.. _booting_linux: Booting Linux ============= Introduction ------------ The lowlevel boot command in barebox is :ref:`command_bootm`. This command can be used directly, but there is also a :ref:`command_boot` command which offers additional features like a boot sequence which tries to boot different entries until one succeeds. The bootm command ----------------- The :ref:`command_bootm` command is the lowlevel boot command. Depending on the architecture the bootm command handles different image types. On ARM the following images are supported: * ARM Linux zImage * ARM64 Linux Image, plain or compressed * U-Boot uImage * barebox images * FIT images, containing a zImage or Image The images can either be passed directly to the bootm command as argument or in the ``global.bootm.image`` variable: .. code-block:: sh bootm /path/to/zImage # same as: global.bootm.image=/path/to/zImage bootm When barebox has an internal devicetree it is passed to the kernel. You can specify an alternative devicetree with the ``-o DTS`` option or the ``global.bootm.oftree`` variable: .. code-block:: sh bootm -o /path/to/dtb /path/to/zImage # same as: global.bootm.oftree=/path/to/dtb global.bootm.image=/path/to/zImage bootm FIT image configurations will be matched by comparing the ``compatible`` property inside the configuration node with the barebox live tree's ``/compatible``. It's also possible to select a specific configuration explicitly: .. code-block:: sh global.bootm.image=/dev/mmc0.fit@conf-imx8mm-evk.dtb **NOTE:** it may happen that barebox is probed from the devicetree, but you have want to start a Kernel without passing a devicetree. In this case set the ``global.bootm.boot_atag`` variable to ``true``. Passing Kernel Arguments ^^^^^^^^^^^^^^^^^^^^^^^^ The simple method to pass bootargs to the kernel is with ``CONFIG_FLEXIBLE_BOOTARGS`` disabled: in this case the bootm command takes the bootargs from the ``bootargs`` environment variable. With ``CONFIG_FLEXIBLE_BOOTARGS`` enabled, the bootargs are composed from different :ref:`global device` variables. All variables beginning with ``global.linux.bootargs.`` will be concatenated to the bootargs: .. code-block:: sh global linux.bootargs.base="console=ttyO0,115200" global linux.bootargs.debug="earlyprintk ignore_loglevel" bootm zImage ... Kernel command line: console=ttymxc0,115200n8 earlyprintk ignore_loglevel Additionally all variables starting with ``global.linux.mtdparts.`` are concatenated to a ``mtdparts=`` parameter to the kernel. This makes it possible to consistently partition devices with the :ref:`command_addpart` command and pass the same string as used with addpart to the Kernel: .. code-block:: sh norparts="512k(bootloader),512k(env),4M(kernel),-(root)" nandparts="1M(bootloader),1M(env),4M(kernel),-(root)" global linux.mtdparts.nor0="physmap-flash.0:$norparts" global linux.mtdparts.nand0="mxc_nand:$nandparts" addpart /dev/nor0 $norparts addpart /dev/nand0 $nandparts ... bootm zImage ... Kernel command line: mtdparts=physmap-flash.0:512k(bootloader),512k(env),4M(kernel),-(root); mxc_nand:1M(bootloader),1M(env),4M(kernel),-(root) Creating root= options for the Kernel ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ It's a common case that the Linux Kernel is loaded from a filesystem that later becomes the root filesystem for the Kernel. For several filesystems barebox can automatically append a suitable root= option to the Kernel command line. This is done when ``global.bootm.appendroot`` is true. How the root= option is appended depends on the device type and filesystem the kernel is booted from. For disk like devices (SD/MMC, ATA) the partition UUID will be used, the root= option will be something like ``root=PARTUUID=deadbeef-1``. For UBIFS fileystems it will be ``root=ubi0:volname ubi.mtd=mtdpartname rootfstype=ubifs``. NFS filesystems will result in ``root=/dev/nfs nfsroot=ip:/path/to/nfsroot,v3,tcp``. The ``v3,tcp`` part is configurable in ``global.linux.rootnfsopts``. The boot command ---------------- The :ref:`command_boot` command offers additional convenience for the :ref:`command_bootm` command. It works with :ref:`boot_entries` and :ref:`bootloader_spec` entries. Boot entries are located under /env/boot/ and are scripts which setup the bootm variables so that the ``boot`` command can run ``bootm`` without further arguments. .. _boot_entries: Boot entries ^^^^^^^^^^^^ A simple boot entry in ``/env/boot/mmc`` could look like this: .. code-block:: sh #!/bin/sh global.bootm.image=/mnt/mmc1/zImage global.bootm.oftree=/env/oftree global linux.bootargs.dyn.root="root=PARTUUID=deadbeef:01" This takes the kernel from ``/mnt/mmc1/zImage`` (which could be an :ref:`automount` path registered earlier). The devicetree will be used from ``/env/oftree``. The Kernel gets the command line ``root=PARTUUID=deadbeef:01``. Note the ``.dyn`` in the bootargs variable name. boot entries should always add Kernel command line parameters to variables with ``.dyn`` in it. These will be cleared before booting different boot entries. This is done so that following boot entries do not leak command line parameters from the previous boot entries. This entry can be booted with ``boot mmc``. It can also be made the default by setting the ``global.boot.default`` variable to ``mmc`` and then calling ``boot`` without arguments. .. _bootloader_spec: Boot Loader Specification ^^^^^^^^^^^^^^^^^^^^^^^^^ barebox supports booting according to the `Boot Loader Specification `__ (sometimes also known as *Bootloader Spec*, *bootspec* or *blspec*). It follows another philosophy than the :ref:`boot_entries`. With Boot Entries booting is completely configured in the bootloader. Bootloader Spec Entries on the other hand are part of the boot medium. This gives a boot medium the possibility to describe where a kernel is located and which parameters are needed to boot it. All Bootloader Spec Entries are located in a partition on the boot medium under ``/loader/entries/*.conf``. According to the Bootloader Spec, a boot medium has to use a dedicated partition for boot entries. barebox is less strict, it accepts Bootloader Spec Entries on every partition that barebox can read. A Bootloader Spec Entry consists of key value pairs:: /loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf: title Fedora 19 (Rawhide) version 3.8.0-2.fc19.x86_64 machine-id 6a9857a393724b7a981ebb5b8495b9ea options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd All paths are absolute paths in the partition. Bootloader Spec Entries can be created manually, but there also is the ``scripts/kernel-install`` tool to create/list/modify entries directly on a MMC/SD card or other media. To use it, create an SD card / USB memory stick with a ``/boot`` partition with type ``0xea``. The partition can be formatted with FAT or EXT4 filesystem. If you wish to write to it from barebox later you must use FAT. The following creates a Bootloader Spec Entry on a SD card: .. code-block:: sh scripts/kernel-install --device=/dev/mmcblk0 -a \ --machine-id=11ab7c89d02c4f66a4e2474ea25b2b84 --kernel-version="3.15" \ --kernel=/home/sha/linux/arch/arm/boot/zImage --add-root-option \ --root=/dev/mmcblk0p1 -o "console=ttymxc0,115200" The entry can be listed with the ``-l`` option: .. code-block:: none scripts/kernel-install --device=/dev/mmcblk0 -l Entry 0: title: Linux-3.15 version: 3.15 machine_id: 11ab7c89d02c4f66a4e2474ea25b2b84 options: console=ttymxc0,115200 root=PARTUUID=0007CB20-01 linux: 11ab7c89d02c4f66a4e2474ea25b2b84.15/linux When the SD card shows up as ``mmc1`` in barebox, this entry can be booted with ``boot mmc1`` or by setting ``global.boot.default`` to ``mmc1``. A bootloader spec entry can also reside on an NFS server, in which case an `RFC 2224 `__-compatible NFS URI must be passed to the boot command: .. code-block:: sh boot nfs://nfshost[:port]//path/ Additional notes about keys in the bootloader spec entries: ``machine-id`` This key is optional. If the ``global.boot.machine_id`` variable is set to a non-empty value, barebox will only boot the Bootloader Spec Entry whose ``machine-id`` key matches the ``global.boot.machine_id`` variable. All other Bootloader Spec entries will be ignored. ``linux-appendroot`` This boolean option is understood by Barebox although it is not part of the original specification. If set to ``true``, barebox will automatically append a ``root=`` string to the Linux commandline based on the device where the entry is found on. This makes it possible to use the same rootfs image on different devices without having to specify a different ``root=`` option each time. .. _booting_linux_net: Network boot ------------ With the following steps, barebox can start the kernel and root filesystem over the network, a standard development case. See :ref:`networking` for informations how to configure your network interfaces. Note that barebox will pass the same IP settings to the kernel, i.e. it passes ``ip=$ipaddr:$serverip:$gateway:$netmask:::`` for a static IP setup and ``ip=dhcp`` for a dynamic DHCP setup. ```` is a configurable value. set ``nv.dev..linuxdevname`` to the name the device has in Linux. By default, barebox uses the variables ``global.user`` and ``global.hostname`` to retrieve its kernel image over TFTP, which makes it possible to use multiple boards for multiple users with one single server. You can adjust those variables using nvvars with these commands:: nv user=sha nv hostname=efikasb Copy the kernel (and devicetree if needed) to the root directory of your TFTP server, and name them accordingly; for example:: cp zImage /tftpboot/sha-linux-efikasb cp myboard.dtb /tftpboot/sha-oftree-efikasb (In this example, the directory ``/tftpboot`` represents the root directory of the TFTP server. That directory depends on the configuration of your TFTP server, some servers may also use ``/srv/tftp`` instead.) barebox will pass ``nfsroot=/home/${global.user}/nfsroot/${global.hostname}`` to the kernel. This causes the kernel to mount its root filesystem from a NFS server, which is detected through the DHCP reply. To choose a different server, simply prepend its IP address to the mount path, e.g. ``nfsroot=192.168.23.5:/home/...``. In any case, make sure that the specified mountpoint is exported by your NFS server. For more information about booting with ``nfsroot``, see `Documentation/admin-guide/nfs/nfsroot.rst `__ in the Linux kernel documentation. If the preconfigured paths or names are not suitable, they can be adjusted in ``/env/boot/net``: .. code-block:: sh #!/bin/sh path="/mnt/tftp" global.bootm.image="${path}/${global.user}-linux-${global.hostname}" oftree="${path}/${global.user}-oftree-${global.hostname}" if [ -f "${oftree}" ]; then global.bootm.oftree="$oftree" fi nfsroot="/home/${global.user}/nfsroot/${global.hostname}" ip_route_get -b ${global.net.server} global.linux.bootargs.dyn.ip global.linux.bootargs.dyn.root="root=/dev/nfs nfsroot=$nfsroot,v3,tcp" ``boot net`` will then retrieve the kernel (and also the device tree and initramfs, if used) over TFTP and boot it.