| Flash Management and JFFS2 File System |
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Linux provides management of the on-module NOR Flash, including the ability to partition the physical Flash onto separate logical partitions and mount a JFFS2 flash filesystem on a Flash partition. The Flash management and JFFS2 filesystem require enabling appropriate kernel options in the kernel configuration and adding certain device nodes and Linux tools to the target file system. The networking project included in the Emcraft distribution at projects/networking provides a sample Linux configuration that has the Flash management and JFFS2 functionality fully enabled. The examples in the text below make use of the networking project. Linux splits the external Flash onto several partitions. Even though the partitioning is purely logical in the software, the user view is that each Flash partition can be accessed as a separate "Flash disk" device, independent of the other Flash partitions. Each Flash partitioned can be accessed as a "raw" disk or, alternatively, a Flash file system can be mounted on a partition. The Flash partitioning is defined in the following kernel platform file: linux/arch/arm/mach-stm32/flash.c. There are certain C data structures that define the number, offset and size of the configured Flash partitions, for instance: static struct mtd_partition flash_partitions[] = { flash.c included in the distribution provides a reasonable Flash partitioning for the on-module Flash device, however, depending on your application needs, you might want to change the number and size of configured Flash partitions. When Linux boots on the target, there will be the following messages in the kernel print-out indicating that the Flash driver is enabled and then describing the specific partitions created by the kernel: stm32f4 platform flash device: 01000000 at 64000000 In the above example, the first partition is used to hold the U-Boot environment variables. The second partition is allocated to the bootable Linux image (uImage). Finally, the third partition is dedicated to the JFFS2 file system. Let's start with creating a JFFS2 file system on the third partition. The following command erases the Flash partition and marks it up as a JFFS2 file system. JFFS2 provides transparent management of bad blocks and Flash wearing capabilities ensuring the robustness and longevity of the file system in Flash. Notice use of the -j flag as the indication to flash_eraseall that the partition must not only be erased by also marked up for JFFS2: ~ # flash_eraseall -j /dev/mtd2 Now everything is ready to mount a JFFS2 file system on that partition. As expected, the initial file system is empty: ~ # mkdir /m Let's add some files to the JFFS2 file system and validate that they are correct. In the example below, we copy the busybox binary from the initramfs root file system to Flash and then run it from Flash: ~ # /m/busybox echo Hello from JFFFS2 Now, let's reboot the system and make sure that any updates we make in the JFFS2 are indeed persistent: ~ # reboot -f As a next step, let's update the Linux uImage in the second partition from a host directory mounted over NFS. This provides a simple demonstration of a possible software upgrade sequence using a Linux image pulled from the network. First step is to prepare the partition for a new uImage by erasing the Flash: ~ # flash_eraseall /dev/mtd1 ~ # mount -o nolock,rsize=1024 172.17.0.1:/home/vlad/test /mnt Now, we are ready to reboot and validate the target is indeed running the newly installed Linux project: ~ # reboot -f At this point, you will have to use the U-Boot command interface to install a different uImage to the Flash. Refer to Installing Linux images to Flash for detailed instructions on how to do that.
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