[U-Boot] [RFC PATCH v2 2/6] fdt: Add support for embedded device tree (CONFIG_OF_EMBED)

Wed Sep 14 18:45:28 CEST 2011

On Mon, Sep 12, 2011 at 03:04:23PM -0700, Simon Glass wrote:
> This new option allows U-Boot to embed a binary device tree into its image
> to allow run-time control of peripherals. This device tree is for U-Boot's
> own use and is not necessarily the same one as is passed to the kernel.
> 
> The device tree compiler output should be placed in the $(obj)
> rooted tree. Since $(OBJCOPY) insists on adding the path to the
> generated symbol names, to ensure consistency it should be
> invoked from the directory where the .dtb file is located and
> given the input file name without the path.
> 
> This commit contains my entry for the ugliest Makefile / shell interaction
> competition.
> 
> Signed-off-by: Simon Glass <sjg at chromium.org>

May I suggest an alternate approach?  Rather than hard linking the dtb
into the u-boot image, this would be so much more useful if the dtb
can be concatenated to the u-boot binary so that it can be configured
at install time.  Otherwise, switching to .dtb doesn't seem to
actually buy much when it still requires a recompile of u-boot to
change the dtb configuration data.

The linker script would need to be modified to make sure the end of
the binary image is aligned, and that there is a label indicating the
beginning of the .dtb section.  The init code will also need to read
the .dtb header to get the dtb length so that it can be relocated into
RAM with the rest of u-boot.

Targets could even be added as u-boot.%.bin so that a single build
could spit out several variants as needed, but still produce a bare
u-boot.bin binary that can have the .dtb appended manually.

Using CONFIG_OF_EMBED may actually be harmful if it starts encouraging
developers to put .dts files into the u-boot tree.  Especially when
right now the plan for the kernel is to actually move .dts file out of
the Linux tree and into a separate & neutral repository.

> ---
>  Makefile         |    4 ++
>  README           |   11 +++++-
>  config.mk        |    1 +
>  dts/Makefile     |  100 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/common.h |    1 +
>  5 files changed, 115 insertions(+), 2 deletions(-)
>  create mode 100644 dts/Makefile
> 
> diff --git a/Makefile b/Makefile
> index d5a1f0a..658a622 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -224,6 +224,9 @@ endif
>  ifeq ($(CPU),ixp)
>  LIBS += arch/arm/cpu/ixp/npe/libnpe.o
>  endif
> +ifeq ($(CONFIG_OF_EMBED),y)
> +LIBS += dts/libdts.o
> +endif
>  LIBS += arch/$(ARCH)/lib/lib$(ARCH).o
>  LIBS += fs/cramfs/libcramfs.o fs/fat/libfat.o fs/fdos/libfdos.o fs/jffs2/libjffs2.o \
>  	fs/reiserfs/libreiserfs.o fs/ext2/libext2fs.o fs/yaffs2/libyaffs2.o \
> @@ -962,6 +965,7 @@ clobber:	clean
>  	@rm -f $(obj)u-boot.kwb
>  	@rm -f $(obj)u-boot.imx
>  	@rm -f $(obj)u-boot.ubl
> +	@rm -f $(obj)u-boot.dtb
>  	@rm -f $(obj)tools/{env/crc32.c,inca-swap-bytes}
>  	@rm -f $(obj)arch/powerpc/cpu/mpc824x/bedbug_603e.c
>  	@rm -fr $(obj)include/asm/proc $(obj)include/asm/arch $(obj)include/asm
> diff --git a/README b/README
> index 812805f..5a2f060 100644
> --- a/README
> +++ b/README
> @@ -803,8 +803,15 @@ 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. This will
> -		be enabled in a future patch.
> +		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.
> +
> +		CONFIG_OF_EMBED
> +		If this variable is defined, U-Boot will embed a device tree
> +		binary in its image. This device tree file should be in the
> +		board directory and called <soc>-<board>.dts. The binary file
> +		is then picked up in board_init_f() and made available through
> +		the global data structure as gd->blob.
>  
>  - Watchdog:
>  		CONFIG_WATCHDOG
> diff --git a/config.mk b/config.mk
> index e2b440d..6e61eb6 100644
> --- a/config.mk
> +++ b/config.mk
> @@ -124,6 +124,7 @@ STRIP	= $(CROSS_COMPILE)strip
>  OBJCOPY = $(CROSS_COMPILE)objcopy
>  OBJDUMP = $(CROSS_COMPILE)objdump
>  RANLIB	= $(CROSS_COMPILE)RANLIB
> +DTC	= dtc
>  
>  #########################################################################
>  
> diff --git a/dts/Makefile b/dts/Makefile
> new file mode 100644
> index 0000000..e70a16b
> --- /dev/null
> +++ b/dts/Makefile
> @@ -0,0 +1,100 @@
> +#
> +# 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
> +#
> +
> +# This Makefile builds the internal U-Boot fdt if CONFIG_OF_CONTROL is
> +# enabled. See doc/README.fdt-control for more details.
> +
> +include $(TOPDIR)/config.mk
> +
> +LIB	= $(obj)libdts.o
> +
> +$(if $(CONFIG_DEFAULT_DEVICE_TREE),,\
> +$(error Please define CONFIG_DEFAULT_DEVICE_TREE in your board header file))
> +DEVICE_TREE = $(subst ",,$(CONFIG_DEFAULT_DEVICE_TREE))
> +
> +all:	$(obj).depend $(LIB)
> +
> +# Use a constant name for this so we can access it from C code.
> +# objcopy doesn't seem to allow us to set the symbol name independently of
> +# the filename.
> +DT_BIN	:= $(obj)dt.dtb
> +
> +$(DT_BIN): $(TOPDIR)/board/$(BOARDDIR)/$(DEVICE_TREE).dts
> +	$(DTC) -R 4 -p 0x1000 -O dtb -o ${DT_BIN} $<
> +
> +process_lds = \
> +	$(1) | sed -r -n 's/^OUTPUT_$(2)[ ("]*([^")]*).*/\1/p'
> +
> +# Run the compiler and get the link script from the linker
> +GET_LDS = $(CC) $(CFLAGS) $(LDFLAGS) -Wl,--verbose 2>&1
> +
> +$(obj)dt.o: $(DT_BIN)
> +	# We want the output format and arch.
> +	# We also hope to win a prize for ugliest Makefile / shell interaction

:-)

> +	# We look in the LDSCRIPT first.
> +	# Then try the linker which should give us the answer.
> +	# Then check it worked.
> +	oformat=`$(call process_lds,cat $(LDSCRIPT),FORMAT)` ;\
> +	oarch=`$(call process_lds,cat $(LDSCRIPT),ARCH)` ;\
> +	\
> +	[ -z $${oformat} ] && \
> +		oformat=`$(call process_lds,$(GET_LDS),FORMAT)` ;\
> +	[ -z $${oarch} ] && \
> +		oarch=`$(call process_lds,$(GET_LDS),ARCH)` ;\
> +	\
> +	[ -z $${oformat} ] && \
> +		echo "Cannot read OUTPUT_FORMAT from lds file $(LDSCRIPT)" && \
> +		exit 1 || true ;\
> +	[ -z $${oarch} ] && \
> +		echo "Cannot read OUTPUT_ARCH from lds file $(LDSCRIPT)" && \
> +		exit 1 || true ;\
> +	\
> +	cd $(dir ${DT_BIN}) && \
> +	$(OBJCOPY) -I binary -O $${oformat} -B $${oarch} \
> +		$(notdir ${DT_BIN}) $@
> +	rm $(DT_BIN)

Or, you could use the assembler to do the heavy lifting for you (and
should be more reliable).  Look at the Linux piggy*.S files in
arch/arm/boot/compressed.  (Of course this point is moot if you switch
to appending the dtb).

> +
> +OBJS-$(CONFIG_OF_EMBED)	:= dt.o
> +
> +COBJS	:= $(OBJS-y)
> +
> +OBJS	:= $(addprefix $(obj),$(COBJS))
> +
> +binary:	$(DT_BIN)
> +
> +$(LIB):	$(OBJS) $(DTB)
> +	$(call cmd_link_o_target, $(OBJS))
> +
> +clean:
> +	rm -f $(OBJS) $(LIB)
> +	rm -f $(DT_BIN)
> +
> +distclean:	clean
> +
> +#########################################################################
> +
> +# defines $(obj).depend target
> +include $(SRCTREE)/rules.mk
> +
> +sinclude $(obj).depend
> +
> +#########################################################################
> diff --git a/include/common.h b/include/common.h
> index bd10f31..6cdcc50 100644
> --- a/include/common.h
> +++ b/include/common.h
> @@ -249,6 +249,7 @@ int	checkdram     (void);
>  int	last_stage_init(void);
>  extern ulong monitor_flash_len;
>  int mac_read_from_eeprom(void);
> +extern u8 _binary_dt_dtb_start[];	/* embedded device tree blob */
>  
>  /* common/flash.c */
>  void flash_perror (int);
> -- 
> 1.7.3.1
> 