doc/arch/x86/manual_boot.rst

Booting Ubuntu Manually
-----------------------

This shows a manual approach to booting Ubuntu without standard boot or the EFI
interface.

As an example of how to set up your boot flow with U-Boot, here are
instructions for starting Ubuntu from U-Boot. These instructions have been
tested on Minnowboard MAX with a SATA drive but are equally applicable on
other platforms and other media. There are really only four steps and it's a
very simple script, but a more detailed explanation is provided here for
completeness.

Note: It is possible to set up U-Boot to boot automatically using syslinux.
It could also use the grub.cfg file (/efi/ubuntu/grub.cfg) to obtain the
GUID. If you figure these out, please post patches to this README.

Firstly, you will need Ubuntu installed on an available disk. It should be
possible to make U-Boot start a USB start-up disk but for now let's assume
that you used another boot loader to install Ubuntu.

Use the U-Boot command line to find the UUID of the partition you want to
boot. For example our disk is SCSI device 0::

   => part list scsi 0

   Partition Map for SCSI device 0  --   Partition Type: EFI

      Part	Start LBA	End LBA		Name
        Attributes
        Type GUID
        Partition GUID
      1	0x00000800	0x001007ff	""
        attrs:	0x0000000000000000
        type:	c12a7328-f81f-11d2-ba4b-00a0c93ec93b
        guid:	9d02e8e4-4d59-408f-a9b0-fd497bc9291c
      2	0x00100800	0x037d8fff	""
        attrs:	0x0000000000000000
        type:	0fc63daf-8483-4772-8e79-3d69d8477de4
        guid:	965c59ee-1822-4326-90d2-b02446050059
      3	0x037d9000	0x03ba27ff	""
        attrs:	0x0000000000000000
        type:	0657fd6d-a4ab-43c4-84e5-0933c84b4f4f
        guid:	2c4282bd-1e82-4bcf-a5ff-51dedbf39f17
      =>

This shows that your SCSI disk has three partitions. The really long hex
strings are called Globally Unique Identifiers (GUIDs). You can look up the
'type' ones `here`_. On this disk the first partition is for EFI and is in
VFAT format (DOS/Windows)::

   => fatls scsi 0:1
               efi/

   0 file(s), 1 dir(s)


Partition 2 is 'Linux filesystem data' so that will be our root disk. It is
in ext2 format::

   => ext2ls scsi 0:2
   <DIR>       4096 .
   <DIR>       4096 ..
   <DIR>      16384 lost+found
   <DIR>       4096 boot
   <DIR>      12288 etc
   <DIR>       4096 media
   <DIR>       4096 bin
   <DIR>       4096 dev
   <DIR>       4096 home
   <DIR>       4096 lib
   <DIR>       4096 lib64
   <DIR>       4096 mnt
   <DIR>       4096 opt
   <DIR>       4096 proc
   <DIR>       4096 root
   <DIR>       4096 run
   <DIR>      12288 sbin
   <DIR>       4096 srv
   <DIR>       4096 sys
   <DIR>       4096 tmp
   <DIR>       4096 usr
   <DIR>       4096 var
   <SYM>         33 initrd.img
   <SYM>         30 vmlinuz
   <DIR>       4096 cdrom
   <SYM>         33 initrd.img.old
   =>

and if you look in the /boot directory you will see the kernel::

   => ext2ls scsi 0:2 /boot
   <DIR>       4096 .
   <DIR>       4096 ..
   <DIR>       4096 efi
   <DIR>       4096 grub
            3381262 System.map-3.13.0-32-generic
            1162712 abi-3.13.0-32-generic
             165611 config-3.13.0-32-generic
             176500 memtest86+.bin
             178176 memtest86+.elf
             178680 memtest86+_multiboot.bin
            5798112 vmlinuz-3.13.0-32-generic
             165762 config-3.13.0-58-generic
            1165129 abi-3.13.0-58-generic
            5823136 vmlinuz-3.13.0-58-generic
           19215259 initrd.img-3.13.0-58-generic
            3391763 System.map-3.13.0-58-generic
            5825048 vmlinuz-3.13.0-58-generic.efi.signed
           28304443 initrd.img-3.13.0-32-generic
   =>

The 'vmlinuz' files contain a packaged Linux kernel. The format is a kind of
self-extracting compressed file mixed with some 'setup' configuration data.
Despite its size (uncompressed it is >10MB) this only includes a basic set of
device drivers, enough to boot on most hardware types.

The 'initrd' files contain a RAM disk. This is something that can be loaded
into RAM and will appear to Linux like a disk. Ubuntu uses this to hold lots
of drivers for whatever hardware you might have. It is loaded before the
real root disk is accessed.

The numbers after the end of each file are the version. Here it is Linux
version 3.13. You can find the source code for this in the Linux tree with
the tag v3.13. The '.0' allows for additional Linux releases to fix problems,
but normally this is not needed. The '-58' is used by Ubuntu. Each time they
release a new kernel they increment this number. New Ubuntu versions might
include kernel patches to fix reported bugs. Stable kernels can exist for
some years so this number can get quite high.

The '.efi.signed' kernel is signed for EFI's secure boot. U-Boot has its own
secure boot mechanism - see `this`_ & `that`_. It cannot read .efi files
at present.

To boot Ubuntu from U-Boot the steps are as follows:

1. Set up the boot arguments. Use the GUID for the partition you want to boot::

   => setenv bootargs root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro

Here root= tells Linux the location of its root disk. The disk is specified
by its GUID, using '/dev/disk/by-partuuid/', a Linux path to a 'directory'
containing all the GUIDs Linux has found. When it starts up, there will be a
file in that directory with this name in it. It is also possible to use a
device name here, see later.

2. Load the kernel. Since it is an ext2/4 filesystem we can do::

   => ext2load scsi 0:2 03000000 /boot/vmlinuz-3.13.0-58-generic

The address 30000000 is arbitrary, but there seem to be problems with using
small addresses (sometimes Linux cannot find the ramdisk). This is 48MB into
the start of RAM (which is at 0 on x86).

3. Load the ramdisk (to 64MB)::

   => ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic

4. Start up the kernel. We need to know the size of the ramdisk, but can use
   a variable for that. U-Boot sets 'filesize' to the size of the last file it
   loaded::

   => zboot 03000000 0 04000000 ${filesize}

Type 'help zboot' if you want to see what the arguments are. U-Boot on x86 is
quite verbose when it boots a kernel. You should see these messages from
U-Boot::

   Valid Boot Flag
   Setup Size = 0x00004400
   Magic signature found
   Using boot protocol version 2.0c
   Linux kernel version 3.13.0-58-generic (buildd@allspice) #97-Ubuntu SMP Wed Jul 8 02:56:15 UTC 2015
   Building boot_params at 0x00090000
   Loading bzImage at address 100000 (5805728 bytes)
   Magic signature found
   Initial RAM disk at linear address 0x04000000, size 19215259 bytes
   Kernel command line: "root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro"

   Starting kernel ...

U-Boot prints out some bootstage timing. This is more useful if you put the
above commands into a script since then it will be faster::

   Timer summary in microseconds:
          Mark    Elapsed  Stage
             0          0  reset
       241,535    241,535  board_init_r
     2,421,611  2,180,076  id=64
     2,421,790        179  id=65
     2,428,215      6,425  main_loop
    48,860,584 46,432,369  start_kernel

   Accumulated time:
                  240,329  ahci
                1,422,704  vesa display

Now the kernel actually starts (if you want to examine kernel boot up message on
the serial console, append "console=ttyS0,115200" to the kernel command line)::

   [    0.000000] Initializing cgroup subsys cpuset
   [    0.000000] Initializing cgroup subsys cpu
   [    0.000000] Initializing cgroup subsys cpuacct
   [    0.000000] Linux version 3.13.0-58-generic (buildd@allspice) (gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) ) #97-Ubuntu SMP Wed Jul 8 02:56:15 UTC 2015 (Ubuntu 3.13.0-58.97-generic 3.13.11-ckt22)
   [    0.000000] Command line: root=/dev/disk/by-partuuid/965c59ee-1822-4326-90d2-b02446050059 ro console=ttyS0,115200

It continues for a long time. Along the way you will see it pick up your
ramdisk::

   [    0.000000] RAMDISK: [mem 0x04000000-0x05253fff]
   ...
   [    0.788540] Trying to unpack rootfs image as initramfs...
   [    1.540111] Freeing initrd memory: 18768K (ffff880004000000 - ffff880005254000)
   ...

Later it actually starts using it::

   Begin: Running /scripts/local-premount ... done.

You should also see your boot disk turn up::

   [    4.357243] scsi 1:0:0:0: Direct-Access     ATA      ADATA SP310      5.2  PQ: 0 ANSI: 5
   [    4.366860] sd 1:0:0:0: [sda] 62533296 512-byte logical blocks: (32.0 GB/29.8 GiB)
   [    4.375677] sd 1:0:0:0: Attached scsi generic sg0 type 0
   [    4.381859] sd 1:0:0:0: [sda] Write Protect is off
   [    4.387452] sd 1:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
   [    4.399535]  sda: sda1 sda2 sda3

Linux has found the three partitions (sda1-3). Mercifully it doesn't print out
the GUIDs. In step 1 above we could have used::

   setenv bootargs root=/dev/sda2 ro

instead of the GUID. However if you add another drive to your board the
numbering may change whereas the GUIDs will not. So if your boot partition
becomes sdb2, it will still boot. For embedded systems where you just want to
boot the first disk, you have that option.

The last thing you will see on the console is mention of plymouth (which
displays the Ubuntu start-up screen) and a lot of 'Starting' messages::

   * Starting Mount filesystems on boot                                   [ OK ]

After a pause you should see a login screen on your display and you are done.

If you want to put this in a script you can use something like this::

   setenv bootargs root=UUID=b2aaf743-0418-4d90-94cc-3e6108d7d968 ro
   setenv boot zboot 03000000 0 04000000 \${filesize}
   setenv bootcmd "ext2load scsi 0:2 03000000 /boot/vmlinuz-3.13.0-58-generic; ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic; run boot"
   saveenv

The \ is to tell the shell not to evaluate ${filesize} as part of the setenv
command.

You can also bake this behaviour into your build by hard-coding the
environment variables if you add this to minnowmax.h:

.. code-block:: c

	#undef CONFIG_BOOTCOMMAND
	#define CONFIG_BOOTCOMMAND	\
		"ext2load scsi 0:2 03000000 /boot/vmlinuz-3.13.0-58-generic; " \
		"ext2load scsi 0:2 04000000 /boot/initrd.img-3.13.0-58-generic; " \
		"run boot"

	#undef CFG_EXTRA_ENV_SETTINGS
	#define CFG_EXTRA_ENV_SETTINGS "boot=zboot 03000000 0 04000000 ${filesize}"

and change CONFIG_BOOTARGS value in configs/minnowmax_defconfig to::

   CONFIG_BOOTARGS="root=/dev/sda2 ro"

.. _here: https://en.wikipedia.org/wiki/GUID_Partition_Table
.. _this: http://events.linuxfoundation.org/sites/events/files/slides/chromeos_and_diy_vboot_0.pdf
.. _that: http://events.linuxfoundation.org/sites/events/files/slides/elce-2014.pdf