bootflow command

Synopis

bootflow scan [-abelGH] [bootdev]
bootflow list [-e]
bootflow select [<num|name>]
bootflow info [-ds]
bootflow read
bootflow boot
bootflow cmdline [set|get|clear|delete|auto] <param> [<value>]
bootfloe menu [-t]

Description

The bootflow command is used to manage bootflows. It can scan bootdevs to locate bootflows, list them and boot them.

See U-Boot Standard Boot for more information.

Note that CONFIG_BOOTSTD_FULL (which enables CONFIG_CMD_BOOTFLOW_FULL) must be enabled to obtain full functionality with this command. Otherwise, it only supports `bootflow scan which scans and boots the first available bootflow.

bootflow scan

Scans for available bootflows, optionally booting the first valid one it finds. This operates in two modes:

  • If no bootdev is selected (see bootdev select) it scans bootflows one by one, extracting all the bootdevs from each

  • If a bootdev is selected, it just scans that one bootflow

Flags are:

-a

Collect all bootflows, even those that cannot be loaded. Normally if a file is not where it is expected, then the bootflow fails and so is dropped during the scan. With this option you can see why each bootflow would be dropped.

-b

Boot each valid bootflow as it is scanned. Typically only the first bootflow matters, since by then the system boots in the OS and U-Boot is no-longer running. bootflow scan -b is a quick way to boot the first available OS. A valid bootflow is one that made it all the way to the loaded state. Note that if -m is provided as well, booting is delayed until the user selects a bootflow.

-e

Used with -l to also show errors for each bootflow. The shows detailed error information for each bootflow that failed to make it to the loaded state.

-l

List bootflows while scanning. This is helpful when you want to see what is happening during scanning. Use it with the -b flag to see which bootdev and bootflows are being tried.

-G

Skip global bootmeths when scanning. By default these are tried first, but this flag disables them.

-H

Don’t use bootdev hunters. By default these are used before each boot priority or label is tried, to see if more bootdevs can be discovered, but this flag disables that process.

-m

Show a menu of available bootflows for the user to select. When used with -b it then boots the one that was selected, if any.

The optional argument specifies a particular bootdev to scan. This can either be the name of a bootdev or its sequence number (both shown with bootdev list). Alternatively a convenience label can be used, like mmc0, which is the type of device and an optional sequence number. Specifically, the label is the uclass of the bootdev’s parent followed by the sequence number of that parent. Sequence numbers are typically set by aliases, so if you have ‘mmc0’ in your devicetree alias section, then mmc0 refers to the bootdev attached to that device.

bootflow list

Lists the previously scanned bootflows. You must use bootflow scan before this to see anything.

If you scanned with -a and have bootflows with errors, -e can be used to show those errors.

The list looks something like this:

Seq

Method

State

Uclass

Part

Name

Filename

0

distro

ready

mmc

2

mmc@7e202000.bootdev.part_2

/boot/extlinux/extlinux.conf

1

pxe

ready

ethernet

0

smsc95xx_eth.bootdev.0

rpi.pxe/extlinux/extlinux.conf

The fields are as follows:

Seq:

Sequence number in the scan, used to reference the bootflow later

Method:

The boot method (bootmeth) used to find the bootflow. Several methods are included in U-Boot.

State:

Current state of the bootflow, indicating how far the bootdev got in obtaining a valid one. See Bootflow states for a list of states.

Uclass:

Name of the media device’s Uclass. This indicates the type of the parent device (e.g. MMC, Ethernet).

Part:

Partition number being accesseed, numbered from 1. Normally a device will have a partition table with a small number of partitions. For devices without partition tables (e.g. network) this field is 0.

Name:

Name of the bootflow. This is generated from the bootdev appended with the partition information

Filename:

Name of the bootflow file. This indicates where the file is on the filesystem or network device.

bootflow select

Use this to select a particular bootflow. You can select it by the sequence number or name, as shown in bootflow list.

Once a bootflow is selected, you can use bootflow info and bootflow boot.

If no bootflow name or number is provided, then any existing bootflow is unselected.

bootflow info

This shows information on the current bootflow, with the format looking like this:

Name

mmc@7e202000.bootdev.part_2

Device

mmc@7e202000.bootdev

Block dev

mmc@7e202000.blk

Type

distro

Method:

extlinux

State

ready

Partition

2

Subdir

(none)

Filename

/extlinux/extlinux.conf

Buffer

3db7ad48

Size

232 (562 bytes)

FDT:

<NULL>

Error

0

Most of the information is the same as bootflow list above. The new fields are:

Device

Name of the bootdev

Block dev

Name of the block device, if any. Network devices don’t have a block device.

Subdir

Subdirectory used for retrieving files. For network bootdevs this is the directory of the ‘bootfile’ parameter passed from DHCP. All file retrievals when booting are relative to this.

Buffer

Buffer containing the bootflow file. You can use the md command to look at it, or dump it with bootflow info -d.

Size

Size of the bootflow file

FDT:

Filename of the device tree, if supported. The EFI bootmeth uses this to remember the filename to load. If <NULL> then there is none.

Error

Error number returned from scanning for the bootflow. This is 0 if the bootflow is in the ‘loaded’ state, or a negative error value on error. You can look up Linux error codes to find the meaning of the number.

Use the -d flag to dump out the contents of the bootfile file.

The -s flag shows any x86 setup block, instead of the above.

bootflow read

This reads any files related to the bootflow. Some bootflows with large files avoid doing this when the bootflow is scanned, since it uses a lot of memory and takes extra time. The files are then automatically read when bootflow boot is used.

This command reads these files immediately. Typically this fills in the bootflow buf property, which can be used to examine the bootflow.

Note that reading the files does not result in any extra parsing, nor loading of images in the files. This is purely used to read in the data ready for booting, or examination.

bootflow boot

This boots the current bootflow, reading any required files first.

bootflow cmdline

Some bootmeths can obtain the OS command line since it is stored with the OS. In that case, you can use bootflow cmdline to adjust this. The command line is assumed to be in the format used by Linux, i.e. a space-separated set of parameters with optional values, e.g. “noinitrd console=/dev/tty0”.

To change or add a parameter, use:

bootflow cmdline set <param> <value>

To clear a parameter value to empty you can use “” for the value, or use:

bootflow cmdline clear <param>

To delete a parameter entirely, use:

bootflow cmdline delete <param>

Automatic parameters are available in a very few cases. You can use these to add parmeters where the value is known by U-Boot. For example:

bootflow cmdline auto earlycon
bootflow cmdline auto console

can be used to set the early console (or console) to a suitable value so that output appears on the serial port. This is only supported by the 16550 serial driver so far.

bootflow menu

This shows a menu with available bootflows. The user can select a particular bootflow, which then becomes the current one.

The -t flag requests a text menu. Otherwise, if a display is available, a graphical menu is shown.

Example

Here is an example of scanning for bootflows, then listing them:

U-Boot> bootflow scan -l
Scanning for bootflows in all bootdevs
Seq  Type         State   Uclass    Part  Name                      Filename
---  -----------  ------  --------  ----  ------------------------  ----------------
Scanning bootdev 'mmc@7e202000.bootdev':
  0  distro       ready   mmc          2  mmc@7e202000.bootdev.p    /extlinux/extlinux.conf
Scanning bootdev 'sdhci@7e300000.bootdev':
Card did not respond to voltage select! : -110
Scanning bootdev 'smsc95xx_eth.bootdev':
Waiting for Ethernet connection... done.
BOOTP broadcast 1
DHCP client bound to address 192.168.4.30 (4 ms)
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe/'.
Load address: 0x200000
Loading: *
TFTP error: 'Is a directory' (0)
Starting again

missing environment variable: pxeuuid
Retrieving file: rpi.pxe/pxelinux.cfg/01-b8-27-eb-a6-61-e1
Waiting for Ethernet connection... done.
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe/pxelinux.cfg/01-b8-27-eb-a6-61-e1'.
Load address: 0x2500000
Loading: ##################################################  566 Bytes
     45.9 KiB/s
done
Bytes transferred = 566 (236 hex)
  1  distro       ready   ethernet     0  smsc95xx_eth.bootdev.0 rpi.pxe/extlinux/extlinux.conf
No more bootdevs
---  -----------  ------  --------  ----  ------------------------  ----------------
(2 bootflows, 2 valid)
U-Boot> bootflow l
Showing all bootflows
Seq  Type         State   Uclass    Part  Name                      Filename
---  -----------  ------  --------  ----  ------------------------  ----------------
  0  distro       ready   mmc          2  mmc@7e202000.bootdev.p    /extlinux/extlinux.conf
  1  pxe          ready   ethernet     0  smsc95xx_eth.bootdev.0     rpi.pxe/extlinux/extlinux.conf
---  -----------  ------  --------  ----  ------------------------  ----------------
(2 bootflows, 2 valid)

The second one is then selected by name (we could instead use bootflow sel 0), displayed and booted:

U-Boot> bootflow info
No bootflow selected
U-Boot> bootflow sel mmc@7e202000.bootdev.part_2
U-Boot> bootflow info
Name:      mmc@7e202000.bootdev.part_2
Device:    mmc@7e202000.bootdev
Block dev: mmc@7e202000.blk
Method:    distro
State:     ready
Partition: 2
Subdir:    (none)
Filename:  extlinux/extlinux.conf
Buffer:    3db7ae88
Size:      232 (562 bytes)
OS:        Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
Cmdline:   (none)
Logo:      (none)
FDT:       <NULL>
Error:     0
U-Boot> bootflow boot
** Booting bootflow 'smsc95xx_eth.bootdev.0'
Ignoring unknown command: ui
Ignoring malformed menu command:  autoboot
Ignoring malformed menu command:  hidden
Ignoring unknown command: totaltimeout
1:  Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
Retrieving file: rpi.pxe/initramfs-5.3.7-301.fc31.armv7hl.img
get 2700000 rpi.pxe/initramfs-5.3.7-301.fc31.armv7hl.img
Waiting for Ethernet connection... done.
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe/initramfs-5.3.7-301.fc31.armv7hl.img'.
Load address: 0x2700000
Loading: ###################################T ###############  57.7 MiB
     1.9 MiB/s
done
Bytes transferred = 60498594 (39b22a2 hex)
Retrieving file: rpi.pxe//vmlinuz-5.3.7-301.fc31.armv7hl
get 80000 rpi.pxe//vmlinuz-5.3.7-301.fc31.armv7hl
Waiting for Ethernet connection... done.
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe//vmlinuz-5.3.7-301.fc31.armv7hl'.
Load address: 0x80000
Loading: ##################################################  7.2 MiB
     2.3 MiB/s
done
Bytes transferred = 7508480 (729200 hex)
append: ro root=UUID=9732b35b-4cd5-458b-9b91-80f7047e0b8a rhgb quiet LANG=en_US.UTF-8 cma=192MB cma=256MB
Retrieving file: rpi.pxe//dtb-5.3.7-301.fc31.armv7hl/bcm2837-rpi-3-b.dtb
get 2600000 rpi.pxe//dtb-5.3.7-301.fc31.armv7hl/bcm2837-rpi-3-b.dtb
Waiting for Ethernet connection... done.
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe//dtb-5.3.7-301.fc31.armv7hl/bcm2837-rpi-3-b.dtb'.
Load address: 0x2600000
Loading: ##################################################  13.8 KiB
     764.6 KiB/s
done
Bytes transferred = 14102 (3716 hex)
Kernel image @ 0x080000 [ 0x000000 - 0x729200 ]
## Flattened Device Tree blob at 02600000
   Booting using the fdt blob at 0x2600000
   Using Device Tree in place at 02600000, end 02606715

Starting kernel ...

[  OK  ] Started Show Plymouth Boot Screen.
[  OK  ] Started Forward Password R…s to Plymouth Directory Watch.
[  OK  ] Reached target Local Encrypted Volumes.
[  OK  ] Reached target Paths.
....

Here we scan for bootflows and boot the first one found:

U-Boot> bootflow scan -bl
Scanning for bootflows in all bootdevs
Seq  Method       State   Uclass    Part  Name                    Filename
---  -----------  ------  --------  ----  ----------------------  ----------------
Scanning bootdev 'mmc@7e202000.bootdev':
  0  distro       ready   mmc          2  mmc@7e202000.bootdev.p  /extlinux/extlinux.conf
** Booting bootflow 'mmc@7e202000.bootdev.part_2'
Ignoring unknown command: ui
Ignoring malformed menu command:  autoboot
Ignoring malformed menu command:  hidden
Ignoring unknown command: totaltimeout
1:  Fedora-KDE-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
Retrieving file: /initramfs-5.3.7-301.fc31.armv7hl.img
getfile 2700000 /initramfs-5.3.7-301.fc31.armv7hl.img
Retrieving file: /vmlinuz-5.3.7-301.fc31.armv7hl
getfile 80000 /vmlinuz-5.3.7-301.fc31.armv7hl
append: ro root=UUID=b8781f09-e2dd-4cb8-979b-7df5eeaaabea rhgb LANG=en_US.UTF-8 cma=192MB console=tty0 console=ttyS1,115200
Retrieving file: /dtb-5.3.7-301.fc31.armv7hl/bcm2837-rpi-3-b.dtb
getfile 2600000 /dtb-5.3.7-301.fc31.armv7hl/bcm2837-rpi-3-b.dtb
Kernel image @ 0x080000 [ 0x000000 - 0x729200 ]
## Flattened Device Tree blob at 02600000
   Booting using the fdt blob at 0x2600000
   Using Device Tree in place at 02600000, end 02606715

Starting kernel ...

[    0.000000] Booting Linux on physical CPU 0x0

Here is am example using the -e flag to see all errors:

U-Boot> bootflow scan -a
Card did not respond to voltage select! : -110
Waiting for Ethernet connection... done.
BOOTP broadcast 1
DHCP client bound to address 192.168.4.30 (4 ms)
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe/'.
Load address: 0x200000
Loading: *
TFTP error: 'Is a directory' (0)
Starting again

missing environment variable: pxeuuid
Retrieving file: rpi.pxe/pxelinux.cfg/01-b8-27-eb-a6-61-e1
Waiting for Ethernet connection... done.
Using smsc95xx_eth device
TFTP from server 192.168.4.1; our IP address is 192.168.4.30
Filename 'rpi.pxe/pxelinux.cfg/01-b8-27-eb-a6-61-e1'.
Load address: 0x2500000
Loading: ##################################################  566 Bytes
     49.8 KiB/s
done
Bytes transferred = 566 (236 hex)
U-Boot> bootflow l -e
Showing all bootflows
Seq  Type         State   Uclass    Part  Name                   Filename
---  -----------  ------  --------  ----  ---------------------  ----------------
  0  distro       fs      mmc          1  mmc@7e202000.bootdev.p /extlinux/extlinux.conf
     ** File not found, err=-2
  1  distro       ready   mmc          2  mmc@7e202000.bootdev.p /extlinux/extlinux.conf
  2  distro       fs      mmc          3  mmc@7e202000.bootdev.p /extlinux/extlinux.conf
     ** File not found, err=-1
  3  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  4  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  5  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  6  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  7  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  8  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  9  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  a  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  b  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  c  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  d  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  e  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
  f  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
 10  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
 11  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
 12  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
 13  distro       media   mmc          0  mmc@7e202000.bootdev.p <NULL>
     ** No partition found, err=-2
 14  distro       ready   ethernet     0  smsc95xx_eth.bootdev.0 rpi.pxe/extlinux/extlinux.conf
---  -----------  ------  --------  ----  ---------------------  ----------------
(21 bootflows, 2 valid)
U-Boot>

Here is an example of booting ChromeOS, adjusting the console beforehand. Note that the cmdline is word-wrapped here and some parts of the command line are elided:

=> bootfl list
Showing all bootflows
Seq  Method       State   Uclass    Part  Name                      Filename
---  -----------  ------  --------  ----  ------------------------  ----------------
0  cros         ready   nvme         0  5.10.153-20434-g98da1eb2c <NULL>
1  efi          ready   nvme         c  nvme#0.blk#1.bootdev.part efi/boot/bootia32.efi
2  efi          ready   usb_mass_    2  usb_mass_storage.lun0.boo efi/boot/bootia32.efi
---  -----------  ------  --------  ----  ------------------------  ----------------
(3 bootflows, 3 valid)
=> bootfl sel 0
=> bootfl inf
Name:      5.10.153-20434-g98da1eb2cf9d (chrome-bot@chromeos-release-builder-us-central1-b-x32-12-xijx) #1 SMP PREEMPT Tue Jan 24 19:38:23 PST 2023
Device:    nvme#0.blk#1.bootdev
Block dev: nvme#0.blk#1
Method:    cros
State:     ready
Partition: 0
Subdir:    (none)
Filename:  <NULL>
Buffer:    737a1400
Size:      c47000 (12873728 bytes)
OS:        ChromeOS
Cmdline:   console= loglevel=7 init=/sbin/init cros_secure drm.trace=0x106
    root=/dev/dm-0 rootwait ro dm_verity.error_behavior=3
    dm_verity.max_bios=-1 dm_verity.dev_wait=1
    dm="1 vroot none ro 1,0 6348800
      verity payload=PARTUUID=799c935b-ae62-d143-8493-816fa936eef7/PARTNROFF=1
      hashtree=PARTUUID=799c935b-ae62-d143-8493-816fa936eef7/PARTNROFF=1
      hashstart=6348800 alg=sha256
      root_hexdigest=78cc462cd45aecbcd49ca476587b4dee59aa1b00ba5ece58e2c29ec9acd914ab
      salt=8dec4dc80a75dd834a9b3175c674405e15b16a253fdfe05c79394ae5fd76f66a"
    noinitrd vt.global_cursor_default=0
    kern_guid=799c935b-ae62-d143-8493-816fa936eef7 add_efi_memmap
    noresume i915.modeset=1 ramoops.ecc=1 tpm_tis.force=0
    intel_pmc_core.warn_on_s0ix_failures=1 i915.enable_guc=3 i915.enable_dc=4
    xdomain=0 swiotlb=65536 intel_iommu=on i915.enable_psr=1
    usb-storage.quirks=13fe:6500:u
X86 setup: 742e3400
Logo:      (none)
FDT:       <NULL>
Error:     0
=> bootflow cmdline auto earlycon
=> bootflow cmd auto console
=> print bootargs
bootargs=console=ttyS0,115200n8 loglevel=7 ...
    usb-storage.quirks=13fe:6500:u earlycon=uart8250,mmio32,0xfe03e000,115200n8
=> bootflow cmd del console
=> print bootargs
bootargs=loglevel=7 ... earlycon=uart8250,mmio32,0xfe03e000,115200n8
=> bootfl boot
** Booting bootflow '5.10.153-20434-g98da1eb2cf9d (chrome-bot@chromeos-release-builder-us-central1-b-x32-12-xijx) #1 SMP PREEMPT Tue Jan 24 19:38:23 PST 2023' with cros
Kernel command line: "loglevel=7 ... earlycon=uart8250,mmio32,0xfe03e000,115200n8"

Starting kernel ...

[    0.000000] Linux version 5.10.153-20434-g98da1eb2cf9d (chrome-bot@chromeos-release-builder-us-central1-b-x32-12-xijx) (Chromium OS 15.0_pre465103_p20220825-r4 clang version 15.0.0 (/var/tmp/portage/sys-devel/llvm-15.0_pre465103_p20220825-r4/work/llvm-15.0_pre465103_p20220825/clang db1978b67431ca3462ad8935bf662c15750b8252), LLD 15.0.0) #1 SMP PREEMPT Tue Jan 24 19:38:23 PST 2023
[    0.000000] Command line: loglevel=7 ... usb-storage.quirks=13fe:6500:u earlycon=uart8250,mmio32,0xfe03e000,115200n8
[    0.000000] x86/split lock detection: warning about user-space split_locks

This shows looking at x86 setup information:

=> bootfl sel 0
=> bootfl i -s
Setup located at 77b56010:

ACPI RSDP addr      : 0
E820: 2 entries
        Addr        Size  Type
           0        1000  RAM
    fffff000        1000  Reserved
Setup sectors       : 1e
Root flags          : 1
Sys size            : 63420
RAM size            : 0
Video mode          : ffff
Root dev            : 0
Boot flag           : 0
Jump                : 66eb
Header              : 53726448
                      Kernel V2
Version             : 20d
Real mode switch    : 0
Start sys seg       : 1000
Kernel version      : 38cc
   @00003acc:
Type of loader      : ff
                      unknown
Load flags          : 1
                    : loaded-high
Setup move size     : 8000
Code32 start        : 100000
Ramdisk image       : 0
Ramdisk size        : 0
Bootsect kludge     : 0
Heap end ptr        : 5160
Ext loader ver      : 0
Ext loader type     : 0
Command line ptr    : 735000
Initrd addr max     : 7fffffff
Kernel alignment    : 200000
Relocatable kernel  : 1
Min alignment       : 15
                    : 200000
Xload flags         : 3
                    : 64-bit-entry can-load-above-4gb
Cmdline size        : 7ff
Hardware subarch    : 0
HW subarch data     : 0
Payload offset      : 26e
Payload length      : 612045
Setup data          : 0
Pref address        : 1000000
Init size           : 1383000
Handover offset     : 0

This shows reading a bootflow to examine the kernel:

=> bootfl i 0
Name:
Device:    emmc@1c,0.bootdev
Block dev: emmc@1c,0.blk
Method:    cros
State:     ready
Partition: 2
Subdir:    (none)
Filename:  <NULL>
Buffer:    0
Size:      63ee00 (6548992 bytes)
OS:        ChromeOS
Cmdline:   console= loglevel=7 init=/sbin/init cros_secure oops=panic panic=-1 root=PARTUUID=35c775e7-3735-d745-93e5-d9e0238f7ed0/PARTNROFF=1 rootwait rw dm_verity.error_behavior=3 dm_verity.max_bios=-1 dm_verity.dev_wait=0 dm="1 vroot none rw 1,0 3788800 verity payload=ROOT_DEV hashtree=HASH_DEV hashstart=3788800 alg=sha1 root_hexdigest=55052b629d3ac889f25a9583ea12cdcd3ea15ff8 salt=a2d4d9e574069f4fed5e3961b99054b7a4905414b60a25d89974a7334021165c" noinitrd vt.global_cursor_default=0 kern_guid=35c775e7-3735-d745-93e5-d9e0238f7ed0 add_efi_memmap boot=local noresume noswap i915.modeset=1 tpm_tis.force=1 tpm_tis.interrupts=0 nmi_watchdog=panic,lapic disablevmx=off
X86 setup: 77b56010
Logo:      (none)
FDT:       <NULL>
Error:     0

Note that Buffer is 0 so it has not be read yet. Using bootflow read:

=> bootfl read
=> bootfl info
Name:
Device:    emmc@1c,0.bootdev
Block dev: emmc@1c,0.blk
Method:    cros
State:     ready
Partition: 2
Subdir:    (none)
Filename:  <NULL>
Buffer:    77b7e400
Size:      63ee00 (6548992 bytes)
OS:        ChromeOS
Cmdline:   console= loglevel=7 init=/sbin/init cros_secure oops=panic panic=-1 root=PARTUUID=35c775e7-3735-d745-93e5-d9e0238f7ed0/PARTNROFF=1 rootwait rw dm_verity.error_behavior=3 dm_verity.max_bios=-1 dm_verity.dev_wait=0 dm="1 vroot none rw 1,0 3788800 verity payload=ROOT_DEV hashtree=HASH_DEV hashstart=3788800 alg=sha1 root_hexdigest=55052b629d3ac889f25a9583ea12cdcd3ea15ff8 salt=a2d4d9e574069f4fed5e3961b99054b7a4905414b60a25d89974a7334021165c" noinitrd vt.global_cursor_default=0 kern_guid=35c775e7-3735-d745-93e5-d9e0238f7ed0 add_efi_memmap boot=local noresume noswap i915.modeset=1 tpm_tis.force=1 tpm_tis.interrupts=0 nmi_watchdog=panic,lapic disablevmx=off
X86 setup: 781b4400
Logo:      (none)
FDT:       <NULL>
Error:     0

Now the buffer can be accessed:

=> md 77b7e400
77b7e400: 1186f6fc 40000002 b8fa0c75 00000018  .......@u.......
77b7e410: c08ed88e a68dd08e 000001e8 000000e8  ................
77b7e420: ed815d00 00000021 62c280b8 89e80100  .]..!......b....
77b7e430: 22f7e8c4 c0850061 22ec850f eb890061  ..."a......"a...
77b7e440: 0230868b 01480000 21d0f7c3 00fb81c3  ..0...H....!....
77b7e450: 7d010000 0000bb05 c3810100 00d4f000  ...}............
77b7e460: 8130858d 85890061 00618132 3095010f  ..0.a...2.a....0
77b7e470: 0f006181 c883e020 e0220f20 e000bb8d  .a.. ... .".....
77b7e480: c0310062 001800b9 8dabf300 62e000bb  b.1............b
77b7e490: 07878d00 89000010 00bb8d07 8d0062f0  .............b..
77b7e4a0: 00100787 0004b900 07890000 00100005  ................
77b7e4b0: 08c78300 8df37549 630000bb 0183b800  ....Iu.....c....
77b7e4c0: 00b90000 89000008 00000507 c7830020  ............ ...
77b7e4d0: f3754908 e000838d 220f0062 0080b9d8  .Iu.....b.."....
77b7e4e0: 320fc000 08e8ba0f c031300f b8d0000f  ...2.....01.....
77b7e4f0: 00000020 6ad8000f 00858d10 50000002   ......j.......P

This shows using a text menu to boot an OS:

=> bootflow scan
=> bootfl list
=> bootfl menu -t
U-Boot    :    Boot Menu

UP and DOWN to choose, ENTER to select

  >    0  mmc1        mmc1.bootdev.whole
       1  mmc1        Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
       2  mmc1        mmc1.bootdev.part_1
       3  mmc4        mmc4.bootdev.whole
       4  mmc4        Armbian
       5  mmc4        mmc4.bootdev.part_1
       6  mmc5        mmc5.bootdev.whole
       7  mmc5        ChromeOS
       8  mmc5        ChromeOS
U-Boot    :    Boot Menu

UP and DOWN to choose, ENTER to select

       0  mmc1        mmc1.bootdev.whole
  >    1  mmc1        Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
       2  mmc1        mmc1.bootdev.part_1
       3  mmc4        mmc4.bootdev.whole
       4  mmc4        Armbian
       5  mmc4        mmc4.bootdev.part_1
       6  mmc5        mmc5.bootdev.whole
       7  mmc5        ChromeOS
       8  mmc5        ChromeOS
U-Boot    :    Boot Menu

Selected: Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
=> bootfl boot
** Booting bootflow 'mmc1.bootdev.part_1' with extlinux
Ignoring unknown command: ui
Ignoring malformed menu command:  autoboot
Ignoring malformed menu command:  hidden
Ignoring unknown command: totaltimeout
Fedora-Workstation-armhfp-31-1.9 Boot Options.
1:  Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
Enter choice: 1
1:  Fedora-Workstation-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
Retrieving file: /vmlinuz-5.3.7-301.fc31.armv7hl
Retrieving file: /initramfs-5.3.7-301.fc31.armv7hl.img
append: ro root=UUID=9732b35b-4cd5-458b-9b91-80f7047e0b8a rhgb quiet LANG=en_US.UTF-8 cma=192MB cma=256MB
Retrieving file: /dtb-5.3.7-301.fc31.armv7hl/sandbox.dtb
...

Return value

On success bootflow boot normally boots into the Operating System and does not return to U-Boot. If something about the U-Boot processing fails, then the return value $? is 1. If the boot succeeds but for some reason the Operating System returns, then $? is 0, indicating success.

For bootflow menu the return value is $? is 0 (true) if an option was choses, else 1.

For other subcommands, the return value $? is always 0 (true).