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5.3 Linux Support 23
5.3.8 Notes about userland root filesystems
Section 5.3.6 refers to the cramfs target without really indicating its’ use.
The cramfs option is one of three primary ’bundled’ targets:
1. cramfs - Creates a single file as a read-only, gzip-compressed image of
a filesystem tree. When you can nail down the content of your filesys-
tem, this is a great choice, providing the fastest boot time (around 7
seconds on a PXA-255) and complete immunity to corruption. This
filesystem is often used in conjunction with read-write filesystems (ram
disk for volatile data, or VFAT for semi-static data).
Requires cramfs support in the kernel (Miscellaneous Filesystems—Compressed
ROM file system support).
2. jffs2 - Creates a single file as a read-write, gzip-compressed image of
a filesystem tree. This is useful for placement in flash, and is fairly
immune to corruption at the cost of extra time for validation at boot
(typically 30-45 seconds for a 32MB filesystem).
Requires JFFS2 support in the kernel (Miscellaneous Filesystems—Journalling
Flash File System v2 ).
3. mmcinitrd/mmcinitrd.u-boot - Creates a single file as a read-
write, uncompressed image of a filesystem tree suitable for use as an
initial RAM disk (initrd).
It requires the following options in the kernel:
Loopback device support Device Drivers—Block Devices
Initial RAM Disk support Device Drivers—Block Devices
In addition, this target makes a bunch of other choices for you. Since
this is a bit involved, discussion of the steps is deferred to Section
5.3.9.
The Makefile instructions for each of these is at the tail-end of the
userland Makefile (userland/Makefile).
Refer to that file for details, but the bundled image for each is created
by performing a single command specifying an output file (the image), a
path name to a directory tree, and the devices.txt file.
Typical usage for the initrd target is to have the boot loader load the
image into RAM. Das U-Bootprovides support for handing the load address
to the Linux kernel through the bootm command.
Both the cramfs and JFFS2 images may also be mounted directly from
flash EEPROM using Linux MTD block devices. U-Boot’s support for passing
December 28, 2005 Revision 2.8