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

6.4.3. Bootloader Support for initrd

6.4.3. Bootloader Support for initrd

Let's look at a simple example based on the popular U-Boot bootloader running on an ARM processor. This bootloader has been designed with Linux kernel support. Using U-Boot, it is easy to include an initrd image with the kernel image. Listing 6-10 examines a typical boot sequence containing an initial ramdisk image.

Listing 6-10. Booting Kernel with Ramdisk Support

# tftpboot 0x10000000 kernel-uImage
...
Load address: 0x10000000
Loading: ############################ done
Bytes transferred = 1069092 (105024 hex)
# tftpboot 0x10800000 initrd-uboot
...
Load address: 0x10800000
Loading: ########################################### done
Bytes transferred = 282575 (44fcf hex)
 # bootm 0x10000000 0x10800040
Uncompressing kernel.................done.
...
RAMDISK driver initialized: 16 RAM disks of 16384K size 1024 blocksize
...
RAMDISK: Compressed image found at block 0
VFS: Mounted root (ext2 filesystem).
Greetings: this is linuxrc from Initial RAMDisk
Mounting /proc filesystem
BusyBox v1.00 (2005.03.14-16:37+0000) Built-in shell (ash)
Enter 'help' for a list of built-in commands.
# (<<<< Busybox command prompt) 

Here in Listing 6-10, we get a glimpse of the U-Boot bootloader, which we examine in more detail in the next chapter. The tftpboot command causes U-Boot to download the kernel image from a tftp server. The kernel image is downloaded and placed into the base of this target system's memory at the 256MB address (0x10000000 hex[53]). Then a second image, the initial ramdisk image, is downloaded from a tftp server into memory at a higher memory address (256MB + 8MB, in this example). Finally, we issue the U-Boot bootm command, which is the "boot from memory" command. The bootm command takes two arguments: the address of the Linux kernel image, optionally followed by an address representing the location of the initial ramdisk image.

Take special note of one feature of the U-Boot bootloader. It fully supports loading kernel and ramdisk images over an Ethernet connection. This is a very useful development configuration. You can get a kernel and ramdisk image onto your board in other ways as well. You can flash them into your Flash memory using a hardware-based flash programming tool, or you can use a serial port and download the kernel and file system images via RS-232. However, because these images are typically large (a kernel can be about a megabyte, and a ramdisk can be tens of megabytes), you will save a significant amount of engineering time if you invest in this Ethernet-based tftp download method. Whatever bootloader you choose, make sure it supports network download of development images.

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