EBC Exercise 21a Boot Sequence
From eLinux.org
Embedded Linux Class by Mark A. Yoder
Up to this point we've always booted the Beagle into Linux and worked from there. Now it's time to learn what has to happen before we can run Linux. To do this you need to look at the output of the serial port so see the boot sequence.
Contents |
Booting Up
Boot up your bone and connect to the serial port using the screen command. Hit return and log in.
host$ screen /dev/ttyUSB0 115200
.---O---.
| | .-. o o
| | |-----.-----.-----.| | .----..-----.-----.
| | | __ | ---'| '--.| .-'| | |
| | | | | |--- || --'| | | ' | | | |
'---'---'--'--'--. |-----------' '-----'-'-'-'
-' |
'---'
The Angstrom Distribution beaglebone ttyO0
Angstrom v2012.05 - Kernel 3.2.25
beaglebone login: root
root@beaglebone:~#
Shutdown
Now shutdown and be ready to hit return when you get the u-boot prompt.
beagle$ shutdown -r now Sending SIGTERM to remaining processes... Sending SIGKILL to remaining processes... Unmounting file systems. Unmounted /dev/mqueue. Unmounted /sys/kernel/debug. Disabling swaps. Detaching loop devices. Detaching DM devices. [ 599.018005] Restarting system.
These first messages are from the kernel shutting down.
U-Boot Starting
The following messages are the boot sequence.
U-Boot SPL 2011.09-00053-gb423c52 (Aug 10 2012 - 11:26:55) Texas Instruments Revision detection unimplemented No daughter card present No AC power, disabling frequency switch OMAP SD/MMC: 0 reading u-boot.img reading u-boot.img U-Boot 2011.09-00053-gb423c52 (Aug 10 2012 - 11:26:55) I2C: ready DRAM: 256 MiB WARNING: Caches not enabled No daughter card present NAND: HW ECC Hamming Code selected No NAND device found!!! 0 MiB MMC: OMAP SD/MMC: 0, OMAP SD/MMC: 1 *** Warning - readenv() failed, using default environment Net: cpsw Hit any key to stop autoboot: 0 U-Boot#
U-Boot help
You are now in the u-boot boot loader. Try help
U-Boot# help ? - alias for 'help' askenv - get environment variables from stdin base - print or set address offset bdinfo - print Board Info structure boot - boot default, i.e., run 'bootcmd' bootd - boot default, i.e., run 'bootcmd' bootm - boot application image from memory bootp - boot image via network using BOOTP/TFTP protocol cmp - memory compare coninfo - print console devices and information cp - memory copy crc32 - checksum calculation dcache - enable or disable data cache dhcp - boot image via network using DHCP/TFTP protocol echo - echo args to console editenv - edit environment variable env - environment handling commands exit - exit script ext2load- load binary file from a Ext2 filesystem ext2ls - list files in a directory (default /) ext4load- load binary file from a Ext2 filesystem ext4ls - list files in a directory (default /) false - do nothing, unsuccessfully fatinfo - print information about filesystem fatload - load binary file from a dos filesystem fatls - list files in a directory (default /) go - start application at address 'addr' help - print command description/usage i2c - I2C sub-system icache - enable or disable instruction cache iminfo - print header information for application image imxtract- extract a part of a multi-image itest - return true/false on integer compare loadb - load binary file over serial line (kermit mode) loads - load S-Record file over serial line loady - load binary file over serial line (ymodem mode) loop - infinite loop on address range md - memory display mm - memory modify (auto-incrementing address) mmc - MMC sub system mmcinfo - display MMC info mtest - simple RAM read/write test mw - memory write (fill) nand - NAND sub-system nandecc - Switch NAND ECC calculation algorithm b/w hardware and software nboot - boot from NAND device nfs - boot image via network using NFS protocol nm - memory modify (constant address) ping - send ICMP ECHO_REQUEST to network host printenv- print environment variables reset - Perform RESET of the CPU run - run commands in an environment variable saveenv - save environment variables to persistent storage setenv - set environment variables sf - SPI flash sub-system showvar - print local hushshell variables sleep - delay execution for some time source - run script from memory test - minimal test like /bin/sh tftpboot- boot image via network using TFTP protocol true - do nothing, successfully version - print monitor, compiler and linker version
Discovering what boot does
Your goal is to discover what happens when the boot command is run.
U-Boot# help boot
boot - boot default, i.e., run 'bootcmd'
U-boot# print bootcmd
bootcmd=if mmc rescan; then echo SD/MMC found on device ${mmc_dev};
if run loadbootenv; then echo Loaded environment from ${bootenv};run importbootenv;fi;
if test -n $uenvcmd; then echo Running uenvcmd ...;run uenvcmd;fi;
if run mmc_load_uimage_ext4; then run mmc_args;bootm ${kloadaddr};fi;fi;run nand_boot;
Figure out what's happening above by 'pretty printing' bootcmd. That is, properly format the code so you can follow the flow.
if mmc rescan; then
echo SD/MMC found on device ${mmc_dev}; Find out what mmc_dev is and fill it in
if run loadbootenv; then
echo Loaded environment from ${bootenv};
run importbootenv;
fi;
... and so on.
U-Boot print
It might be helpful to print the contents of all the variables and save it for future reference.
U-boot# print
autoload=yes
baudrate=115200
bootargs_defaults=setenv bootargs console=${console} ${optargs}
bootcmd=if mmc rescan; then echo SD/MMC found on device ${mmc_dev};if run loadbootenv; then echo Loaded
environment from ${bootenv};run importbootenv;fi;if test -n $uenvcmd; then echo Running uenvcmd ...;run
uenvcmd;fi;if run mmc_load_uimage_ext4; then run mmc_args;bootm ${kloadaddr};fi;fi;run nand_boot;
bootdelay=1
bootenv=uEnv.txt
bootfile=uImage
console=ttyO0,115200n8
ethact=cpsw
ethaddr=d4:94:a1:39:ed:0c
importbootenv=echo Importing environment from mmc ...; env import -t $loadaddr $filesize
ip_method=none
kloadaddr=0x80007fc0
loadaddr=0x82000000
loadbootenv=fatload mmc ${mmc_dev} ${loadaddr} ${bootenv}
mmc_args=run bootargs_defaults;setenv bootargs ${bootargs} root=${mmc_root} rootfstype=${mmc_root_fs_type}
ip=${ip_method}
mmc_boot=run mmc_args; run mmc_load_uimage_ext4; bootm ${kloadaddr}
mmc_dev=0
mmc_load_uimage=fatload mmc ${mmc_dev}:1 ${kloadaddr} ${bootfile}
mmc_load_uimage_ext4=ext4load mmc ${mmc_dev}:2 ${kloadaddr} /boot/${bootfile}
mmc_root=/dev/mmcblk0p2 ro
mmc_root_fs_type=ext4 rootwait
nand_args=run bootargs_defaults;setenv bootargs ${bootargs} root=${nand_root} noinitrd
rootfstype=${nand_root_fs_type} ip=${ip_method}
nand_boot=echo Booting from nand ...; run nand_args; nandecc hw 2; nand read.i ${kloadaddr}
${nand_src_addr} ${nand_img_siz}; bootm ${kloadaddr}
nand_img_siz=0x500000
nand_root=ubi0:rootfs rw ubi.mtd=7,2048
nand_root_fs_type=ubifs rootwait=1
nand_src_addr=0x280000
net_args=run bootargs_defaults;setenv bootargs ${bootargs} root=/dev/nfs
nfsroot=${serverip}:${rootpath},${nfsopts} rw ip=dhcp
net_boot=echo Booting from network ...; setenv autoload no; dhcp; tftp ${kloadaddr} ${bootfile}; run
net_args; bootm ${kloadaddr}
nfsopts=nolock
nor_args=run bootargs_defaults;setenv bootargs ${bootargs} root={nor_root} rootfstype=${nor_root_fs_type}
ip=${ip_method}
nor_boot=echo Booting from NOR ...; run nor_args; cp.b ${0x08080000} ${kloadaddr} ${nor_img_siz}; bootm
${kloadaddr}
nor_img_siz=0x280000
nor_root=/dev/mtdblock3 rw
nor_root_fs_type=jffs2
nor_src_addr=0x08080000
rootpath=/export/rootfs
script_addr=0x81900000
spi_args=run bootargs_defaults;setenv bootargs ${bootargs} root=${spi_root} rootfstype=${spi_root_fs_type}
ip=${ip_method}
spi_boot=echo Booting from spi ...; run spi_args; sf probe ${spi_bus_no}:0; sf read ${kloadaddr}
${spi_src_addr} ${spi_img_siz}; bootm ${kloadaddr}
spi_bus_no=0
spi_img_siz=0x380000
spi_root=/dev/mtdblock4 rw
spi_root_fs_type=jffs2
spi_src_addr=0x62000
static_ip=${ipaddr}:${serverip}:${gatewayip}:${netmask}:${hostname}::off
stderr=serial
stdin=serial
stdout=serial
Based on your investigation above, predict what will happen at boot time, then boot and check yourself.
U-boot boot
U-Boot# boot
SD/MMC found on device 0
reading uEnv.txt
33 bytes read
Loaded environment from uEnv.txt
Importing environment from mmc ...
Loading file "/boot/uImage" from mmc device 0:2 xxa2
3319440 bytes read
## Booting kernel from Legacy Image at 80007fc0 ...
Image Name: Angstrom/3.2.25/beaglebone
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 3319376 Bytes = 3.2 MiB
Load Address: 80008000
Entry Point: 80008000
Verifying Checksum ... OK
XIP Kernel Image ... OK
OK
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
systemd-fsck[56]: Angstrom-Cloud9: clean, 51868/218592 files, 304346/873534 blocks
.---O---.
| | .-. o o
| | |-----.-----.-----.| | .----..-----.-----.
| | | __ | ---'| '--.| .-'| | |
| | | | | |--- || --'| | | ' | | | |
'---'---'--'--'--. |-----------' '-----'-'-'-'
-' |
'---'
The Angstrom Distribution beaglebone ttyO0
Angstrom v2012.05 - Kernel 3.2.25
beaglebone login: root
Looking on the FAT partition
The SD card you have been using has 2 partitions on it. Partition 1 is a small FAT partition. These are the files you see appear when you first boot the bone. This contains the boot loaders.
The second partition is ext4 and contains the root file system. These are all the other files.
You can see the FAT partition with:
beagle$ mkdir /media/fat beagle$ ls /media/fat # Hmm.... nothing there beagle$ mount /dev/mmcblk0p1 /media/fat beagle$ ls /media/fat # and now there is MLO U-BOOT.BIN UIMAGE uEnv uEnv.txt
- MLO
- This is the x-loader that the ROM-based loader loads.
- u-boot.bin
- This is is U-Boot!
- uImage
- The kernel
- uEnv.txt
- Kernel command line arguments
- uEnv
- a directory of uEnv.txt files for various screen sizes
Look in uEnv.txt
beagle$ cat uEnv.txt optargs=run_hardware_tests quiet
These are the arguments that are passed to the kernel when it boots. Try editing the file and removing quiet, then reboot. You should see many more boot messages.
Embedded Linux Class by Mark A. Yoder
