common/image.c: align usage of fdt_high with initrd_high

The commit message of a28afca (Add uboot "fdt_high" enviroment variable)
states that fdt_high behaves similarly to the existing initrd_high.
But fdt_high actually has an outstanding difference from initrd_high.
The former specifies the start address, while the later specifies the
end address.

As fdt_high and initrd_high will likely be used together, it'd be nice
to have them behave same.  The patch changes the behavior of fdt_high
to have it aligned with initrd_high.

The document of fdt_high in README is updated with an example to
demonstrate the usage of this environment variable.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/README b/README
index eba6378536..89646725a0 100644
--- a/README
+++ b/README
@@ -3665,6 +3665,14 @@ List of environment variables (most likely not complete):
 
   fdt_high	- if set this restricts the maximum address that the
 		  flattened device tree will be copied into upon boot.
+		  For example, if you have a system with 1 GB memory
+		  at physical address 0x10000000, while Linux kernel
+		  only recognizes the first 704 MB as low memory, you
+		  may need to set fdt_high as 0x3C000000 to have the
+		  device tree blob be copied to the maximum address
+		  of the 704 MB low memory, so that Linux kernel can
+		  access it during the boot procedure.
+
 		  If this is set to the special value 0xFFFFFFFF then
 		  the fdt will not be copied at all on boot.  For this
 		  to work it must reside in writable memory, have