.. title: Loop device partition scanning on Ubuntu 14.04
.. date: 2015-04-17 17:49
.. tags: linux, kernel
.. slug: ubuntu-loop-max_part
.. type: text

In Embedded Computing, we often face the problem to prepare images for
entire storage media. Nowadays this applies especially to SD cards.
Whereas other operating systems usually need special utilities for
such situations, Linux has all the means already built-in to deal with
this.  The ``loopback`` device plays a central role here as it can
expose a regular file as a kernel level block device.  This block
device can then be used in place of a "real" SD card together with
regular programs.

.. TEASER_END
   
One recurring problem however is how to deal with images that contain
partitioned data, i.e. that need to be exposed through "sub devices"
on a partition level.  Searching the internet for solutions on this
usually turns up advice on how to manually use losetup multiple times
with manually calculated partition offsets.  There are even scripts
encapsulating this nicely.  Recently I discovered that the kernel
already has support for that and makes such scripts completely
unnecessary.  When you use a ``util-linux`` version of 2.21 or later,
it is as easy to use as specifying ``--partscan`` to ``losetup`` and
voila, minor number device node are created for the partitions
automatically.

Unfortunately, one of the more widely used GNU/Linux distributions,
e.g. Ubuntu 14.04 still carries util-linux in version 2.20 and we thus
don't have that easy way available.  Researching the situation some
more shows a mildly complicated workaround however - setting the
``max_part`` module parameter for the loop device to some number
greater than 0. As the driver is built into the Ubuntu kernel, we have
to change it through the kernel command line.  Edit
``/etc/default/grub`` and change the following line::

  GRUB_CMDLINE_LINUX="loop.max_part=63"

Now rerun ``update-grub`` and reboot.  With this parameter the loop
device will do an automatic partition scan and immediately create up
to 63 devices.  Here's an example from a terminal session:

.. code-block:: console

  $ sudo losetup /dev/loop0 core-image-minimal-imx6qsabresd.sdcard 
  [sudo] password for dzu: 
  $ sudo fdisk -l /dev/loop0
  Disk /dev/loop0: 25 MB, 25165824 bytes
  4 heads, 32 sectors/track, 384 cylinders, total 49152 sectors
  Units = sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disk identifier: 0x00019a5c
  
        Device Boot      Start         End      Blocks   Id  System
  /dev/loop0p1            8192       24575        8192    c  W95 FAT32 (LBA)
  /dev/loop0p2           24576       40959        8192   83  Linux
  $ ls -l /dev/loop0*
  brw-rw---- 1 root disk 7, 0 Apr 17 10:47 /dev/loop0
  brw-rw---- 1 root disk 7, 1 Apr 17 10:47 /dev/loop0p1
  brw-rw---- 1 root disk 7, 2 Apr 17 10:47 /dev/loop0p2