Revision as of 07:20, 1 February 2013

UNDER CONSRUCTION!

PAGE STILL UNDER CONSTRUCTION...

Basic assumptions / scope

• This guide is meant for converting a working u-boot to boot to mer / nemo from SD card or internal eMMC.
  • "loadaddr" environment variable is defined
• It is assumed that you have some way of accessing u-boot console (serial / adb / etc)
• Mer / Nemo / etc image with separate FAT or Ext2 boot partition and a root partition on your boot media (SD or eMMC)
  • in case of device internal eMMC you need HW specific flashing tools to transfer the image to it

• u-boot features needed:
  • working mmc/SD card driver for your board
  • "fat load" or ext2load support (CONFIG_CMD_FAT, CONFIG_CMD_EXT2)
  • hush shell support (CONFIG_SYS_HUSH_PARSER)

Minimum path to boot

What are the absolute minimum things to get u-boot boot a linux-kernel from a SD card or eMMC for mer/nemo?

Here is the list:

• load the kernel uImage from a disk to memory
  • fat load / ext2load / mmc load
• start the kernel
  • bootm ${loadaddr}
• pass right parameters to kernel in "bootargs" environment variable
  • parameters specific for the HW (typically just re-use what is there already in bootargs)
  • Mer/Nemo root FS location
  • systemd.unit=start.getty

Quick & generic SD card boot

1. Insert SD card with nemo image (uImage on ext2 or vfat boot partition, and rootfs on second partition)
2. Boot the device to u-boot console

Then we need to setup the above minimum things in u-boot environment in place. Start with running "printenv"

$ printenv
bootcmd=run emmcboot
bootdelay=1
baudrate=115200
verify=n
rdinit=init
loadaddr=0x00100000
console=ttyAMA2,115200n8
...
<clip><clip>

Now you see what is the current environment, it's good idea to copy-paste this somewhere safe just in case you make a mess of it ;)

Now then, the steps to success were to load the kernel and boot it, so we set up a generic script to do that (copy paste below command to u-boot prompt):

setenv nemo_bootcmd 'for j in 2 1 0; do \
	echo stepj $j; \
		mmc rescan $j;
		for i in 0 1 2 3 4 5 6 7 8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19; do \
			mmc rescan $j; \
			if fat load mmc $j:$i ${loadaddr} /uImage; then \
				bootm ${loadaddr}; \
			fi; \
			if fat load mmc $j:$i ${loadaddr} /boot/uImage; then \
				bootm ${loadaddr}; \
			fi; \
			if ext2load mmc $j:$i ${loadaddr} /uImage; then \
				bootm ${loadaddr}; \
			fi; \
			if ext2load mmc $j:$i ${loadaddr} /boot/uImage; then \
				bootm ${loadaddr}; \
			fi; \
			echo no device found in mmc $j partition $i; \
		done; \
	done'

The command above sets up a new environment variable "nemo_bootcmd", that, if run, goes through mmc devices 2, 1, and 0 and tries loading uImage from partitions 1 to 25 on each of those devices. It tries to load the kernel uImage with fat load (in case it's a FAT partition) and ext2load (in case it's ext2 partition) from / and /boot/ folders (the most likely locations). If a image is found and loaded successfully it is started with the bootm command.

Next, before we boot, we need to make sure kernel gets right bootargs passed so it knows where the root filesystem is. On my SD card the boot partition is partition 1 and root fs partition is 2. Check what does is set for bootargs when you run "printenv". Here is a example bootargs:

bootargs=cachepolicy=writealloc noinitrd hwtoolonuart= rdinit=init init=init board_id=1 crashkernel=crashkernel=1M@0x5600000 logo.nologo startup_graphics=1 root=/dev/mmcblk2p2 rootwait console=ttyAMA2,115200n8

Now then, let's try running our script:

$ run nemo_bootcmd
U8500 $ run nemo_bootcmd
Writing partition block (if needed)...
 eMMC partition block exists now
MMC Device 2 not found
MMC Device 2 not found

** Invalid boot device **
MMC Device 2 not found

** Invalid boot device **
MMC Device 2 not found
** Block device mmc 2 not supported
MMC Device 2 not found
** Block device mmc 2 not supported
no device found in mmc 2 partition 0

<clip-clip>
....
<clip-clip\>

** Invalid boot device **
MMC Device 2 not found
** Block device mmc 2 not supported
MMC Device 2 not found
** Block device mmc 2 not supported
no device found in mmc 2 partition 19
** Partition 0 not valid on device 1 **

** Unable to use mmc 1:0 for fatload **
** Partition 0 not valid on device 1 **

** Unable to use mmc 1:0 for fatload **
Loading file "/uImage" from mmc device 1
Failed to mount ext2 filesystem...
** Bad ext2 partition or disk - mmc 1:0 **
Loading file "/boot/uImage" from mmc device 1
Failed to mount ext2 filesystem...
** Bad ext2 partition or disk - mmc 1:0 **
no device found in mmc 1 partition 0
Reading /uImage

4764960 bytes read
## Booting kernel from Legacy Image at 00100000 ...
   Image Name:   Linux-3.0.8.20121218.2
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    4764896 Bytes =  4.5 MB
   Load Address: 00008000
   Entry Point:  00008000
   Loading Kernel Image ... OK
OK

Starting kernel ...  arch 3293

Uncompressing Linux... done, booting the kernel.

So now the kernel was found and booted from partition 1 of mmc dev 1 (which was my SD card).

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