Advanced Configuration

Target-Specific Configuration

Different target boards have different layouts for GPIO, LEDs, and more. Often, this requires that configurations be specified per-target. In this example, we will be looking at how to configure the LEDs for two different targets. First, let’s start by modifying our config.exs to include configs for each target.

# config/config.exs

use Mix.Config

import_config "#{Mix.Project.config[:target]}.exs"

This will load a different Mix config for each target. Let’s say we plan to support targets rpi3 and bbb. These target devices have different numbers of user-controlled LEDs and we want each to blink all of its LEDs. The configuration files would look like this:

# config/rpi3.exs

config :blinky, led_list: [ :green ]
config :nerves_leds, names: [ green: "led0" ]
# config/bbb.exs

config :blinky, led_list: [ :led0, :led1, :led2, :led3 ]

config :nerves_leds, names: [
  led0: "beaglebone:green:usr0",
  led1: "beaglebone:green:usr1",
  led2: "beaglebone:green:usr2",
  led3: "beaglebone:green:usr3"
]

Root Filesystem Additions

Sometimes, you want to ship additional files and configurations with your firmware. This is done by telling the firmware assembler where to find a directory to use as an overlay on the root mount point:

# config/config.exs

config :nerves, :firmware,
  rootfs_additions: "config/rootfs-additions"

This declares that the contents of the folder at config/rootfs-additions will be merged into the root file system when mix firmware is called. You can also specify different rootfs additions per target, as shown in the previous section.

Overwriting Files in the Root File System

Any files in the rootfs_additions directory will overwrite those present in the underlying filesystem. This can be useful if you want to change the contents of included files in the underlying Nerves system. Let’s say, for example, that you want to change the behavior of erlinit. You can include your own erlinit.config:

# config/rootfs-additions/etc/erlinit.config

# Uncomment to hang the board rather than rebooting when Erlang exits
#--hang-on-exit

# Enable UTF-8 filename handling in Erlang and custom inet configuration
-e LANG=en_US.UTF-8;LANGUAGE=en;ERL_INETRC=/etc/erl_inetrc

# Mount the configdata partition
# See http://www.linuxfromscratch.org/lfs/view/6.3/chapter08/fstab.html about
# ignoring warning the Linux kernel warning about using UTF8 with vfat.
-m /dev/mmcblk0p3:/root:vfat::

# Erlang release search path
-r /srv/erlang

# Hostname
-d "/usr/bin/boardid -b bbb -n 4"
-n nerves-%.4s

It is important to note that if you replace a config file, the entire file is replaced, rather than merging the contents. Therefore, you should first obtain and modify the original file. A trick for doing this is to expand the rootfs.squashfs. You can do this using unsquashfs:

$ unsquashfs ~/.nerves/artifacts/<cached_system_name>/images/rootfs.squashfs

It will be expanded into the current directory under squashfs-root

Overwriting Files in the Boot Partition

Different targets have different boot partition contents. To overwrite files in the boot partition, you will need to use your own fwup.conf file:

# config/config.exs

config :nerves, :firmware,
  fwup_conf: "config/fwup.conf"

In your included fwup.conf file, you can use absolute paths or environment variables to point to the location of included files.

Let’s say you have a Raspberry Pi and you want to change the contents of the cmdline.txt file. You can do this by editing the fwup.conf as follows:

# fwup.conf

file-resource cmdline.txt {
    host-path = "${NERVES_APP}/config/cmdline.txt"
}

You can use the NERVES_APP environment variable to point to the root of your Elixir app. This variable is automatically managed for you by nerves_bootstrap.

Partitions

Nerves firmware uses Master Boot Record partition layout, which only supports 4 primary partitions. By default, the root filesystem partition is mounted in read-only mode. This prevents corruption of the root filesystem due to “improper shutdowns”. With embedded systems, it is assumed that power can be removed from the device at any time. This could be problematic if you are performing a write operation on the filesystem. Because the root filesystem is read-only, we also add a read/write partition by default, called app_data and mounted at /root (the root user’s home directory). These settings are defined in etc/erlinit.config.

+----------------------------+
| MBR                        |
+----------------------------+
| Boot partition (FAT32)     |
+----------------------------+
| p1*: Rootfs A (squashfs)   |
+----------------------------+
| p1*: Rootfs B (squashfs)   |
+----------------------------+
| p2: App Data  (FAT32)      |
+----------------------------+

You can enable and mount an additional read/write partition by modifying the fwup.conf file. This strategy is typically used to define two locations where data can be written. Let’s say you want to persist some infrequently-written configuration data and some frequently-written log data. These use-cases could be segmented into separate partitions so that the important, infrequently-written configuration data is not corrupted due to a loss of power while writing the more-frequent, but less-critical, log data.

First, define a new space on the disk for the partition:

# fwup.conf

# The boot partition
define(BOOT_PART_OFFSET, 63)
define(BOOT_PART_COUNT, 16321)

# Let the rootfs have room to grow up to 128 MiB and align
# it to the nearest 1 MB boundary
define(ROOTFS_A_PART_OFFSET, 16384)
define(ROOTFS_A_PART_COUNT, 289044)
define(ROOTFS_B_PART_OFFSET, 305428)
define(ROOTFS_B_PART_COUNT, 289044)

# Config partition
define(CONFIG_PART_OFFSET, 594472)
define(CONFIG_PART_COUNT, 1048576)

# Log partition
define(LOG_PART_OFFSET, 1643048)
define(LOG_PART_COUNT, 1048576)

In this example, we are changing the app data partition to CONFIG_PART and adding LOG_PART.

Next, we change the mapping to include these two new partitions:

# fwup.conf

...
mbr mbr-a {
    partition 0 {
        block-offset = ${BOOT_PART_OFFSET}
        block-count = ${BOOT_PART_COUNT}
        type = 0xc # FAT32
        boot = true
    }
    partition 1 {
        block-offset = ${ROOTFS_A_PART_OFFSET}
        block-count = ${ROOTFS_A_PART_COUNT}
        type = 0x83 # Linux
    }
    partition 2 {
        block-offset = ${CONFIG_PART_OFFSET}
        block-count = ${CONFIG_PART_COUNT}
        type = 0xc # FAT32
    }
    partition 3 {
        block-offset = ${LOG_PART_OFFSET}
        block-count = ${LOG_PART_COUNT}
        type = 0x83 # Linux
    }
}

mbr mbr-b {
    partition 0 {
        block-offset = ${BOOT_PART_OFFSET}
        block-count = ${BOOT_PART_COUNT}
        type = 0xc # FAT32
        boot = true
    }
    partition 1 {
        block-offset = ${ROOTFS_B_PART_OFFSET}
        block-count = ${ROOTFS_B_PART_COUNT}
        type = 0x83 # Linux
    }
    partition 2 {
        block-offset = ${CONFIG_PART_OFFSET}
        block-count = ${CONFIG_PART_COUNT}
        type = 0xc # FAT32
    }
    partition 3 {
        block-offset = ${LOG_PART_OFFSET}
        block-count = ${LOG_PART_COUNT}
        type = 0x83 # Linux
    }
}

This layout defines our system as follows:

+----------------------------+
| MBR                        |
+----------------------------+
| Boot partition (FAT32)     |
+----------------------------+
| p1*: Rootfs A (squashfs)   |
+----------------------------+
| p1*: Rootfs B (squashfs)   |
+----------------------------+
| p2: Config      (FAT32)    |
+----------------------------+
| p3: Log         (EXT4)     |
+----------------------------+

Mounting the Partition

Mounting your new partition can either be handled by erlinit or by your Elixir application. To have erlinit mount the partition for you, you will need to supply your own erlinit.config file to set the required -m option:

# Mount the configdata partition
# See http://www.linuxfromscratch.org/lfs/view/6.3/chapter08/fstab.html about
# ignoring warning the Linux kernel warning about using UTF8 with vfat.
-m /dev/mmcblk0p3:/root:vfat::;/dev/mmcblk0p4:/mnt/log:ext4::

The other option is to handle it in your Elixir code. This can be useful if you want to scan the disk for corruption and reformat or seed it. erlinit can only attempt to mount the partition.

First, we initialize the filesystem on the partition:

System.cmd("mke2fs", ["-t", "ext4", "-L", "LOGDATA", "/dev/mmcblk0p4"])

Then, we mount it:

System.cmd("mount", ["-t", "ext4", "/dev/mmcblk0p4", "/mnt/log"])