dm: doc: add documentation for pre-reloc properties in SPL and TPL

Add documentation for the pre-reloc property in SPL and TPL device-tree:
- u-boot,dm-pre-proper
- u-boot,dm-pre-reloc
- u-boot,dm-spl
- u-boot,dm-tpl

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

3 files changed, 24 insertions, 0 deletions

diff --git a/doc/README.SPL b/doc/README.SPL
index 7a30fef..6eed83f 100644
--- a/doc/README.SPL
+++ b/doc/README.SPL
@@ -66,6 +66,22 @@ CONFIG_SPL_SPI_LOAD (drivers/mtd/spi/spi_spl_load.o)
 CONFIG_SPL_RAM_DEVICE (common/spl/spl.c)
 CONFIG_SPL_WATCHDOG_SUPPORT (drivers/watchdog/libwatchdog.o)
 
+Device tree
+-----------
+The U-Boot device tree is filtered by the fdtgrep tools during the build
+process to generate a much smaller device tree used in SPL (spl/u-boot-spl.dtb)
+with:
+- the mandatory nodes (/alias, /chosen, /config)
+- the nodes with one pre-relocation property:
+  'u-boot,dm-pre-reloc' or 'u-boot,dm-spl'
+
+ftgrep is also used to remove:
+- the properties defined in CONFIG_OF_SPL_REMOVE_PROPS
+- all the pre-relocation properties
+  ('u-boot,dm-pre-reloc', 'u-boot,dm-spl' and 'u-boot,dm-tpl')
+
+All the nodes remaining in the SPL devicetree are bound
+(see driver-model/README.txt).
 
 Debugging
 ---------
diff --git a/doc/README.TPL b/doc/README.TPL
index 980debe..c94129f 100644
--- a/doc/README.TPL
+++ b/doc/README.TPL
@@ -34,6 +34,10 @@ determine which SPL options to choose based on whether CONFIG_TPL_BUILD
 is set. Source files can be compiled for TPL with options choosed in the
 board config file.
 
+TPL use a small device tree (u-boot-tpl.dtb), containing only the nodes with
+the pre-relocation properties: 'u-boot,dm-pre-reloc' and 'u-boot,dm-tpl'
+(see README.SPL for details).
+
 For example:
 
 spl/Makefile:
diff --git a/doc/driver-model/README.txt b/doc/driver-model/README.txt
index 07b120d..532a771 100644
--- a/doc/driver-model/README.txt
+++ b/doc/driver-model/README.txt
@@ -849,6 +849,10 @@ in the device tree node. For U-Boot proper you can use 'u-boot,dm-pre-proper'
 which means that it will be processed (and a driver bound) in U-Boot proper
 prior to relocation, but will not be available in SPL or TPL.
 
+To reduce the size of SPL and TPL, only the nodes with pre-relocation properties
+('u-boot,dm-pre-reloc', 'u-boot,dm-spl' or 'u-boot,dm-tpl') are keept in their
+device trees (see README.SPL for details); the remaining nodes are always bound.
+
 Then post relocation we throw that away and re-init driver model again.
 For drivers which require some sort of continuity between pre- and
 post-relocation devices, we can provide access to the pre-relocation