Mounting a Samsung Galaxy Player on Linux (Shallow Thoughts)

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

Wed, 18 Apr 2012

Mounting a Samsung Galaxy Player on Linux

My new toy: a Samsung Galaxy Player 5.0!

So far I love it. It's everything my old Archos 5 wanted to be when it grew up, except the Archos never grew up. It's the same size, a little lighter weight, reliable hardware (no random reboots), great battery life, fast GPS, modern Android 2.3, and the camera is even not too bad (though it certainly wouldn't tempt me away from my Canon).

For the record, Dave got a Galaxy Player 4.0, and it's very nifty too, and choosing between them was a tough choice -- the 4-inch is light and so easy to hold, and it uses replaceable batteries, while the 5-inch's screen is better for reading and maps.

USB-storage devices don't register

I love the Galaxy ... but there's one thing that bugs me about it. When I plug it in to Linux, dmesg reports two new storage devices, one for main storage and one for the SD card. Just like most Android devices, so far.

The difference is that these Samsung devices aren't fully there. They don't show up in /proc/partitions or in /dev/disk/by-uuid, dmesg doesn't show sizes for them, and, most important, they can't be mounted by UUID from an fstab entry, like

UUID=62B0-C667   /droidsd    vfat   user,noauto,exec,fmask=111,shortname=lower 0 0
That meant I couldn't mount it as myself -- I had to become root, figure out where it happened to show up this time (/dev/sdf or wherever, depending on what else might be plugged in), mount it, then do all my file transfers as root.

I found that if I mounted it explicitly using the device pathname -- mount /dev/sdf /mnt -- then subsequently the device shows up normally, even after I unmount it. So I could check dmesg to find the device name, mount it as root, unmount as root, then mount it again as myself using my fstab entry. What a pain!

A kernel expert I asked thought it looked like the Samsung is pretending to be a removable device, but only "plugging in" when the system actually tries to access it. Annoying. So how do you get Linux to "access" it?

Udev: still an exercise in frustration

The obvious solution is a udev rule. Some scrutiny of /lib/udev/rules.d/60-persistent-storage.rules found some rules that did this intriguing thing: IMPORT{program}="ata_id --export $tempnode".

Naturally, this mysterious ata_id is undocumented. It's hidden in /lib/udev/ata_id, and I found this tiny ata_id man page online since there's none available in Ubuntu. Running ata_id /dev/sdf seemed to do what I needed: it made the device show up in /proc/partitions and /dev/disk/by-uuid, and after that, I could mount it without being root.

I created a file named /etc/udev/rules.d/59-galaxy.rules, with the rule:

KERNEL=="sd[b-g]", SUBSYSTEMS=="usb", ATTRS{idVendor}=="04e8", SYMLINK+="galaxy-%k-%n", IMPORT{program}="ata_id --export $tempnode"

When I tested it with udevadm test /block/sdf, not only did the rule trigger correctly, but it actually ran ata_id and the device became visible -- even though udevadm test states clearly no programs will be run. How do I love udev -- let me count the ways.

But a reboot revealed that udev was not actually running the rule when I actually plugged the Galaxy in -- the devices did not become visible. Did I mention how much I love udev?

Much simpler: a shell alias

But one thing I'd noticed in all this: side by side with /dev/disk/by-uuid is a directory called /dev/disk/by-id. And the Samsung devices did show up there, with names like usb-Android_UMS_Composite_c197022a2b41c1ae-0:0.

Faced with the prospect of spending the rest of the day trying random udev rules, rebooting each time since that's the only way to test udev, I decided to cheat and find another way. I made a shell alias:

alias galaxy='sudo sh -c "for d in /dev/disk/by-id/usb-Android*; do /lib/udev/ata_id --export \$d; done"'

Typing galaxy will now cause the Samsung to register both devices; and from then on I can mount and unmount them without needing root, using my normal fstab entries.

Update: This works for the Nook's main storage, too -- just add x/dev/disk/by-id/usb-B_N_Ebook_Disk* to the list -- but it doesn't work reliably for the Nook's SD card. The SD card does show up in /dev/disk/by-id along with main storage; but running ata_id on it doesn't make its UUID appear. I may just change my fstab entry to refer to the /dev/disk/by-id device directly.

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[ 13:43 Apr 18, 2012    More linux/kernel | permalink to this entry | comments ]
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