DENX . DULG . DiskOnChip

Topic DiskOnChip not in WebOrder Using Disk On Chip "Drives"

Before you can use a new, uninitialized Disk-On-Chip (DOC) flash disk, you must initialize it. In this process you can reserve a certain area of the disk for a binary image, and create normal partitions and filesystems on the remaining space - like you would do on a harddisk drive.

The separate binary image partition is visible from U-Boot, but cannot (easily) be accessed from within Linux. Such a binary partition is intended to hold boot images - U-Boot can use it to load a Linux kernel image, so we recommend to create such a partition if you intend to boot from the DOC drive.

First you have to make sure that no conflicting data is present on the DOC drive. Under Linux, the eraseall command can be used to completely erase the whole drive. Please note that eraseall operates on the MTD character device.

In the second step the nftl_format command is used to write the necessary administrative information and (optionally) to reserve a certain amount of the DOC drive as binary partition - the size is given as third argument to the nftl_format command.

The next step is to create standard partitions on the DOC drive; as with normal harddisk drives the fdisk command is called for this purpose. In most cases you will create only a single (primary) partition on the DOC drive.

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Now we have to create a filesystem on the DOC partition, and populate it with the files necessary for a Linux root filesystem. We use the mke2fs command to create a standard EXT2 Linux filesystem on the device - we reserve no space ro the root user here (Option "-m0") as this makes little sense in an embedded system.

We use a trick here to populate the filesystem: we use the contents of the known-to-work ramdisk image from DENX' SELF package: first we use tftp to download the image from our TFTP server, then we skip the 64 bytes U-Boot header with a dd command with the appropriate options and uncompress the image with gunzip, writing the output into one of the Linux ramdisk devices (/dev/ram1). Then we mount both the ramdisk and the DOC drive, and copy the contents with a combination of find and cpio commands:

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After unmounting the filesystems (don't forget this!), we can reboot the system and verify in U-Boot that our new format is present. This can be done with the doc info:

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Now we are ready to store a Linux kernel image in the binary partition of the DOC drive. First we make sure that the binary partition is empty using the doc erase command, then we TFTP a kernel image from the TFTP server, and store it on the DOC drive using the doc write command.

For our convenience later we also define a new environment variable "docargs" which sets kernel boot arguments to mount the root filesystem from the DOC drive, and save it to the persistent storage:

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Finally, we demonstrate how to boot from the DOC drive: with docboot we load the Linux kernel drom the DOC device, then we initialize the bootargs environment variable by running the commands stored in the "docargs" (set root device) and "addip" (append network configuration) variables, before we boot the kernel with bootm:

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