Instructions for building and booting Linux: Difference between revisions

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These instructions explain how to build a toolchain, root filesystem and kernel for Linux running on an Xtensa processor.  These instructions apply specifically to the XTAV60 (LX60) board.
This page describes what parts are necessary in order to run GNU/Linux OS on Xtensa, their interaction and setup.


Some general notes on these instructions:
= GNU/Linux on Xtensa =


*  They are a work in progress.  If you are using the latest development sources, the build process may change, so you should be sure to get the latest version of this document at [http://wiki.linux-xtensa.org/index.php/Buildroot_Build_Instructions http://wiki.linux-xtensa.org/index.php/Buildroot_Build_Instructions].  If you are using one of the development snapshot releases, you should use the version of the build instructions included with that snapshot.
GNU is a collection of free software that provides POSIX environment.
Linux is an operating system kernel.
In order to boot GNU/Linux OS on Xtensa the following parts are needed:


* The following was tested on x86 machines running RedHat Enterprise Linux 4 (RHEL4) and Fedora Core 3 (FC3).  It is expected to work on newer Fedora releases (e.g., was tested on FC6) but to maintain host compatibility with Xtensa Tools, it is best to avoid Fedora releases beyond FC5.  Other host distributions likely work but have not been tested.
* a board with Xtensa core supported by Linux, or Xtensa simulator
** '''NOTE:''' this assumes the default selection of packages in buildroot.  Selecting extra packages may require a more recent host OS or more recent version of certain tools.  For example, the default version of <tt>find</tt> on RHEL4 is too old to allow building all X11 packages in buildroot, so it is necessary to upgrade the host version of <tt>findutils</tt> on RHEL4 to build these packages.
* bootable Linux kernel image
* GNU software packaged in a form suitable for booting on Linux, the so called root file system
* a bootloader or on-chip debugger capable of loading Linux kernel image into the board RAM


*  Lines prefixed with "sudo" need to be executed as root. The rest is best executed as a non-root user. (It is possible to install <tt>git</tt> as a non-root user if root access is an issue.  How to do this isn't shown here.)
To build Linux image and root file system a toolchain correctly configured for used Xtensa core is needed. Toolchain configuration is performed by application of [[Toolchain_Overlay_File|Xtensa configuration overlay]]. Correct toolchain configuration is critical for building other pieces of software correctly.


*  Lines that set environment variables assume a Bourne compatible shell (e.g., /bin/sh or bash), but are easily adapted to other shells.
== Xtensa cores supported by Linux ==


*  Instructions are assumed executed all in order (e.g., commands assume current directory and environment variables set earlier).
Official requirements for Xtensa core to run Linux are listed in the Xtensa OSKit Guide document.
The absolute minimum set of options required to run Linux is windowed registers, exception option and one level-1 or medium level timer interrupt.
* Full MMU option is required for full featured Linux kernel that includes process address space isolation, virtual memory and fork syscall.
* Full MMU option, thread pointer option and conditional store option are required for running userspace with NPTL or TLS.


== Xtensa boards supported by Linux ==


The following boards collectively referred as XTFPGA family are readily supported by Linux: Avnet LX60, LX110, LX200; Xilinx ML605, KC705. For board details/setup guide please see corresponding Board Guide or Board Prototyping Guide provided with Xtensa software release.


== Setup your host system ==
== Xtensa simulators ==


If you are using one of the snapshot releases, you can skip this step.
* Xtensa software provides Instruction Set Simulator (xt-run) capable of running Linux configured for ISS.
* QEMU is a free system emulator capable of emulating any XTFPGA board or ISS. It can be configured to emulate any Xtensa core, but it does not currently support TIE extensions to the core architecture. Please see [[Xtensa_on_QEMU|this]] page for more details.


If you want to work with the latest development sources, you will need
== Linux kernel ==
[[Installing GIT|<tt>git</tt> installed on your system]].


== Download buildroot and the Linux kernel ==
Linux kernel needs to be configured for the specific Xtensa core. For that reason Linux is usually configured and built separately from the userspace software. Please see [[Linux_Kernel|this]] page for more details.


The following section describes how to download the latest
== Root file system ==
versions of buildroot and of the Linux kernel.
Skip this step if you are using one of the
[http://wiki.linux-xtensa.org/index.php/Buildroot_Snapshots snapshot releases].


For most users, just execute the following:
Currently we support building GNU software and packaging it into root file system with the [[Buildroot]].
There are several types of root file system that Linux can boot with:


  $ cd <workdir>
=== CPIO archive ===
  $ git clone git://git.linux-xtensa.org/git/buildroot/buildroot-xtensa buildroot
* CPIO archive may be built into the kernel (so called 'built-in initramfs'), the resulting single image will contain both Linux and GNU software. It's easy to boot, but in order to change anything the kernel needs to be rebuilt.
  $ git clone git://git.linux-xtensa.org/git/kernel/xtensa-2.6.29-smp linux
* CPIO archive may also be loaded separately from the kernel, but used in the same way as the built-in initramfs. It's harder to boot (two files instead of one), but easier to change.


where <tt><workdir></tt> is the path to an empty directory on a disk with
Both cases of CPIO use RAM for the root file system, so they're very fast. The downsides are that they take some time at startup to unpack, reserve some RAM (which may be an issue on low-memory boards like LX60) and are not persistent.
at least 4 GB available space. You end up with the following file structure,
which is assumed by the rest of this document:


  <workdir>/
=== EXT2/3/4 file system image ===
            /buildroot
EXT2/3/4 file system image may be used directly with ISS or QEMU through the Simulated block device Linux feature. Please see [[Linux_Kernel#ISS_platform|this]] chapter for more details.
            /linux


The MASTER branch of the buildroot repository is often is a state of developmet.
=== TAR image ===
It's recommended to use the last snapshot for application developers. In the
Tar image may be unpacked into a directory shared through NFS. Linux on XTFPGA boards and on QEMU can be configured to use NFS root file system.
case of the HiFi-2 development board a 'snapshot_2+SMP' branch is recomended
in the currently experimental SMP git repository:


    $ git clone git://git.linux-xtensa.org/git/buildroot/buildroot-xtensa-smp buildroot
=== UBIFS image ===
    $ git branch --track snapshot_2+SMP
UBIFS image may be written directly to the daughterboard FLASH on XTFPGA boards. Please see [[Linux_Kernel#Rootfs_in_FLASH_.28UBIFS.29|this]] chapter for more details.
    $ git checkout snapshot_2+SMP
   


If you happen to have write access to either repository, or want more details,
== Bootloader ==
see the [http://wiki.linux-xtensa.org/index.php/Repository_Access repository access] page.
U-Boot can be used as a bootloader on XTFPGA boards. Please see [[U-Boot|this]] page for more details about installing it into the daughterboard FLASH.


== Install any required processor specific overlay ==
== On-chip debugger ==
On-chip debugger is good for debugging lowest level software: bootloader and Linux kernel itself. It also allows loading executable images into new board with empty FLASH.
It consists of JTAG probe connected to the XTFPGA daughterboard (ML605 and KC705 daughterboards also have built-in JTAG chip), OCD monitor (xt-ocd) running on developer machine where the other end of JTAG probe is connected, and xt-gdb, the modified version of GNU debugger from the Xtensa software release. Please see Tensilica Debug User's Guide for more information about OCD setup and configuration.


This step is required if you are targeting a custom Xtensa processor.
= Building GNU/Linux OS =
If you use a Diamond 232L Standard Core, you can skip this step.
In order to follow these instruction a build machine with GNU/Linux and a set of development tools (native binutils and gcc, make, wget, tar, ...) is needed. In the following example all software will be downloaded and built in the current directory. Xtensa tools used in the examples are installed into $XT_ROOT, tools release is $XT_RELEASE and selected core is $XT_CORE.


To configure buildroot and the kernel to use a custom Xtensa processor
== Prepare the overlay ==
configuration, you need to overlay a set of files customized for your
Please see [[Toolchain_Overlay_File|this]] page for overlay repacking script and details of the overlay source location.
processor configuration onto toolchain sources (gcc, binutils, gdb, etc)
Create conversion script by pasting [[Toolchain_Overlay_File#Conversion_Script|this]] into the file overlay-script and making it executable:
and the Linux kernel.  This process has been automated using a script
  $ '''cat > overlay-script'''
located in the buildroot source tree.
  '''....'''
For usage information, you can invoke it without arguments:
  ^D
  $ '''chmod +x overlay-script'''


  $ cd <workdir>
Run it and rename the resulting file:
  $ ./buildroot/target/xtensa/xt-buildroot-overlay-install
  $ '''./overlay-script ${XT_ROOT}/XtDevTools/install/builds/${XT_RELEASE}-linux/${XT_CORE}/src/xtensa-config-overlay.tar.gz'''
  $ '''mv xtensa-overlay.tar xtensa_${XT_CORE}.tar'''


which displays a help message similar to the following:
== Get, configure and build the toolchain and root file system ==
Get the buildroot git tree:
  $ '''git clone git://git.buildroot.net/buildroot'''


xt-buildroot-overlay-install version 1.3
Check out some released version:
Usage: xt-buildroot-overlay-install <parameters> [<options>]
  $ '''git checkout 2014.11'''
Where <parameters> are:
  -t file.tgz    Specify path to the Xtensa Linux overlay tarball, typically
                  <xtensa_root>/src/xtensa-config-overlay.tar.gz
  -b dir          Path to the base of the buildroot source tree, in which
                  package specific overlay tarballs get installed.
  -k dir          Path to the base of the Linux kernel source tree, in which
                  the Linux kernel specific overlay gets installed.
  -c config_name  Name for the Xtensa processor configuration as it will be
                  known to the open source community.  Must be a lowercase
                  identifier, starting with a letter, consisting of letters
                  and numbers and underscores, not ending with underscore
                  and not containing consecutive underscores.  For examples:
                      dc232b , dc232b_be , mmubasele , fsf , s5000 .
  -l long_name    Long name for the Xtensa processor configuration, human-
                  readable with spaces etc allowed (must be quoted).
                  For example:  'Diamond 232L Standard Core Rev.B (LE)'
                  Try to keep it within approximately 40 characters.
And <options> are:
  -f              If package specific overlay tarballs already exist in
                  the destination source tree, overwrite them without asking.
  --help          Show this usage message.


Note that the Xtensa Linux overlay tarball MUST be obtained from an Xtensa
Create build directory and configure it:
core package built using release RB-2008.3 or later.
  $ '''mkdir buildroot-build'''
  $ '''make -C buildroot O=$(pwd)/buildroot-build menuconfig'''


Here is an example invocation of the overlay installation script:
Please see [[Buildroot#Configuring_and_building_Buildroot|this]] chapter for more details about configuring the Buildroot.


  $ ./buildroot/target/xtensa/xt-buildroot-overlay-install -t blinkcore-config-overlay.tar.gz \
Select 'Target Architecture' = Xtensa, 'Target Architecture Variant' = Custom Xtensa processor configuration, set 'Overlay file for custom configuration ' to the location of the overlay archive (it may be a path to a local file or an URL).
        -b ./buildroot -k ./linux -c superzip -l "ChipCorp SuperZIP Blink Accelerator Core"


The script extracts some relevant information from the overlay tarball,
Select toolchain options, packages to be built and final root file system image type. Select whether the kernel need to be built, kernel version and configuration.
presents it along with relevant parameters to the user, and prompts
for confirmation before proceeding.


The chosen processor name (-c option) must be unique among known Xtensa processors.
Build the toolchain, kernel and the root file system:
It is also probably wise to avoid names confusingly similar to other (non-Xtensa) processors.
  $ '''make -C buildroot O=$(pwd)/buildroot-build'''
If you expect at some point to contribute support for your custom processor to the
open-source community, this name will likely be long-lived.
(One way to contribute support for a custom processor might be to submit the overlay tarball
or files to the linux-xtensa.org maintainers.)


In the steps below, buildroot and the Linux kernel must be configured
== The kernel may be built on its own ==
(in their respective <code>''make menuconfig''</code> steps) to select
Get stable Linux release tree:
the <code>''custom''</code> Xtensa processor option.
  $ '''wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.19.1.tar.xz'''
This presents an extra configuration menu entry where you must provide
  $ '''tar -xf linux-3.19.1.tar.xz'''
the name of the Xtensa processor. Set it to the same name
as was specified in the <code>-c</code> option above.


== Build a toolchain and root filesystem using buildroot ==
Create variant directory for the specific core and copy core configuration files there:
  $ '''mkdir -p linux-3.19.1/arch/xtensa/variants/${XT_CORE}/include/variant'''
  $ '''cp ${XT_ROOT}/XtDevTools/install/builds/${XT_RELEASE}-linux/${XT_CORE}/config/*.h linux-3.19.1/arch/xtensa/variants/${XT_CORE}/include/variant'''


'''Note:''' for more details on generic buildroot topics not covered in this simple how-to,
Create build directory and configure it:
see [http://buildroot.uclibc.org/ buildroot.uclibc.org].
  $ '''mkdir linux-build'''
  $ '''ARCH=xtensa CROSS_COMPILE=$(pwd)/buildroot-build/host/usr/bin/xtensa-buildroot-linux-uclibc- make -C linux-3.19.1 O=$(pwd)/linux-build menuconfig'''


=== Configure buildroot ===
Select 'Xtensa Processor Configuration' = Custom Xtensa processor configuration, set 'Xtensa Processor Custom Core Variant Name' to the contents of ${XT_CORE}. Select desired 'Xtensa System Type'.


The first time you configure buildroot, start with defaults for your Xtensa processor:
Please see [[Linux_Kernel#Configuring_the_kernel|this]] chapter for more details about configuring Linux.


  $ cd <workdir>/buildroot
Build the kernel:
  $ ./target/xtensa/setup-config <i><corename></i>
  $ '''ARCH=xtensa CROSS_COMPILE=$(pwd)/buildroot-build/host/usr/bin/xtensa-buildroot-linux-uclibc- make -C linux-3.19.1 O=$(pwd)/linux-build all'''


where ''<corename>'' is the overlay name of your selected Xtensa or Diamond core
= Booting GNU/Linux =
(e.g., '''dc232b''').  If you omit ''<corename>'', a help message and a list
TBD
of currently installed Tensilica core overlays is displayed.
 
Optionally, you can then customize your buildroot configuration, such as
selecting additional packages, setting various parameters, and so on.
The following make targets provide a ''curses'' (text-based graphical) interface
for configuring buildroot, the uClibc C library, and busybox, respectively.
('''Note:'''  You need the <tt>TERM</tt> environment variable properly set for
these curses based tools to work.)
 
  $ make menuconfig
  $ make uclibc-menuconfig
  $ make busybox-menuconfig
 
=== Build buildroot ===
 
Just type:
 
  $ make
 
THIS WILL TAKE A LONG TIME (from about 30 minutes with defaults on a 3 GHz Pentium 4,
to perhaps an hour or many depending on your system and on whether
you enabled extra packages in your buildroot configuration).
 
Wait patiently for the build to complete.  This builds an entire toolchain
as well as some basic packages, and constructs a root filesystem.
It might build many more packages if you selected them earlier with menuconfig.
 
==== If Something Goes Wrong ====
 
Perhaps you missed some step above and the build fails.  Or you're just switching to a different processor configuration, or making some change with unknown dependencies.  Either way, you'll probably need to fix the error and/or make the change, and redo the whole build.  The above <tt>make</tt> command usually works fine if you
just added a package using menuconfig, but otherwise does not check many dependencies, so it will usually NOT rebuild things that depend on whatever you have fixed or changed.  Here's one way to retry the make from scratch without having to wipe out the <tt>buildroot</tt> directory tree completely and start again from the git cloning.
 
  $ rm -rf *build_xtensa* binaries
  $ make
 
It is not necessary to empty the 'dl' subdirectory, which contains tarballs downloaded from the Internet, nor the .config file, which contains the buildroot configuration.  Customizations to the uClibc and <tt>busybox</tt> configurations should have ended up getting saved in target/xtensa/uClibc.config and target/xtensa/busybox-config, respectively, so hopefully are preserved by the above sequence.
 
== Build and Run a Linux Kernel on the XTAV60 or XTAV200 Board ==
 
References to the ''XTAV60 board'' refer to the combination of
the Avnet [http://www.em.avnet.com/tensilica1 LX60 (Xilinx) Development Board]
and a Tensilica-provided FPGA bitstream containing a configured Tensilica
processor and basic peripheral IP.  For more details, please refer to the
''Tensilica Avnet LX60 (XT-AV60) Board User's Guide''.
 
Similarly, references to the ''XTAV200 board'' refers the combination of
the Avnet [http://www.em.avnet.com/tensilica2 LX200 (Xilinx) Development Board]
and a Tensilica-provided FPGA bitstream containing a configured Tensilica
processor and basic peripheral IP.  For more details, please refer to the
''Tensilica Avnet LX200 (XT-AV200) Board User's Guide''.
 
 
=== Configure the kernel ===
 
The kernel build is a two-step process:  configure the kernel, and build it.
First, before we can do anything with the kernel, set your PATH to
point to the toolchain built using buildroot.
 
  $ export PATH="''<workdir>''/buildroot/build_xtensa_''<cname>''/staging_dir/usr/bin:$PATH"
 
where ''<cname>'' is the name of the Xtensa processor configuration you selected
when building the toolchain (e.g., <code>dc232b</code>).
 
Now setup a destination for kernel builds.
We usually like to have the build directory separate from
the source directory, so we use 'O=</destination/path>' in
kernel make commands.  If you'll be routinely building multiple
configurations of the Linux kernel, it's good to have some
naming conventions for the build directory.  For example,
to experiment with multiple target platforms but only one
processor configuration, we include just the platform name here:
 
  $ mkdir build-xtav60
 
Let's start with the default kernel configuration for the Avnet board
(which works on both XTAV60 (LX60) and XTAV200 (LX200) boards)
as follows:
 
  $ cd linux
  $ make O=../build-xtav60 ARCH=xtensa KBUILD_DEFCONFIG=lx60_defconfig defconfig
 
This configures the kernel using the default configuration found in
arch/xtensa/configs/lx60_defconfig.
 
NOTE:  This kernel configuration has nothing to do with Xtensa
processor configuration.  That comes next.
 
Let's now customize this a little bit to select the correct Xtensa processor
configuration, and to bundle the root filesystem into the kernel:
 
  $ make O=../build-xtav60 ARCH=xtensa menuconfig
 
Under '''General setup''',
make sure the '''Initial RAM filesystem and RAM disk (initramfs/initrd) support'''
entry is enabled.  Underneath that, edit '''Initramfs source file(s)''',
and enter the path to the cpio formatted root filesystem
generated by buildroot:
 
      ''<workdir>''/buildroot/binaries/uclibc/rootfs.xtensa_''<cname>''.cpio.gz
 
(Don't forget to replace ''<workdir>'' with the appropriate full path
and ''<cname>'' with your Xtensa processor configuration name.)
 
Under '''Processor type and features''',
make sure the '''Xtensa Processor Configuration''' is correct and matches
the processor configuration name you used to build the toolchain with buildroot
(for example, ''dc232b'' for Diamond 232L Rev.B).
The kernel build selects a toolchain and processor variant specific headers
based on this selection.
If the desired processor configuration does not appear explicitly in the list,
simply select '''Custom Xtensa processor configuration''' and enter the
processor configuration name (all lowercase) into the next menu parameter,
'''Xtensa Processor Custom Variant Name'''.
 
Back in the main menu, under '''Platform options''', make sure the
'''CPU clock rate''' entry is set correctly according to the following table:
 
{| border="1" style="text-align:center; margin: 1em auto 1em auto"
|-
! Board !! Frequency selected in XPG <br/>(MHz) !! ''CPU clock rate'' setting <br/>(kHz)
|-
|rowspan="3"| XTAV60 (LX60)
|  30  || 33333
|-
|  40  || 40000
|-
|  50  || 50000
|-
|rowspan="3"| XTAV200 (LX200)
|  30  || 31250
|-
|  40  || 41667
|-
|  50  || 50000
|-
|}
 
Exit menuconfig, saving your configuration changes.
 
Backup your configuration outside the build directory.  For example:
 
  $ cp ../build-xtav60/.config config.xtav60.saved
 
If you do a clean rebuild of the kernel (e.g., "rm -rf ../build-xtav60")
you can now configure it with simply:
  $ mkdir ../build-xtav60 ; cp config.xtav60.saved ../build-xtav60/.config
Of course if you update the kernel, you'll probably again have to do:
  $ make O=../build-xtav60 ARCH=xtensa  menuconfig
and save a new copy of your configuration.
 
=== Build the kernel ===
 
  $ make O=../build-xtav60 ARCH=xtensa
 
The build will issue a few warnings which are normal.
Once complete, the bootable image is in:
      <workdir>/build-xtav60/arch/xtensa/boot/Image.elf
and the uncompressed ELF file containing kernel symbols is in:
      <workdir>/build-xtav60/vmlinux
There is also a RedBoot bootable image (if you use RedBoot) in:
      <workdir>/build-xtav60/arch/xtensa/boot/zImage.redboot
 
=== Install Xtensa Tools for the selected core ===
 
If you opt to download the kernel over OCD, you need <tt>xt-gdb</tt>
which is part of Tensilica's Xtensa Tools package.  This must be obtained
from Tensilica, and is not the same toolchain as built by buildroot.
You also need to install the corresponding Tensilica core package,
so that the Xtensa Tools know about your particular configured
Tensilica core.
 
The toolchain built using buildroot can be configured to
include GDB, but that version of GDB does not support
communicating with Tensilica's Xtensa OCD Daemon, and
thus cannot be used to download the kernel over OCD.
 
In this example, it is assumed that the machine hosting the
Xtensa Tools has access to the <workdir> directory tree.
 
=== Install and setup the Xtensa OCD Daemon ===
 
Setup and connect your JTAG probe, etc.
 
=== Connect a terminal server ===
 
At 38400 bps 8N1 no flow control to the XTAV60 serial port.
 
=== Optionally, setup networking ===
 
This step is optional.
 
Setup the board's MAC address using dipswitches
(see XTAV60 or XTAV200 board docs as appropriate)
and connect the board to a network that has a
DHCP server that will respond to that MAC address.
If you don't do this step, you'll simply not have network
access, and the boot process will take a minute or so longer
while the kernel times out waiting for a BOOTP response.
You can edit the kernel configuration and rebuild the kernel
to avoid using bootp (in particular, remove "ip=bootp" from
the kernel cmdline).
 
=== Download and run the kernel ===
 
Reset the board (see board documentation).
 
Using Xtensa Tools, invoke:
 
  $ xt-gdb <workdir>/build-xtav60/arch/xtensa/boot/Image.elf
 
  (xt-gdb) target remote <ocdhost>:20000 0
  (xt-gdb) reset
  (xt-gdb) load
  (xt-gdb) set $pc = &_ResetVector
  (xt-gdb) symbol-file <workdir>/build-xtav60/vmlinux
  (xt-gdb) c
 
where <ocdhost> is the IP address or DNS name of the machine
running the Xtensa OCD daemon.  The Linux kernel should
start booting as soon as 'c' (continue) is executed.
 
You should eventually get a login prompt.
Just login as root (no password).
 
Try various Linux commands.  Look at <tt>/bin</tt>,
<tt>/sbin</tt>, <tt>/usr/bin</tt>, etc to see what's available.
 
Note:  no need to setup an NFS or TFTP server.  The filesystem
is contained within the kernel image.  You may be able to mount
other filesystems over NFS though, if you wish, after booting.
 
=== Mounting the Root Filesystem Over NFS ===
 
If you build more than the minimal set of packages using
buildroot, the root filesystem can easily grow too large to
fit comfortably (or at all) inside the kernel.  In this case,
rather than follow the instructions exactly as above,
you'll want to mount the root filesystem over NFS.
 
You may also want to mount the root filesystem over NFS to
make changes to the filesystem persistent across runs and
accessible from your development host.
 
First you need to install the root filesystem generated
using buildroot, to a machine that can serve it over NFS.
Assuming this machine runs Linux, one way to install it is
as follows:
 
mkdir ''<exportpath>''
cd ''<exportpath>''
sudo cpio -i -d -m -F ''<workdir>''/buildroot/binaries/uclibc/rootfs.xtensa_''<cname>''.cpio --no-absolute-filenames
 
You then need to enable the NFS server on your system
(if not already active), and export this filesystem.
How you do this depends on your host Linux distribution.
For example, on some Fedora releases, one might:
 
* add the export path to <tt>/etc/exports</tt> using a line such as
 
''<exportpath>''  *(rw,sync,no_root_squash,no_all_squash)
 
* turn on the NFS server if needed:
 
sudo /sbin/chkconfig nfs on
 
* tell the NFS server about the new exported filesystem
 
sudo /usr/sbin/exportfs -a
sudo /usr/sbin/exportfs -r
 
 
 
Second, follow the instructions in the previous sections,
then configure the kernel again as follows.
 
Under '''General setup''', make sure the
'''Initial RAM filesystem and RAM disk (initramfs/initrd) support'''
entry is disabled (rather than enabled as done in previous sections).
 
Under '''Platform options''', under the line '''Default bootloader kernel arguments''',
select the boot parameters line and set it to something like this:
 
console=ttyS0,38400 ip=dhcp root=/dev/nfs rw nfsroot=''<server_ip>'':''<exportpath>''
 
where ''<server_ip>'' is the IP address of your NFS server, and ''<exportpath>''
is the exported filesystem path.
 
Save this configuration, rebuild the kernel, and follow the
instructions in previous sections to download and run the kernel.
If everything goes well and is setup correctly, your kernel should
now boot with a root filesystem mounted over NFS.
 
== Build and Run a Linux Kernel on the Instruction Set Simulator (ISS) ==
 
You can build and run a Linux kernel in the Xtensa Instruction Set
Simulator (ISS).  The following instructions have only been tried
with an ISS from the RB-2008.3 release of Xtensa Tools.  Bear in
mind that the XTAV60 port is likely much more stable than the ISS port
of Linux at this point in time.
 
Note:  Depending on your host OS version, you may need to install
Xtensa Tools on a separate machine.
 
=== Configure and Build the Kernel for ISS ===
 
Repeat all the same instructions as above for building the kernel for the
XTAV60 board, except that when initially configuring the kernel,
start from the ISS platform template instead:
 
  $ mkdir build-iss
  $ cd linux
  $ make O=../build-iss ARCH=xtensa KBUILD_DEFCONFIG=iss_defconfig defconfig
 
This configures the kernel using the default configuration found in
arch/xtensa/configs/iss_defconfig.
 
Also, when running menuconfig:
 
  $ make O=../build-iss ARCH=xtensa  menuconfig
 
in addition to setting up the initramfs filesystem, do the following.
Under '''Bus Options''', deselect '''PCI support'''.
Under '''Platform Options''', deselect '''Default bootloader kernel arguments'''.
 
 
Continue configuring and building the kernel as usual.
 
===  Run the Kernel on ISS ===
 
Using Xtensa Tools, invoke:
 
  $ xt-gdb <workdir>/build-iss/arch/xtensa/boot/Image.elf
 
  (xt-gdb) target sim --turbo
  (xt-gdb) symbol-file <workdir>/build-iss/vmlinux
  (xt-gdb) run
 
Wait patiently while Linux boots ... (maybe a minute)
 
Login as root (no password).
 
Note:  Input via ISS is cooked, so everything typed gets echoed.
Also, time as reported by the kernel does not progress
according to true wall-clock time:  it currently depends on
a simulated processor clock, which advances at various speeds
according to load.

Latest revision as of 01:21, 30 August 2019

This page describes what parts are necessary in order to run GNU/Linux OS on Xtensa, their interaction and setup.

GNU/Linux on Xtensa

GNU is a collection of free software that provides POSIX environment. Linux is an operating system kernel. In order to boot GNU/Linux OS on Xtensa the following parts are needed:

  • a board with Xtensa core supported by Linux, or Xtensa simulator
  • bootable Linux kernel image
  • GNU software packaged in a form suitable for booting on Linux, the so called root file system
  • a bootloader or on-chip debugger capable of loading Linux kernel image into the board RAM

To build Linux image and root file system a toolchain correctly configured for used Xtensa core is needed. Toolchain configuration is performed by application of Xtensa configuration overlay. Correct toolchain configuration is critical for building other pieces of software correctly.

Xtensa cores supported by Linux

Official requirements for Xtensa core to run Linux are listed in the Xtensa OSKit Guide document. The absolute minimum set of options required to run Linux is windowed registers, exception option and one level-1 or medium level timer interrupt.

  • Full MMU option is required for full featured Linux kernel that includes process address space isolation, virtual memory and fork syscall.
  • Full MMU option, thread pointer option and conditional store option are required for running userspace with NPTL or TLS.

Xtensa boards supported by Linux

The following boards collectively referred as XTFPGA family are readily supported by Linux: Avnet LX60, LX110, LX200; Xilinx ML605, KC705. For board details/setup guide please see corresponding Board Guide or Board Prototyping Guide provided with Xtensa software release.

Xtensa simulators

  • Xtensa software provides Instruction Set Simulator (xt-run) capable of running Linux configured for ISS.
  • QEMU is a free system emulator capable of emulating any XTFPGA board or ISS. It can be configured to emulate any Xtensa core, but it does not currently support TIE extensions to the core architecture. Please see this page for more details.

Linux kernel

Linux kernel needs to be configured for the specific Xtensa core. For that reason Linux is usually configured and built separately from the userspace software. Please see this page for more details.

Root file system

Currently we support building GNU software and packaging it into root file system with the Buildroot. There are several types of root file system that Linux can boot with:

CPIO archive

  • CPIO archive may be built into the kernel (so called 'built-in initramfs'), the resulting single image will contain both Linux and GNU software. It's easy to boot, but in order to change anything the kernel needs to be rebuilt.
  • CPIO archive may also be loaded separately from the kernel, but used in the same way as the built-in initramfs. It's harder to boot (two files instead of one), but easier to change.

Both cases of CPIO use RAM for the root file system, so they're very fast. The downsides are that they take some time at startup to unpack, reserve some RAM (which may be an issue on low-memory boards like LX60) and are not persistent.

EXT2/3/4 file system image

EXT2/3/4 file system image may be used directly with ISS or QEMU through the Simulated block device Linux feature. Please see this chapter for more details.

TAR image

Tar image may be unpacked into a directory shared through NFS. Linux on XTFPGA boards and on QEMU can be configured to use NFS root file system.

UBIFS image

UBIFS image may be written directly to the daughterboard FLASH on XTFPGA boards. Please see this chapter for more details.

Bootloader

U-Boot can be used as a bootloader on XTFPGA boards. Please see this page for more details about installing it into the daughterboard FLASH.

On-chip debugger

On-chip debugger is good for debugging lowest level software: bootloader and Linux kernel itself. It also allows loading executable images into new board with empty FLASH. It consists of JTAG probe connected to the XTFPGA daughterboard (ML605 and KC705 daughterboards also have built-in JTAG chip), OCD monitor (xt-ocd) running on developer machine where the other end of JTAG probe is connected, and xt-gdb, the modified version of GNU debugger from the Xtensa software release. Please see Tensilica Debug User's Guide for more information about OCD setup and configuration.

Building GNU/Linux OS

In order to follow these instruction a build machine with GNU/Linux and a set of development tools (native binutils and gcc, make, wget, tar, ...) is needed. In the following example all software will be downloaded and built in the current directory. Xtensa tools used in the examples are installed into $XT_ROOT, tools release is $XT_RELEASE and selected core is $XT_CORE.

Prepare the overlay

Please see this page for overlay repacking script and details of the overlay source location. Create conversion script by pasting this into the file overlay-script and making it executable: 

 $ cat > overlay-script
 ....
 ^D
 $ chmod +x overlay-script

Run it and rename the resulting file: 

 $ ./overlay-script ${XT_ROOT}/XtDevTools/install/builds/${XT_RELEASE}-linux/${XT_CORE}/src/xtensa-config-overlay.tar.gz
 $ mv xtensa-overlay.tar xtensa_${XT_CORE}.tar

Get, configure and build the toolchain and root file system

Get the buildroot git tree: 

 $ git clone git://git.buildroot.net/buildroot

Check out some released version: 

 $ git checkout 2014.11

Create build directory and configure it: 

 $ mkdir buildroot-build
 $ make -C buildroot O=$(pwd)/buildroot-build menuconfig

Please see this chapter for more details about configuring the Buildroot.

Select 'Target Architecture' = Xtensa, 'Target Architecture Variant' = Custom Xtensa processor configuration, set 'Overlay file for custom configuration ' to the location of the overlay archive (it may be a path to a local file or an URL).

Select toolchain options, packages to be built and final root file system image type. Select whether the kernel need to be built, kernel version and configuration.

Build the toolchain, kernel and the root file system: 

 $ make -C buildroot O=$(pwd)/buildroot-build

The kernel may be built on its own

Get stable Linux release tree: 

 $ wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.19.1.tar.xz
 $ tar -xf linux-3.19.1.tar.xz

Create variant directory for the specific core and copy core configuration files there: 

 $ mkdir -p linux-3.19.1/arch/xtensa/variants/${XT_CORE}/include/variant
 $ cp ${XT_ROOT}/XtDevTools/install/builds/${XT_RELEASE}-linux/${XT_CORE}/config/*.h linux-3.19.1/arch/xtensa/variants/${XT_CORE}/include/variant

Create build directory and configure it: 

 $ mkdir linux-build
 $ ARCH=xtensa CROSS_COMPILE=$(pwd)/buildroot-build/host/usr/bin/xtensa-buildroot-linux-uclibc- make -C linux-3.19.1 O=$(pwd)/linux-build menuconfig

Select 'Xtensa Processor Configuration' = Custom Xtensa processor configuration, set 'Xtensa Processor Custom Core Variant Name' to the contents of ${XT_CORE}. Select desired 'Xtensa System Type'.

Please see this chapter for more details about configuring Linux.

Build the kernel: 

 $ ARCH=xtensa CROSS_COMPILE=$(pwd)/buildroot-build/host/usr/bin/xtensa-buildroot-linux-uclibc- make -C linux-3.19.1 O=$(pwd)/linux-build all

Booting GNU/Linux

TBD

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