Difference between revisions of "Fallback mechanism"

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(some options moved to the `Chipset` menu)
 
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* One normal/ image: The image to be tested.
 
* One normal/ image: The image to be tested.
  
This feature is not widely tested on all boards. It also requires it to have a reboot_bits exported in the CMOS layout.
+
This feature is not widely tested on all boards. It also requires it to have a reboot_counter exported in the CMOS layout.
  
 
This also doesn't protect against human errors when using such feature, or bugs in the code responsible for switching between the two images.
 
This also doesn't protect against human errors when using such feature, or bugs in the code responsible for switching between the two images.
Line 30: Line 30:
 
* Then the user bricked her device and has to reflash it externally.
 
* Then the user bricked her device and has to reflash it externally.
  
== Building ==
+
== Fallback build ==
This scrpit takes an existing coreboot image path as argument.
+
To configure it for fallback, do:
* [[Fallback mechanism/fallback.sh|fallback.sh]] script to build the fallback image
+
$ make menuconfig
* [[Fallback mechanism/normal.sh|normal.sh]] script to build the normal (test) image
+
Then in "General setup  --->", near the top use "fallback" in "CBFS prefix to use":
 +
(fallback) CBFS prefix to use
 +
Then near the bottom, make sure to have:
 +
[ ] Update existing coreboot.rom image
 +
And in the "Chipset  --->" menu at the bottom:
 +
Bootblock behaviour (Switch to normal if CMOS says so)  --->
 +
[*] Do not clear reboot count after successful boot
 +
 
 +
You can then build the fallback image with the [[Fallback mechanism/fallback.sh|fallback.sh]] script.
 +
 
 +
== Normal build ==
 +
To configure it for normal, do:
 +
$ make menuconfig
 +
Then in "General setup  --->", near the top use "normal" in "CBFS prefix to use":
 +
(normal) CBFS prefix to use
 +
Then near the bottom, make sure to have:
 +
[*] Update existing coreboot.rom image
 +
And in the "Chipset  --->" menu at the bottom:
 +
Bootblock behaviour (Switch to normal if CMOS says so)  --->
 +
[*] Do not clear reboot count after successful boot
 +
 
 +
You can then build with the normal part with the [[Fallback mechanism/normal.sh|normal.sh]] script. It takes an existing coreboot image as argument.
  
 
== OS configuration ==
 
== OS configuration ==
  
 
=== The manual way ===
 
=== The manual way ===
The most simple way to do it is to run the nvramtool commands below manually.
+
An approach is to run switch-to-normal.sh before trying an image.
 +
It's however more error prone than the systemd approach because:
 +
* you have to do it manually, each time, before testing an image.
 +
* If you then want to use that new image, you have to flash it, again, to fallback.
  
set-normal-0.sh has to be run:
+
==== switch-to-normal.sh ====
* After the boot is completed and is declared a success.
+
#!/bin/sh
* After the resuming is completed.
+
nvramtool -w boot_option=Normal
 +
nvramtool -w reboot_counter=0
  
===== set-fallback-1.sh =====
+
==== switch-to-fallback.sh ====
 
  #!/bin/sh
 
  #!/bin/sh
 
  nvramtool -w boot_option=Fallback
 
  nvramtool -w boot_option=Fallback
  nvramtool -w reboot_bits=1
+
  nvramtool -w reboot_counter=15
===== set-normal-0.sh =====
 
#!/bin/sh
 
nvramtool -w boot_option=Normal
 
nvramtool -w reboot_bits=0
 
  
===== get-nvram.sh =====
+
(Assuming that 15 is the maximum that can be stored in reboot_counter.)
#!/bin/sh
 
nvramtool -a | grep -e boot_option -e last_boot -e reboot_bits
 
  
 
=== Systemd ===
 
=== Systemd ===
Line 62: Line 81:
 
We are then supposed to use the normal image daily and only resort to fallback in case of issues.
 
We are then supposed to use the normal image daily and only resort to fallback in case of issues.
  
 
+
To install it, first install nvramtool (from coreboot sources):
First, in coreboot, install nvramtool with:
 
 
  $ cd util/nvramtool
 
  $ cd util/nvramtool
 
  $ make
 
  $ make
 
  $ sudo make install
 
  $ sudo make install
  
Then add the following systemd unit at their respective paths:
+
Then add the following systemd units at their respective paths:
 
* [[Fallback_mechanism/coreboot@boot.service|/etc/systemd/system/coreboot@boot.service]]
 
* [[Fallback_mechanism/coreboot@boot.service|/etc/systemd/system/coreboot@boot.service]]
 
* [[Fallback_mechanism/coreboot@resume.service|/etc/systemd/system/coreboot@resume.service]]
 
* [[Fallback_mechanism/coreboot@resume.service|/etc/systemd/system/coreboot@resume.service]]
Line 79: Line 97:
  
 
== Current limitations ==
 
== Current limitations ==
 +
* '''Use of the same cmos.layout in fallback and normal !'''
 
* The user may wrongly identify which image booted, and because of that, end up reflashing a non-working image.
 
* The user may wrongly identify which image booted, and because of that, end up reflashing a non-working image.
 
* Some issues can arrise when the nvram layout is not the same between normal/ and fallback/
 
* Some issues can arrise when the nvram layout is not the same between normal/ and fallback/
Line 88: Line 107:
 
** Changing the path of what SeaBIOS loads from cbfs is probably configurable with SeaBIOS cbfs symlinks but not yet tested/documented with the use of the fallback mecanism
 
** Changing the path of what SeaBIOS loads from cbfs is probably configurable with SeaBIOS cbfs symlinks but not yet tested/documented with the use of the fallback mecanism
 
* Tested boards need to be listed somewhere.
 
* Tested boards need to be listed somewhere.
 +
 +
== Issues ==
 +
=== thinkpad_acpi ===
 +
This linux driver can have some bad interactions with the fallback/normal mecanism: when using it with the volume_control=1 option, volume_mode=1 is required, otherwise after shutting down the computer, it will always boot from fallback.
 +
 +
This might be because as the default settings of volume_mode touches the nvram, it probably corrupts it at shutdown when saving the alsa state of the volume buttons "sound card" (called EC Mixer). Then at boot, coreboot will detects a corrupted nvram and restore its valid defaults.
  
 
== references ==
 
== references ==
 
<references/>
 
<references/>

Latest revision as of 20:59, 25 February 2018

Introduction

This mechanism permits to test and recover from certain non-booting coreboot images.

This works by having two coreboot images in the same flash chip:

  • One fallback/ image: The working image.
  • One normal/ image: The image to be tested.

This feature is not widely tested on all boards. It also requires it to have a reboot_counter exported in the CMOS layout.

This also doesn't protect against human errors when using such feature, or bugs in the code responsible for switching between the two images.

Uses cases

  • Test new images way faster: if the image doesn't boot it will fallback on the old known-working image and save a long reflashing procedure. Handy for bisecting faster.
  • Test new images more safely: Despite of the recommendations of having a way to externally reflash, many new user don't. Still, this method is not totally foolproof.
  • More compact testing setup: Since reflashing tools are not mandatory anymore, the tests can be done with less hardware, very useful when traveling.

How it works

Coreboot increments a reboot count at each boot but never clears it. What runs after coreboot is responsible for that.

That way, the count can be cleared by the OS once it's fully booted.

If a certain threshold[1] is attained at boot, coreboot will boot the fallback image.

Warnings

Because we uses two images, it's easy to wrongly identify which image booted:

  • If the user mistakenly thinks the normal image is booting...
  • But the fallback image always boots...
  • And the normal image doesn't work...
  • And the user flashes the normal in fallback because she thinks it boots fine...
  • Then the user bricked her device and has to reflash it externally.

Fallback build

To configure it for fallback, do:

$ make menuconfig

Then in "General setup --->", near the top use "fallback" in "CBFS prefix to use":

(fallback) CBFS prefix to use

Then near the bottom, make sure to have:

[ ] Update existing coreboot.rom image

And in the "Chipset --->" menu at the bottom:

Bootblock behaviour (Switch to normal if CMOS says so)  --->
[*] Do not clear reboot count after successful boot

You can then build the fallback image with the fallback.sh script.

Normal build

To configure it for normal, do:

$ make menuconfig

Then in "General setup --->", near the top use "normal" in "CBFS prefix to use":

(normal) CBFS prefix to use

Then near the bottom, make sure to have:

[*] Update existing coreboot.rom image

And in the "Chipset --->" menu at the bottom:

Bootblock behaviour (Switch to normal if CMOS says so)  --->
[*] Do not clear reboot count after successful boot

You can then build with the normal part with the normal.sh script. It takes an existing coreboot image as argument.

OS configuration

The manual way

An approach is to run switch-to-normal.sh before trying an image. It's however more error prone than the systemd approach because:

  • you have to do it manually, each time, before testing an image.
  • If you then want to use that new image, you have to flash it, again, to fallback.

switch-to-normal.sh

#!/bin/sh
nvramtool -w boot_option=Normal
nvramtool -w reboot_counter=0

switch-to-fallback.sh

#!/bin/sh
nvramtool -w boot_option=Fallback
nvramtool -w reboot_counter=15

(Assuming that 15 is the maximum that can be stored in reboot_counter.)

Systemd

Here we use systemd to automatically reset the boot counter after each successful boot (or resume).

We are then supposed to use the normal image daily and only resort to fallback in case of issues.

To install it, first install nvramtool (from coreboot sources):

$ cd util/nvramtool
$ make
$ sudo make install

Then add the following systemd units at their respective paths:

Then enable them with:

$ sudo systemctl enable coreboot@boot.service
$ sudo systemctl start coreboot@boot.service
$ sudo systemctl enable coreboot@resume.service
$ sudo systemctl start coreboot@resume.service

Current limitations

  • Use of the same cmos.layout in fallback and normal !
  • The user may wrongly identify which image booted, and because of that, end up reflashing a non-working image.
  • Some issues can arrise when the nvram layout is not the same between normal/ and fallback/
  • The number of failed boot is fixed at compilation time.
  • In order to fully boot, some boards do reset conditionally during the boot process resulting in a non-predictable increment of the boot count.
  • Example script exist only for systemd. Still, they are trivial to adapt to other init systems.
  • Payloads sometime have fixed default locations when loading things from cbfs:
    • When using grub as a payload, grub.cfg is at etc/grub.cfg by default, so if you want to test grub as a payload, remember to change grub.cfg's path not to interfer with the fallback's grub configuration.
    • Changing the path of what SeaBIOS loads from cbfs is probably configurable with SeaBIOS cbfs symlinks but not yet tested/documented with the use of the fallback mecanism
  • Tested boards need to be listed somewhere.

Issues

thinkpad_acpi

This linux driver can have some bad interactions with the fallback/normal mecanism: when using it with the volume_control=1 option, volume_mode=1 is required, otherwise after shutting down the computer, it will always boot from fallback.

This might be because as the default settings of volume_mode touches the nvram, it probably corrupts it at shutdown when saving the alsa state of the volume buttons "sound card" (called EC Mixer). Then at boot, coreboot will detects a corrupted nvram and restore its valid defaults.

references

  1. Defined by CONFIG_MAX_REBOOT_CNT, typically 3