeling that I now had the new UDEV, which no longer lets you use the udev all_partitions directive, so that cards inserted into a multi flash card reader will show up as /dev/sdb1 or whatever the appropriate device name is.

Without all_partitions, you get the initial sdb, sdc, sdd and sde for the various slots in the card reader, but since there's no card there when the machine boots, and the reader doesn't send an event when you insert a card later, you never get a mountable /dev/sdb1 device.

But the udev developers in their infinite wisdom removed all_partitions some time last year, apparently without providing any replacement for it. So you can no longer solve this problem through udev rules.

Static udev devices

Fortunately, there's another way, which is actually easier (though less flexible) than udev rules: udev static devices. You can create the devices you need once, and tell udev to create exactly those devices every time.

To begin, first find out what your base devices are. Look through dmesg | more for your card reader. Mine looks something like this:

[    3.304938] scsi 4:0:0:0: Direct-Access     Generic  USB SD Reader    1.00 PQ
: 0 ANSI: 0
[    3.305440] scsi 4:0:0:1: Direct-Access     Generic  USB CF Reader    1.01 PQ
: 0 ANSI: 0
[    3.305939] scsi 4:0:0:2: Direct-Access     Generic  USB xD/SM Reader 1.02 PQ
: 0 ANSI: 0
[    3.306438] scsi 4:0:0:3: Direct-Access     Generic  USB MS Reader    1.03 PQ
: 0 ANSI: 0
[    3.306876] sd 4:0:0:0: Attached scsi generic sg1 type 0
[    3.307020] sd 4:0:0:1: Attached scsi generic sg2 type 0
[    3.307165] sd 4:0:0:2: Attached scsi generic sg3 type 0
[    3.307293] sd 4:0:0:3: Attached scsi generic sg4 type 0
[    3.313181] sd 4:0:0:1: [sdc] Attached SCSI removable disk
[    3.313806] sd 4:0:0:0: [sdb] Attached SCSI removable disk
[    3.314430] sd 4:0:0:2: [sdd] Attached SCSI removable disk
[    3.315055] sd 4:0:0:3: [sde] Attached SCSI removable disk

Notice that the SD reader is scsi 4:0:0:0, and a few lines later, 4:0:0:0 is mapped to sdb. They're out of order, so make sure you match those scsi numbers. If I want to read SD cards, /dev/sdb is where to look.

(Note: sd in "sdb" stands for "SCSI disk", while SD in "SD card" stands for "Secure Digital". Two completely different meanings for the same abbreviation -- just an unfortunate coincidence to make this all extra confusing.)

To create static devices, I'll need the major and minor device numbers for the devices I want to create. Since I know the SD card slot is sdb, I can get those with ls:

$ ls -l /dev/sdb
brw-rw---- 1 root disk 8, 16 2011-04-20 09:43 /dev/sdb
The b at the beginning of the line tells me it's a block device; the major and minor device numbers for the base SD card device are 8 and 16. To get the first partition on that card, use the same major device and add one to the minor device: 8 and 17.

Now you can create new static block devices, as root, using mknod in the /lib/udev/devices directory:

$ sudo mknod /lib/udev/devices/sdb1 b 8 17
$ sudo mknod /lib/udev/devices/sdb2 b 8 18
$ sudo mknod /lib/udev/devices/sdb3 b 8 19

Update: Previously I had here
$ sudo mknod b 8 17 /lib/udev/devices/sdb1
but the syntax seems to have changed as of mid-2012.

Although my camera only uses one partition, sdb1, I created devices for a couple of extra partitions because I sometimes partition cards that way. If you only use flash cards for cameras and MP3 players, you may not need anything beyond sdb1.

You can make devices for the other slots in the card reader the same way. The memory stick reader showed up as scsi 4:0:0:3 or sde, and /dev/sde has device numbers 8, 64 ... so to read the memory stick from Dave's Sony camera, I'd need:

$ sudo mknod /lib/udev/devices/sde1 b 8 65

You don't have to call the devices sdb1, either. You can call them sdcard1 or whatever you like. However, the base device will still be named sdb (unless you write a udev rule to change that).

fstab entry

I like to use fstab entries and keep control over what's mounted, rather than letting the system automatically mount everything it sees. I can do that with this entry in /etc/fstab:

/dev/sdb1 /sdcard vfat user,noauto,exec,fmask=111,shortname=lower 0 0
plus sudo mkdir /sdcard.

Now, whenever I insert an SD card and want to mount it, I type mount /sdcard as myself. There's no need for sudo because of the user directive.

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[ 19:22 Apr 20, 2011    More linux | permalink to this entry | comments ]
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