Initrd
From Texas Instruments Embedded ProcessorsWiki
After kernel booted, it tries to mount a filesystem. Using Linux on DaVinci, there are several options wherethis file system can come from. Options are
- Harddisk or CompactFlash card
- MMC/SD card
- NFS (see DVEVM Getting Started Guide, sprue66c.pdf, section4.3.4)
- (Flash) file system in NOR orNAND
- Ramdisk
The last option, an initial file system in a ram disk is calledinitrd (initialramdisk). Note that using an initrd with recentkernels is still possible and has some advantages, but isn't recommended any more.Using initramfs is the preferred way today.
initrd/initramfs (and NFS) is a file system option notpermanently stored at the target device and thus normally usedwhile development. initrd/initramfs is typically newinstalled/downloaded each time the target board is power cycled(using e.g. boot loader U-Boot). The two other options above (harddisk and flash file system) give the target system permanentlyaccess to its file system without any external debug connection(e.g. network download) and thus are used after developmentin production ready devices.
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initrd
Creation
To create an (initially empty) initrd use the followingsteps:
Note: Change count to your required filesystem size. E.g.with count=8192 you will get a 8MB ramdisk.
host > dd if=/dev/zero of=/dev/ram0 bs=1k count=<count> host > mke2fs -vm0 /dev/ram0 <count> host > tune2fs -c 0 /dev/ram0 host > dd if=/dev/ram0 bs=1k count=<count> | gzip -v9 > ramdisk.gz
Now, we have a (empty) gzipped ramdisk image with (extracted)size of <count>.
Filling
To fill empty ramdisk created above with all files needed forramdisk, mount the image and fill it. Content would be e.g.BusyBox and/or otherapplications and/or libraries.
host > mkdir mnt host > gunzip ramdisk.gz host > mount -o loop ramdisk mnt/ host > ... copy stuff you want to have in ramdisk to mnt... host > umount mnt host > gzip -v9 ramdisk
The resulting ramdisk.gz is now ready for usage. Note its sizeis smaller than <count> causeof compression.
Note: Don't forget to create/copy some basic /dev/xxx nodes toramdisk.
Note: If BusyBox or applications in ramdisk are linkeddynamically, don't forget to copy dynamic libraries (*.so) toramdisk (to correct directory) as well.
Kernel options
To make initrd work, you have to configure kernel properly:
# # General setup # ... CONFIG_BLK_DEV_INITRD=y CONFIG_INITRAMFS_SOURCE="" ... # # UBI - Unsorted block images # ... CONFIG_BLK_DEV_RAM=y CONFIG_BLK_DEV_RAM_COUNT=1 CONFIG_BLK_DEV_RAM_SIZE=8192 CONFIG_BLK_DEV_RAM_BLOCKSIZE=1024 ...
Note: The ramdisk size e.g. 8192 above has to be configured foryour individual setup.
Installation
Now, you can install the ramdisk via u-boot e.g. in NOR flash.For this copy filled ramdisk created above to your tftpbootdirectory on host (e.g. /tftpboot/ramdisk.gz). Then start targetand copy the data into RAM and flash:
UBOOT # tftp 0x87000000 ramdisk.gz UBOOT # erase 0x2200000 +0x<filesize> UBOOT # cp.b 0x87000000 0x2200000 0x<filesize>
Note: Replace filesize above by the value the tftpdownload command gives you as Bytes transferred.
Now, last step is to update kernel boot parameters and savethem
UBOOT # setenv bootargs ... root=/dev/ram0 rw initrd=0x87000000,8M UBOOT # setenv bootcmd cp.b 0x2200000 0x87000000 0x<filesize>; bootm UBOOT # saveenv
Note: In example above with "8M" we assume that your ramdisk is8MBytes. Adapt this to your needs.
Note: Your ramdisk filled above should have a /dev/ram0 node
brw-rw---- 1 root disk 1, 0 Sep 11 1999 /dev/ram0
to make this work properly.
Now you should be able to start your kernel and it should findand mount the initrd:
Linux version 2.6.23-davinci1 ... ... checking if image is initramfs...it isn't (no cpio magic); looks like an initrd Freeing initrd memory: 8192K ... RAMDISK driver initialized: 1 RAM disks of 8192K size 1024 blocksize ... RAMDISK: Compressed image found at block 0 VFS: Mounted root (ext2 filesystem). Freeing init memory: ... ...
initramfs
To use initramfs a cpio archive is embedded directly into thekernel. I.e. you don't create an additional (ramdisk) image.Instead, the initial file system is directly incorporated into thekernel. With this, the kernel size increases by the file systemsize. It's like you embed above ramdisk directly into thekernel.
Creation
Cause initramfs is directly embedded in the the kernel, itscreation is simpler. No dd & mount& gzip stuff like with ramdisk above. You simplyhave to fill a directory on your host with the target filesystemyou like and then pass the path to this directory to the kernelbuild process.
Create target file system
host > mkdir target_fs host > ... copy stuff you want to have in initramfs to target_fs...
Note: cpio system used for initramfs can't handle hardlinks. If you e.g. created your BusyBox using hard links, youwill get a quite large initramfs cause each command is taken withits size and not as hard link. In cpio initramfs use symbolic/soft links instead.
Note: To be able to detect initramfs by kernel properly, the toplevel directory has to contain a program called init. Thiscan be done by e.g. using a soft link from top level init to/bin/busybox
/init -> /bin/busybox
if you use BusyBox in your initramfs.
Kernel options
The only difference from creating an initrd is to give thekernel the path to the target file system you like to embed:
# # General setup # ... CONFIG_BLK_DEV_INITRD=y CONFIG_INITRAMFS_SOURCE="<path_to>/target_fs>" ... # # UBI - Unsorted block images # ... CONFIG_BLK_DEV_RAM=y CONFIG_BLK_DEV_RAM_COUNT=1 CONFIG_BLK_DEV_RAM_SIZE=8192 CONFIG_BLK_DEV_RAM_BLOCKSIZE=1024 ...
Then, if you compile the kernel, e.g. by make uImage, the cpioarchive is generated and embedded into the kernel:
... CHK include/linux/compile.h GEN usr/initramfs_data.cpio.gz AS usr/initramfs_data.o LD usr/built-in.o ...
Installation
No special installation like above with initrd isnecessary. The initramfs is already in the kernel. If you start thekernel, the initramfs is already there. Therefore, there isno root=/dev/ram0 rw initrd=0x87000000,8M bootargsoption necessary. Remove this if you still have it!
initrd vs. initramfs
- Using initrd, kernel and initial file system are splitted.Making changes to kernel or filesystem doesn't touch the other one.The download size (e.g. while development) of one component issmaller.
- Creating and modifying an initramfs is easier than with initrd(unzip & mount & unmount& zip)
- Having one big image (kernel & initramfs) iseasier to handle (e.g. download or flashing) than having twosplitted images.