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Creating RAM Disk U-Boot Images

With U-Boot it is possible to store a ramdisk image in flash and use that as a root filesystem for the Linux kernel. The following example shows how to build, and download a ramdisk image to flash and boot the kernel in flash using this ramdisk image.

Creating The RAM Disk Image

Instructions for creating the ramdisk image see Booting with a ramdisk root file system. Once this filesystem image has been created it needs to be converted into a U-Boot image file: 

host% ./tools/mkimage -A sh -O linux -T ramdisk -a 0x8C800000 -n "ST40 Linux ramdisk" -d initrd.img /export/ramdisk.ub
Image Name:   ST40 Linux ramdisk
Created:      Fri Nov  7 17:00:42 2008
Image Type:   SuperH Linux RAMDisk Image (gzip compressed)
Data Size:    4739544 Bytes = 4628.46 KiB = 4.52 MiB
Load Address: 0x8C800000
Entry Point:  0x8C800000

Note: the address given for the "-a" argument should be located shortly beyond the address where the Linux kernel resides in SDRAM. Typically, add about 8MiB to the kernel's load addresses.

Downloading The RAM Disk Image

The U-Boot ramdisk image can then be downloaded to the target using NFS download: 
MB618> nfs $load_addr /export/ramdisk.ub
 
Warning: MAC addresses don't match:
        HW MAC address:  FF:FF:FF:FF:FF:FF
        "ethaddr" value: AA:00:04:00:0F:0F
Using MAC Address AA:00:04:00:0F:0F
STM-GMAC: SMSC LAN8700 found
STM-GMAC: 100Mbs full duplex link detected
File transfer via NFS from server 164.129.15.56; our IP address is 164.129.15.15
Filename '/export/ramdisk.ub'.
Load address: 0x8c000000
Loading: #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         ################
done
Bytes transferred = 4739608 (485218 hex)
MB618> iminfo $fileaddr
 
## Checking Image at 8c000000 ...
   Image Name:   ST40 Linux ramdisk
   Image Type:   SuperH Linux RAMDisk Image (gzip compressed)
   Data Size:    4739544 Bytes =   4.5 MiB
   Load Address: 8c800000
   Entry Point:  8c800000
   Verifying Checksum ... OK

Burning The RAM Disk Image To Flash

The ramdisk image can then by copied in to flash: 

MB618> prot off 1:27-63
Un-Protect Flash Sectors 27-63 in Bank # 1
..................................... done
MB618> erase 1:27-63
Erase Flash Sectors 27-63 in Bank # 1
..................................... done
MB618> cp.b $fileaddr A0300000 $filesize
Copy to Flash
................................................................................
................................................................................
................................................................................
................................................. done
MB618> iminfo A0300000
 
## Checking Image at a0300000 ...
   Image Name:   ST40 Linux ramdisk
   Image Type:   SuperH Linux RAMDisk Image (gzip compressed)
   Data Size:    4739544 Bytes =   4.5 MiB
   Load Address: 8c800000
   Entry Point:  8c800000
   Verifying Checksum ... OK

At this point, there will typically be 2 U-Boot images burned in the flash - one for the kernel, and one for the ramdisk. All U-Boot images in flash can be easily seen (and verified) using the "imls" command: 

MB618> imls
Image at A0060000:
   Image Name:   Linux-2.6.23.17_stm23_0117-mb618
   Image Type:   SuperH Linux Kernel Image (gzip compressed)
   Data Size:    1845268 Bytes =   1.8 MiB
   Load Address: 8c001000
   Entry Point:  8c002000
   Verifying Checksum ... OK
Image at A0300000:
   Image Name:   ST40 Linux ramdisk
   Image Type:   SuperH Linux RAMDisk Image (gzip compressed)
   Data Size:    4739544 Bytes =   4.5 MiB
   Load Address: 8c800000
   Entry Point:  8c800000
   Verifying Checksum ... OK
Note: neither the U-Boot program (u-boot.bin), nor U-Boot's environment variables are displayed using the imls command, as this command only lists ".ub" images in flash.

Booting The Linux Kernel With A RAM Disk Root Filesystem

To use a ramdisk image when booting a kernel, set the appropriate value for "bootargs", and add the address of the ramdisk image (in flash) to the "bootm" command, after the address of the linux kernel image (in flash). See Booting The Linux Kernel With A RAM Disk Root Filesystem for further details.