[PATCH 1/2] README: Document 32-bit / 64-bit images build

Bin Meng bmeng.cn at gmail.com
Tue Jul 23 21:29:58 PDT 2019

At present the exact details of building 32-bit or 64-bit OpenSBI
images are missing in the docs. This adds the missing part.

Signed-off-by: Bin Meng <bmeng.cn at gmail.com>

 README.md | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/README.md b/README.md
index 1f62048..5b505ad 100644
--- a/README.md
+++ b/README.md
@@ -131,6 +131,25 @@ top-level make command line. These options, such as *PLATFORM_<xyz>* or
 *docs/platform/<platform_name>.md* files and
 *docs/firmware/<firmware_name>.md* files.
+Building 32-bit / 64-bit OpenSBI Images
+By default, building OpenSBI genereates 32-bit or 64-bit images based on the
+supplied RISC-V cross-compile toolchain. For example if *CROSS_COMPILE* is set
+to *riscv64-unknown-elf-*, 64-bit OpenSBI images will be generated. If building
+32-bit OpenSBI images, *CROSS_COMPILE* should be set to a toolchain that is
+pre-configured to generate 32-bit RISC-V codes, like *riscv32-unknown-elf-*.
+However it's possible to explicitly specify the image bits we want to build with
+a given RISC-V toolchain. This can be done by setting the environment variable
+*PLATFORM_RISCV_XLEN* to the desired width, for example:
+export CROSS_COMPILE=riscv64-unknown-elf-
+will generate 32-bit OpenSBI images. And vice vesa.

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