Linux: How to map a blockdevice to a USB-device?

Question

if I plugin a USB memory stick, I see a new folder in /sys/bus/usb/devices ... thus a new USB-device.

Also I see a new folder in /sys/block ... thus a new block-device.

My question is: How can I get a waterproof mapping between those two devices? Means: If I get a new device in /sys/bus/usb/devices, how can I programatically (f.i. by checking /sys/...) find out which block device is mapped/related to this usb-device and vice-versa?!

Answer 1

The information in /sys is organized in multiple ways (by driver, by bus, etc.), and there are many symbolic links to go from one hierarchy to another.

Case in point (example seen on kernel 2.6.26): starting from the block device in /sys/block/sdc, the symbolic link /sys/block/sdc/device points inside the per-device-type hierarchy. You can see that it's an USB device because the target of the link is something like

../../devices/pci0000:00/0000:00:1d.7/usb8/8-2/8-2:1.0/host9/target9:0:0/9:0:0:0

Conversely, USB devices are listed in /sys/bus/usb/devices, and we can see that 8-2:1.0 is a disk-like device because /sys/bus/usb/devices/8-2:1.0/driver links to usb-storage. To find out what the associated block device is, it seems we need to go down to the directory /sys/bus/usb/devices/8-2:1.0/host9/target9:0:0/9:0:0:0 which contains a symbolic link block:sdc whose target is /sys/block/sdc.

ADDED: Caution: the exact structure of /sys changes from kernel version to kernel version. For example, with kernel 2.6.32, /sys/block/sdc/device points directly into the /dev/bus/scsi without going through the USB hop.

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A different approach is to call the udevadm info command. udevadm info -p /sys/block/sdc --query=… gives information on a device based on its /sys entry, while udevadm info -n sdc --query=… gives information on the device /dev/sdc.

The information includes bus information, for example udevadm info -p /sys/block/sdc --query=env shows

ID_BUS=usb
ID_PATH=pci-0000:00:1d.7-usb-0:2:1.0-scsi-0:0:0:0

The udev documentation may have more information of interest to you.

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A final word of caution: there are all kinds of complex cases that may make whatever you do not so waterproof. How will your program deal with a single USB device that is an array of disks that are assigned multiple block devices? Conversely, how will your program deal with a RAID array assembled from multiple devices (perhaps some of them USB and some of them not)? Do you care about other removable media types such as Firewire and e-SATA? etc. You won't be able to predict all corner cases, so make sure to fail gracefully.