fdt: Add a Kconfig for boards with a prior stage

When U-Boot is started from another firmware program, not just a prior
phase of U-Boot, special behaviour is typically used. In particular, the
device tree may come from that prior stage.

At present this is sort-of indicated by OF_BOARD, although the
correlation is not 1:1, since that option simply means that the board has
a custom mechanism for obtaining the device tree. For example, sandbox
defines OF_BOARD. Also the board_fdt_blob_setup() function can in fact
make use of the devicetree in U-Boot if it wishes, as used by
dragonboard410c until very recently.

Add an explicit Kconfig for this situation. Update the OF_BOARD option to
more-accurately reflect what it is doing, e.g. for sandbox.

Drop the docs in the README as it is out of date.

Signed-off-by: Simon Glass <sjg@chromium.org>

diff --git a/README b/README
index 056b717..3496bef 100644 (file)
--- a/README
+++ b/README
@@ -646,35 +646,6 @@ The following options need to be configured:
                which adds regex support to some commands, as for
                example "env grep" and "setexpr".
 
-- Device tree:
-               CONFIG_OF_CONTROL
-               If this variable is defined, U-Boot will use a device tree
-               to configure its devices, instead of relying on statically
-               compiled #defines in the board file. This option is
-               experimental and only available on a few boards. The device
-               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 three options below:
-
-               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
-
-               and in fact, U-Boot does this for you, creating a file called
-               u-boot-dtb.bin which is useful in the common case. You can
-               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/dts/Kconfig b/dts/Kconfig
index 5dcc79d..e57ce84 100644 (file)
--- a/dts/Kconfig
+++ b/dts/Kconfig
@@ -98,13 +98,36 @@ endchoice
 
 config OF_BOARD
        bool "Provided by the board (e.g a previous loader) at runtime"
-       default y if SANDBOX
+       default y if SANDBOX || OF_HAS_PRIOR_STAGE
        help
-         If this option is enabled, the device tree will be provided by
-         the board at runtime if the board supports it. The device tree bundled
-         with the image (if any) will be overridden / ignored.
+         If this option is enabled, the device tree is provided at runtime by
+         a custom function called board_fdt_blob_setup(). The board must
+         implement this function if it wishes to provide special behaviour.
 
-         A device tree file must be provided in the tree.
+         With this option, the device tree build by U-Boot may be overridden or
+         ignored. See OF_HAS_PRIOR_STAGE.
+
+         Note: Boards which use this to handle a device tree passed from an
+         earlier stage should enable OF_HAS_PRIOR_STAGE.
+
+config OF_HAS_PRIOR_STAGE
+       bool
+       help
+         Indicates that a prior stage of the firmware (before U-Boot proper)
+         makes use of device tree and this board normally boots with that prior
+         stage, that provides a devicetree to U-Boot.
+
+         This means that the device tree built in U-Boot should not be packaged
+         in the firmware image. Instead, the prior stage's device tree should
+         be so packaged. At runtime, the prior stage reads this, does any
+         necessary fix-ups, then passes it to U-Boot. See OF_BOARD.
+
+         This option does not preclude using the U-Boot device tree, e.g. for
+         development purposes, but it is not recommended, and likely will not
+         even work, for production systems.
+
+         Note: This option must be set in Kconfig and cannot be enabled or
+         disabled in the board's defconfig file.
 
 config DEFAULT_DEVICE_TREE
        string "Default Device Tree for DT control"

diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 118c100..280cda6 100644 (file)
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -1595,6 +1595,7 @@ int fdtdec_set_carveout(void *blob, const char *node, const char *prop_name,
        return 0;
 }
 
+/* TODO(sjg@chromium.org): This function should not be weak */
 __weak int fdtdec_board_setup(const void *fdt_blob)
 {
        return 0;