[U-Boot] [RFC PATCH v2 3/6] fdt: Add support for a separate device tree (CONFIG_OF_SEPARATE)

Wed Sep 14 18:48:30 CEST 2011

On Mon, Sep 12, 2011 at 03:04:24PM -0700, Simon Glass wrote:
> This adds support for an FDT to be build as a separate binary file called
> u-boot.dtb. This can be concatenated with the U-Boot binary to provide a
> device tree located at run-time by U-Boot.

Hahaha, I should the entire thread of patches before commenting.  I
strongly feel that this should be the only method and that
CONFIG_OF_EMBED should be dropped.

> 
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>  .gitignore             |    1 +
>  Makefile               |    5 ++
>  README                 |   11 +++-
>  doc/README.fdt-control |  168 ++++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 183 insertions(+), 2 deletions(-)
>  create mode 100644 doc/README.fdt-control
> 
> diff --git a/.gitignore b/.gitignore
> index dbf545f..c4ebd34 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -34,6 +34,7 @@
>  /u-boot.dis
>  /u-boot.lds
>  /u-boot.ubl
> +/u-boot.dtb
>  
>  #
>  # Generated files
> diff --git a/Makefile b/Makefile
> index 658a622..d73efa1 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -352,9 +352,14 @@ ALL-$(CONFIG_ONENAND_U_BOOT) += $(obj)u-boot-onenand.bin
>  ONENAND_BIN ?= $(obj)onenand_ipl/onenand-ipl-2k.bin
>  ALL-$(CONFIG_MMC_U_BOOT) += $(obj)mmc_spl/u-boot-mmc-spl.bin
>  ALL-$(CONFIG_SPL) += $(obj)spl/u-boot-spl.bin
> +ALL-$(CONFIG_OF_SEPARATE) += $(obj)u-boot.dtb
>  
>  all:		$(ALL-y)
>  
> +$(obj)u-boot.dtb:	$(obj)u-boot
> +		$(MAKE) -C dts binary
> +		mv $(obj)dts/dt.dtb $@
> +
>  $(obj)u-boot.hex:	$(obj)u-boot
>  		$(OBJCOPY) ${OBJCFLAGS} -O ihex $< $@
>  
> diff --git a/README b/README
> index 5a2f060..0b8f338 100644
> --- a/README
> +++ b/README
> @@ -803,8 +803,8 @@ The following options need to be configured:
>  		experimental and only available on a few boards. The device
>  		tree is available in the global data as gd->blob.
>  
> -		U-Boot needs to get its device tree from somewhere. At present
> -		the only way is to embed it in the image with CONFIG_OF_EMBED.
> +		U-Boot needs to get its device tree from somewhere. This can
> +		be done using one of the two options below:
>  
>  		CONFIG_OF_EMBED
>  		If this variable is defined, U-Boot will embed a device tree
> @@ -813,6 +813,13 @@ The following options need to be configured:
>  		is then picked up in board_init_f() and made available through
>  		the global data structure as gd->blob.
>  
> +		CONFIG_OF_SEPARATE
> +		If this variable is defined, U-Boot will build a device tree
> +		binary. It will be called u-boot.dtb. Architecture-specific
> +		code will locate it at run-time. Generally this works by:
> +
> +			cat u-boot.bin u-boot.dtb >image.bin
> +
>  - Watchdog:
>  		CONFIG_WATCHDOG
>  		If this variable is defined, it enables watchdog
> diff --git a/doc/README.fdt-control b/doc/README.fdt-control
> new file mode 100644
> index 0000000..dfc8f06
> --- /dev/null
> +++ b/doc/README.fdt-control
> @@ -0,0 +1,168 @@
> +#
> +# Copyright (c) 2011 The Chromium OS Authors.
> +#
> +# See file CREDITS for list of people who contributed to this
> +# project.
> +#
> +# This program is free software; you can redistribute it and/or
> +# modify it under the terms of the GNU General Public License as
> +# published by the Free Software Foundatio; either version 2 of
> +# the License, or (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program; if not, write to the Free Software
> +# Foundation, Inc., 59 Temple Place, Suite 330, Boston,
> +# MA 02111-1307 USA
> +#
> +
> +Device Tree Control in U-Boot
> +=============================
> +
> +This feature provides for run-time configuration of U-Boot via a flat
> +device tree (fdt). U-Boot configuration has traditionally been done
> +using CONFIG options in the board config file. This feature aims to
> +make it possible for a single U-Boot binary to support multiple boards,
> +with the exact configuration of each board controlled by a flat device
> +tree (fdt). This is the approach recently taken by the ARM Linux kernel
> +and has been used by PowerPC for some time.
> +
> +The fdt is a convenient vehicle for implementing run-time configuration
> +for three reasons. Firstly it is easy to use, being a simple text file.
> +It is extensible since it consists of nodes and properties in a nice
> +hierarchical format.
> +
> +Finally, there is already excellent infrastructure for the fdt: a
> +compiler checks the text file and converts it to a compact binary
> +format, and a library is already available in U-Boot (libfdt) for
> +handling this format.
> +
> +The dts directory contains a Makefile for building the device tree blob
> +and embedding it in your U-Boot image. This is useful since it allows
> +U-Boot to configure itself according to what it finds there. If you have
> +a number of similar boards with different peripherals, you can describe
> +the features of each board in the device tree file, and have a single
> +generic source base.
> +
> +To enable this feature, add CONFIG_OF_CONTROL to your board config file.
> +
> +
> +What is a Flat Device Tree?
> +---------------------------
> +
> +An fdt can be specified in source format as a text file. To read about
> +the fdt syntax, take a look at the specification here:
> +
> +https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
> +
> +You also might find this section of the Linux kernel documentation
> +useful: (access this in the Linux kernel source code)
> +
> +	Documentation/devicetree/booting-without-of.txt
> +
> +There is also a mailing list:
> +
> +	http://lists.ozlabs.org/listinfo/devicetree-discuss
> +
> +In case you are wondering, OF stands for Open Firmware.
> +
> +
> +Tools
> +-----
> +
> +To use this feature you will need to get the device tree compiler here:
> +
> +	git://jdl.com/software/dtc.git
> +
> +For example:
> +
> +	$ git clone git://jdl.com/software/dtc.git
> +	$ cd dtc
> +	$ make
> +	$ sudo make install
> +
> +Then run the compiler (your version will vary):
> +
> +	$ dtc -v
> +	Version: DTC 1.2.0-g2cb4b51f
> +	$ make tests
> +	$ cd tests
> +	$ ./run_tests.sh
> +	********** TEST SUMMARY
> +	*     Total testcases:	1371
> +	*                PASS:	1371
> +	*                FAIL:	0
> +	*   Bad configuration:	0
> +	* Strange test result:	0
> +
> +You will also find a useful ftdump utility for decoding a binary file.
> +
> +
> +Where do I get an fdt file for my board?
> +----------------------------------------
> +
> +You may find that the Linux kernel has a suitable file. Look in the
> +kernel source in arch/<arch>/boot/dts.
> +
> +If not you might find other boards with suitable files that you can
> +modify to your needs. Look in the board directories for files with a
> +.dts extension.
> +
> +Failing that, you could write one from scratch yourself!
> +
> +
> +Configuration
> +-------------
> +
> +Use:
> +
> +#define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
> +
> +to set the filename of the device tree source. Then put your device tree
> +file into
> +
> +	board/<vendor>/<board>/<name>.dts
> +
> +If CONFIG_OF_EMBED is defined, then it will be picked up and built into
> +the U-Boot image (including u-boot.bin).
> +
> +If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
> +a u-boot.dtb file alongside u-boot.bin. A common approach is then to
> +join the two:
> +
> +	cat u-boot.bin u-boot.dtb >image.bin
> +
> +and then flash image.bin onto your board.
> +
> +You cannot use both of these options at the same time.
> +
> +
> +Limitations
> +-----------
> +
> +U-Boot is designed to build with a single architecture type and CPU
> +type. So for example it is not possible to build a single ARM binary
> +which runs on your AT91 and OMAP boards, relying on an fdt to configure
> +the various features. This is because you must select one of
> +the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
> +time. Similarly you cannot build for multiple cpu types or
> +architectures.
> +
> +That said the complexity reduction by using fdt to support variants of
> +boards which use the same SOC / CPU can be substantial.
> +
> +It is important to understand that the fdt only selects options
> +available in the platform / drivers. It cannot add new drivers (yet). So
> +you must still have the CONFIG option to enable the driver. For example,
> +you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
> +but can use the fdt to specific the UART clock, peripheral address, etc.
> +In very broad terms, the CONFIG options in general control *what* driver
> +files are pulled in, and the fdt controls *how* those files work.
> +
> +--
> +Simon Glass <sjg at chromium.org>
> +1-Sep-11
> -- 
> 1.7.3.1
> 