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.  They are based on a snapshot of work in progress.
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 =


*  The following was tested on an x86 machine running RedHat Enterprise Linux 4 (RHEL4) and Fedora Core 3 (FC3). It is expected to work on other Fedora releases (e.g. was tested on FC6) but to maintain host compatibility with Tensilica Tools, it is best to avoid Fedora releases beyond FC5.  Other host distributions likely work but have not been tested.
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:


* 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 subversion and git as a non-root user if root access is an issue.  How to do this isn't shown here.)
* 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


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


*  Instructions are assumed executed all in order (e.g. commands assume current directory and environment variables set earlier).
== 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 ==


== Ensure you have all needed packages installed on your system ==
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.


=== Install the 'Subversion' source control tools ===
== Xtensa simulators ==


Download and install the [http://subversion.tigris.org/ Subversion]
* Xtensa software provides Instruction Set Simulator (xt-run) capable of running Linux configured for ISS.
source control tools if you don't have them already installed (FC6 has it, FC3 doesn't):
* 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.


  $ sudo yum install subversion
== Linux kernel ==


This may install dependent packages (e.g. 'apr', etc).
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.


Documentation for Subversion is available online at http://svnbook.red-bean.com/
== Root file system ==
or by typing <code>svn help</code> at a shell prompt.


=== Download and install the 'git' source control tools ===
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:


Download and install the 'git' source control tools if you
=== CPIO archive ===
don't already have it installed.
* 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.


On FC6, you can just do the following
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.
(which installs documentation accessible with <code>man git</code>
or <code>git help</code>):


  $ sudo yum install git
=== 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 [[Linux_Kernel#ISS_platform|this]] chapter for more details.


On FC3, yum doesn't know about git, so you have to do something like:
=== 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.


  $ cd /tmp
=== UBIFS image ===
  $ lwp-download http://kernel.org/pub/software/scm/git/git-1.5.3.1.tar.bz2
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.
  $ tar xfj git-1.5.3.1.tar.bz2
  $ cd git-1.5.3.1
  $ make prefix=/usr/local all
  $ sudo make prefix=/usr/local install


This doesn't install git documentation.  For that, you have
== Bootloader ==
to build 'doc' and 'info' make targets, which require first
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.
installing other packages such as 'asciidoc' and others.
However, for this example, documentation is not necessary.
You can find online docs such as a
[http://www.kernel.org/pub/software/scm/git/docs/tutorial.html tutorial]
and [http://www.kernel.org/pub/software/scm/git/docs/ manual pages].


== Download buildroot and the Linux kernel ==
== 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.


=== Download buildroot sources ===
= 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.


Buildroot is used to build the toolchain and root filesystem.
== Prepare the overlay ==
First setup a work area:
Please see [[Toolchain_Overlay_File|this]] page for overlay repacking script and details of the overlay source location.
Create conversion script by pasting [[Toolchain_Overlay_File#Conversion_Script|this]] into the file overlay-script and making it executable:
  $ '''cat > overlay-script'''
  '''....'''
  ^D
  $ '''chmod +x overlay-script'''


  $ cd <workdir>
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'''


where <workdir> is the path to an empty directory on a disk with
== Get, configure and build the toolchain and root file system ==
at least 4 GB available space.
Get the buildroot git tree:
The following creates and populates the <workdir>/trunk directory:
  $ '''git clone git://git.buildroot.net/buildroot'''


  $ svn co svn://linux-xtensa.org/var/svn/repos/buildroot/trunk/
Check out some released version:
  $ '''git checkout 2014.11'''


=== Download Linux kernel sources ===
Create build directory and configure it:
  $ '''mkdir buildroot-build'''
  $ '''make -C buildroot O=$(pwd)/buildroot-build menuconfig'''


First you need to decide which kernel tree to work with.  The <code>xtensa-devel</code> tree contains the most recent changes but is not very stable -- it gets updated semi-regularly with the very latest Linux kernel tree from <code>kernel.org</code>.  The <code>xtensa-2.6.23</code> tree is a more stable tree based on the 2.6.23.n kernel from <code>kernel.org</code>.  You can also get both kernel source trees of course.
Please see [[Buildroot#Configuring_and_building_Buildroot|this]] chapter for more details about configuring the Buildroot.


To get the stable tree into directory <dir> (which must not already exist):
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).


  $ cd <workdir>
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.
  $ git clone git://git.linux-xtensa.org/kernel/xtensa-2.6.23 <dir>


This creates and populates the <workdir>/<dir> directory.  If <dir> is omitted, it defaults to <code>xtensa-2.6.23</code> (the last non-extension part of the cloned path).
Build the toolchain, kernel and the root file system:
  $ '''make -C buildroot O=$(pwd)/buildroot-build'''


Similarly, to get the development tree:
== 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'''


  $ cd <workdir>
Create variant directory for the specific core and copy core configuration files there:
  $ git clone git://git.linux-xtensa.org/kernel/xtensa-devel <dir>
  $ '''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'''


== Install any required processor specific overlay ==
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'.


This step is required if you are targeting a custom Xtensa processor.
Please see [[Linux_Kernel#Configuring_the_kernel|this]] chapter for more details about configuring Linux.
If you use a Diamond 232L Standard Core, you can skip this step.


To configure buildroot and the kernel to use a custom Xtensa processor
Build the kernel:
configuration, you need to overlay a set of files customized for your
  $ '''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'''
processor configuration onto toolchain sources (gcc, binutils, gdb, etc)
and the Linux kernel. This process has been automated using a script
located in the buildroot source tree.
For usage information, you can invoke it without arguments:


  $ cd <workdir>
= Booting GNU/Linux =
  $ ./trunk/target/xtensa/xt-buildroot-overlay-install
TBD
 
which displays a help message similar to the following:
 
xt-buildroot-overlay-install version 1.1
Usage:  xt-buildroot-overlay-install <parameters> [<options>]
Where <parameters> are:
  -t file.tgz    Specify path to the Xtensa Linux overlay tarball, typically
                  <xtensa_root>/xtensa-elf/src/linux/misc/linux-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
core package built using release RB-2008.3 or later.  Until this release
becomes available (sometime in early 2008), the overlay tarball can be obtained
by providing Tensilica customer support with your processor configuration
and TIE (such as, .xcf and .tdb files); they should be able to generate it
for active customers.
 
Here is an example invocation of the overlay installation script:
 
  $ ./trunk/target/xtensa/xt-buildroot-overlay-install -t blinkcore-linux-overlay.tar.gz \
        -b ./trunk -k ./xtensa-devel -c superzip -l "ChipCorp SuperZIP Blink Accelerator Core"
 
The script extracts some relevant information from the overlay tarball,
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.
It is also probably wise to avoid names confusingly similar to other (non-Xtensa) processors.
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
(in their respective <code>''make menuconfig''</code> steps) to select
the <code>''custom''</code> Xtensa processor option.
This presents an extra configuration menu entry where you must provide
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 ==
 
Note:  for more details on generic buildroot topics not covered in this simple how-to,
see [http://buildroot.uclibc.org/ buildroot.uclibc.org].
 
=== Change some default behavior ===
 
Edit  trunk/target/generic/target_skeleton/etc/inittab
 
Comment out the default getty's:
 
      # Set up a couple of getty's
      #tty1::respawn:/sbin/getty 38400 tty1
      #tty2::respawn:/sbin/getty 38400 tty2
 
We'll use the serial port getty.  Modify the baud rate from 115200 to 38400,
and un-comment the line:
 
      # Put a getty on the serial port
      ttyS0::respawn:/sbin/getty -L ttyS0 38400 vt100
 
=== Configure buildroot ===
 
Note:  You need the TERM environment variable properly set for curses
based tools to work.
 
  $ cd <workdir>/trunk
  $ make menuconfig
 
You should get a curses interface for configuring buildroot.
Make the following changes:
 
* Change '''Target Architecture''' from '''i386''' to '''xtensa'''
* Change '''Target Architecture Variant''' to desired Xtensa or Diamond core (e.g. '''dc232b''')
* Under '''Toolchain Options''', change '''Binutils Version''' from ''''2.17'''' to ''''2.17.50.0.17''''
* Under '''Toolchain Options''', make sure '''uClibc C library Version''' is set to '''(uClibc 0.9.29)'''
* Under '''Target Options''', enable '''cpio the root filesystem'''; this creates a '''Compression method''' submenu; under this submenu, select '''gzip'''
* Again under '''Target Options''', enable '''initramfs for initial ramdisk of linux kernel'''
* Exit and save the configuration
 
=== Configure uClibc ===
 
  $ make uclibc-config
 
=== Build buildroot ===
 
  $ make
 
THIS WILL TAKE A LONG TIME (perhaps an hour or many, depending on your system).
 
Wait patiently for 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 'make' command does not check many dependencies, so it will usually NOT rebuild things that depend on whatever you have fixed.  Here's one possible sequence to retry the make without having to wipe out the 'trunk' directory tree completely and start again from the svn checkout.  (Caveat emptor: this sequence has not yet been tested.)
 
  $ rm -rf *build_xtensa* binaries
  $ make uclibc-config
  $ make
 
It should not be necessary to empty the 'dl' subdirectory, which contains tarballs downloaded from the Internet, or the .config file, which contains the buildroot configuration.  It does seem safer to delete the uClibc configuration.
 
== Build a Linux kernel ==
 
=== Configure the kernel ===
 
The 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>/trunk/build_xtensa/staging_dir/usr/bin:$PATH"
 
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.
 
  $ mkdir build-kernel
 
Let's start with the default configuration for the XTAV60 (LX60)
board as follows:
 
  $ cd xtensa-devel
  $ make O=../build-kernel ARCH=xtensa KBUILD_DEFCONFIG=lx60_defconfig defconfig
 
This configures the kernel using the default configuration found in
arch/xtensa/configs/lx60_defconfig.
 
Let's customize this a little bit to select the correct processor configuration,
and to bundle the root filesystem into the kernel:
 
  $ make O=../build-kernel 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>/trunk/binaries/uclibc/rootfs.xtensa.cpio.gz
 
(Don't forget to replace <workdir> with the appropriate full path.)
 
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.
 
Exit menuconfig, saving your configuration changes.
 
Backup your configuration outside the build directory.  For example:
 
  $ cp ../build-kernel/.config config.saved
 
If you do a clean rebuild of the kernel (e.g. "rm -rf ../build-kernel")
you can now configure it with simply:
          mkdir ../build-kernel ; cp config.saved ../build-kernel/.config
Of course if you update the kernel, you'll probably again have to do:
          make O=../build-kernel ARCH=xtensa  menuconfig
and save a new copy of your configuration.
 
=== Build the kernel ===
 
  $ make O=../build-kernel ARCH=xtensa
 
The build will issue a few warnings which are normal.
Once complete, the bootable image is in:
      <workdir>/build-kernel/arch/xtensa/boot/Image.elf
and the uncompressed ELF file containing kernel symbols is in:
      <workdir>/build-kernel/vmlinux
There is also a RedBoot bootable image (if you use RedBoot) in:
      <workdir>/build-kernel/arch/xtensa/boot/zImage.redboot
 
== Run the resulting kernel ==
 
=== Install Xtensa Tools with the 232L core ===
 
This can be done a a different machine than the one used so far,
as long as it 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 LX60 serial port.
 
=== Optionally, setup networking ===
 
This step is optional.
 
Setup the board's MAC address using dipswitches
(see LX60 docs) and connect the LX60 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 LX60 board (see board documentation).
 
Using Tensilica Tools, invoke:
 
  $ xt-gdb <workdir>/build-kernel/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-kernel/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 /bin, /sbin, /usr/bin,
etc to see what's available.  Not all guaranteed to work,
but basic things like 'ls' and 'ping' seem to work fine.
 
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.
 
== Building and Running a Kernel in Simulation (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 release RB-2007.2 of Tensilica Tools.  Bear in
mind that the LX60 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
Tensilica Tools on a separate machine.
 
=== Building a Kernel for ISS ===
 
Repeat all the same instructions as above for building the kernel for the
XTAV60 (LX60) board, except that when initially configuring the kernel,
start from the ISS platform template instead:
 
  $ cd xtensa-devel
  $ make O=../build-kernel ARCH=xtensa CROSS_COMPILE=xtensa_dc232b-linux-uclibc-  \
        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-kernel ARCH=xtensa CROSS_COMPILE=xtensa_dc232b-linux-uclibc-  \
        menuconfig
 
in addition to setting up the initramfs filesystem, do the following.
Under '''Bus Options''', deselect '''PCI support'''.
Under '''Platform Options''', deselect '''Default booloader kernel arguments'''.
Under '''Processor type and features''', under '''Xtensa Processor Configuration''', select '''dc232l'''.
 
 
Continue configuring and building the kernel as usual.  Note that if you
had built kernel images for another platform such as the XTAV60 (LX60)
board, this will overwrite them.
 
===  Running a Kernel on ISS ===
 
Using Tensilica Tools, invoke:
 
  $ xt-gdb <workdir>/build-kernel/arch/xtensa/boot/Image.elf
 
  (xt-gdb) target sim --turbo
  (xt-gdb) symbol-file <workdir>/build-kernel/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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