Allow boards to initialize the DT at runtime.

In some boards like the Raspberry Pi the initial bootloader will pass
a DT to the kernel. When using U-Boot as such kernel, the board code in
U-Boot should be able to provide U-Boot with this, already assembled
device tree blob.

This patch introduces a new config option CONFIG_OF_BOARD to use instead
of CONFIG_OF_EMBED or CONFIG_OF_SEPARATE which will initialize the DT
from a board-specific funtion instead of bundling one with U-Boot or as
a separated file. This allows boards like the Raspberry Pi to reuse the
device tree passed from the bootcode.bin and start.elf firmware
files, including the run-time selected device tree overlays.

Signed-off-by: Alex Deymo <deymo@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

6 files changed, 38 insertions, 1 deletions

diff --git a/README b/README
index 78173e2d86..685a8222b0 100644
--- a/README
+++ b/README
@@ -968,7 +968,7 @@ The following options need to be configured:
 		tree is available in the global data as gd->fdt_blob.
 
 		U-Boot needs to get its device tree from somewhere. This can
-		be done using one of the two options below:
+		be done using one of the three options below:
 
 		CONFIG_OF_EMBED
 		If this variable is defined, U-Boot will embed a device tree
@@ -989,6 +989,12 @@ The following options need to be configured:
 		still use the individual files if you need something more
 		exotic.
 
+		CONFIG_OF_BOARD
+		If this variable is defined, U-Boot will use the device tree
+		provided by the board at runtime instead of embedding one with
+		the image. Only boards defining board_fdt_blob_setup() support
+		this option (see include/fdtdec.h file).
+
 - Watchdog:
 		CONFIG_WATCHDOG
 		If this variable is defined, it enables watchdog
diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index 82da9e5a5a..d3c6ba580f 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -470,6 +470,16 @@ int board_init(void)
 	return bcm2835_power_on_module(BCM2835_MBOX_POWER_DEVID_USB_HCD);
 }
 
+/*
+ * If the firmware passed a device tree use it for U-Boot.
+ */
+void *board_fdt_blob_setup(void)
+{
+	if (fdt_magic(fw_dtb_pointer) != FDT_MAGIC)
+		return NULL;
+	return (void *)fw_dtb_pointer;
+}
+
 int ft_board_setup(void *blob, bd_t *bd)
 {
 	/*
diff --git a/doc/README.fdt-control b/doc/README.fdt-control
index c965629905..378b06b108 100644
--- a/doc/README.fdt-control
+++ b/doc/README.fdt-control
@@ -130,6 +130,10 @@ u-boot-dtb.bin which does the above step for you also. If you are using
 CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
 tree binary.
 
+If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
+device tree at runtime, for example if an earlier bootloader stage creates
+it and passes it to U-Boot.
+
 If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
 startup. This is only useful for sandbox. Use the -d flag to U-Boot to
 specify the file to read.
diff --git a/dts/Kconfig b/dts/Kconfig
index 6fe7a5bc08..9a0622154a 100644
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -51,6 +51,14 @@ config OF_EMBED
 	  and development only and is not recommended for production devices.
 	  Boards in the mainline U-Boot tree should not use it.
 
+config OF_BOARD
+	bool "Provided by the board at runtime"
+	depends on !SANDBOX
+	help
+	  If this option is enabled, the device tree will be provided by
+	  the board at runtime if the board supports it, instead of being
+	  bundled with the image.
+
 config OF_HOSTFILE
 	bool "Host filed DTB for DT control"
 	depends on SANDBOX
diff --git a/include/fdtdec.h b/include/fdtdec.h
index 2134701c54..b0e5b2767d 100644
--- a/include/fdtdec.h
+++ b/include/fdtdec.h
@@ -1023,4 +1023,10 @@ int fdtdec_setup_memory_banksize(void);
  */
 int fdtdec_setup(void);
 
+/**
+ * Board-specific FDT initialization. Returns the address to a device tree blob.
+ * Called when CONFIG_OF_BOARD is defined.
+ */
+void *board_fdt_blob_setup(void);
+
 #endif
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 94372cc6cd..c072e54cff 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -1255,6 +1255,9 @@ int fdtdec_setup(void)
 	/* FDT is at end of image */
 	gd->fdt_blob = (ulong *)&_end;
 #  endif
+# elif defined(CONFIG_OF_BOARD)
+	/* Allow the board to override the fdt address. */
+	gd->fdt_blob = board_fdt_blob_setup();
 # elif defined(CONFIG_OF_HOSTFILE)
 	if (sandbox_read_fdt_from_file()) {
 		puts("Failed to read control FDT\n");