Книга: Embedded Linux Primer: A Practical, Real-World Approach

10.4.1. JFFS2 Root File System

10.4.1. JFFS2 Root File System

Creating a JFFS2 root file system is a straightforward process. In addition to compression, JFFS2 supports wear leveling, a feature designed to increase Flash lifetime by fairly distributing the write cycles across the blocks of the device. As pointed out in Chapter 9, Flash memory is subject to a limited number of write cycles. Wear leveling should be considered a mandatory feature in any Flash-based file system you employ. As mentioned elsewhere in this book, you should consider Flash memory as a write-occasional medium. Specifically, you should avoid allowing any processes that require frequent writes to target the Flash file system. Be especially aware of any logging programs, such as syslogd.

We can build a JFFS2 image on our development workstation using the ext2 image we used on our Redboot RootFS partition. The compression benefits will be immediately obvious. The image we used in the previous RootFS example was an ext2 file system image. Here is the listing in long (-l) format:

# ls -l rootfs.ext2
-rw-r--r-- 1 root root 6291456 Nov 19 16:21 rootfs.ext2

Now let's convert this file system image to JFFS2 using the mkfs.jffs2 utility found in the MTD package. Listing 10-15 shows the command and results.

Listing 10-15. Converting RootFS to JFFS2

# mount -o loop rootfs.ext2/mnt/flash/
# mkfs.jffs2 -r /mnt/flash -e 128 -b -o rootfs.jffs2
# ls -l rootfs.jffs2
-rw-r--r-- 1 root root 2401512 Nov 20 10:08 rootfs.jffs2
#

First we mount the ext2 file system image on a loopback device on an arbitrary mount point on our development workstation. Next we invoke the MTD utility mkfs.jffs2 to create the JFFS2 file system image. The -r flag tells mkfs.jffs2 where the root file system image is located. The -e instructs mkfs.jffs2 to build the image while assuming a 128KB block size. The default is 64KB. JFFS2 does not exhibit its most efficient behavior if the Flash device contains a different block size than the block size of the image. Finally, we display a long listing and discover that the resulting JFFS2 root file system image has been reduced in size by more than 60 percent. When you are working with limited Flash memory, this is a substantial reduction in precious Flash resource usage.

Take note of an important command-line flag passed to mkfs.jffs2 in Listing 10-15. The -b flag is the -big-endian flag. This instructs the mkfs.jffs2 utility to create a JFFS2 Flash image suitable for use on a big-endian target. Because we are targeting the ADI Engineering Coyote board, which contains an Intel IXP425 processor running in big-endian mode, this step is crucial for proper operation. If you fail to specify big endian, you will get several screens full of complaints from the kernel as it tries to negotiate the superblock of a JFFS2 file system that is essentially gibberish.[76] Anyone care to guess how I remembered this important detail?

In a similar manner to the previous example, we can copy this image to our Redboot RootFS Flash partition using the flashcp utility. Then we can boot the Linux kernel using a JFFS2 root file system. Listing 10-16 provides the details, running the MTD utilities on our target hardware.

Listing 10-16. Copying JFFS2 to RootFS Partition

root@coyote:~# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00060000 00020000 "RedBoot"
mtd1: 00160000 00020000 "MyKernel"
mtd2: 00600000 00020000 "RootFS"
mtd3: 00001000 00020000 "RedBoot config"
mtd4: 00020000 00020000 "FIS directory"
root@coyote:~# flash_erase /dev/mtd2
Erase Total 1 Units
Performing Flash Erase of length 131072 at offset 0x0 done
root@coyote:~# flashcp /rootfs.jffs2 /dev/mtd2
root@coyote:~#

It is important to note that you must have the JFFS2 file system enabled in your kernel configuration. Execute make ARCH=<arch> gconfig and select JFFS2 under File Systems, Miscellaneous File Systems. Another useful hint is to use the -v (verbose) flag on the MTD utilities. This provides progress updates and other useful information during the Flash operations.

We have already seen how to boot a kernel with the Redboot exec command. Listing 10-17 details the sequence of commands to load and boot the Linux kernel with our new JFFS2 file system as root.

Listing 10-17. Booting with JFFS2 as Root File System

RedBoot> load -r -v -b 0x01008000 coyote-zImage
Using default protocol (TFTP)
Raw file loaded 0x01008000-0x0114decb, assumed entry at 0x01008000
RedBoot> exec -c "console=ttyS0,115200 rootfstype=jffs2 root=/dev/mtdblock2"
Using base address 0x01008000 and length 0x00145ecc
Uncompressing Linux...... done, booting the kernel.
... 

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