[U-Boot] [RFC PATCH 0/4] Run-time configuration of U-Boot via a flat device tree (fdt)

[Simon Glass]

At present in U-Boot configuration is mostly done using CONFIG options in the
board file. This patch set 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.

To read about fdts, 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

To use this patch set you will need to get the device tree compiler here:

git://jdl.com/software/dtc.git

and add some defines to your board (only ARM is currently supported):

 #define CONFIG_OF_CONTROL       (to enable run-time config control via fdt)
 #define CONFIG_OF_EMBED or CONFIG_OF_SEPARATE
     (either build the fdt blob into U-Boot, or create a separate u-boot.dtb)
 #define CONFIG_DEFAULT_DEVICE_TREE	"<your name>"
     (to specify the name of the device tree file is
      board/<vendor>/<board>/<your name>.dts)

This patch set does not include any drivers which actually use the fdt. I have
some concerns about spreading fdt code around the U-Boot code base so am
thinking of having a support file which makes this easier. I can provide a
UART driver modified to use fdt if there is interest.

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.

While only ARM is supported in this patch series, it should be easy enough to
add support for other architectures.


Simon Glass (4):
  fdt: ARM: Add device tree control of U-Boot (CONFIG_OF_CONTROL)
  fdt: Add support for embedded device tree (CONFIG_OF_EMBED)
  fdt: Add support for a separate device tree (CONFIG_OF_SEPARATE)
  fdt: ARM: Implement embedded and separate device tree

 .gitignore                         |    1 +
 Makefile                           |    9 ++
 README                             |   25 ++++++
 arch/arm/include/asm/global_data.h |    1 +
 arch/arm/lib/board.c               |   22 +++++
 config.mk                          |    1 +
 doc/README.fdt-control             |  168 ++++++++++++++++++++++++++++++++++++
 dts/Makefile                       |  100 +++++++++++++++++++++
 include/common.h                   |    1 +
 9 files changed, 328 insertions(+), 0 deletions(-)
 create mode 100644 doc/README.fdt-control
 create mode 100644 dts/Makefile

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1.7.3.1