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

Simon Glass
2011-09-12T22:04:24Z

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.

Signed-off-by: Simon Glass <sjg@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@chromium.org>
+1-Sep-11
--
1.7.3.1