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README: Update toolchain information

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Recent FW_PIC=y changes actually require toolchains with PIE support
and it is on by default. Existing doc uses a GNU bare-metal toolchain
as examples but it does not support PIE.

Replace references of bare-metal toolchain prefix with Linux toolchain
prefix everywhere in documentation.

Also indicate that "riscv64-unknown-freebsd-" as an alternative to
"riscv64-linux-" GNU toolchain.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
This commit is contained in:
Bin Meng
2021-07-12 09:20:16 +08:00
committed by Anup Patel
parent 2c74dc3c47
commit 81eb7088b6
1 changed files with 13 additions and 5 deletions
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18
README.md
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@@ -104,6 +104,14 @@ LLVM/Clang toolchain due to LLVM's ability to support multiple backends in the
same binary, so is often an easy way to obtain a working cross-compilation
toolchain.
Basically, we prefer toolchains with Position Independent Executable (PIE)
support like *riscv64-linux-gnu-gcc*, *riscv64-unknown-freebsd-gcc*, or
*Clang/LLVM* as they generate PIE firmware images that can run at arbitrary
address with appropriate alignment. If a bare-metal GNU toolchain (e.g.
*riscv64-unknown-elf-gcc*) is used, static linked firmware images are
generated instead. *Clang/LLVM* can still generate PIE images if a bare-metal
triple is used (e.g. *-target riscv64-unknown-elf*).
Please note that only a 64-bit version of the toolchain is available in
the Bootlin toolchain repository for now.
@@ -116,7 +124,7 @@ architecture than RISC-V.
For cross-compiling, the environment variable *CROSS_COMPILE* must be defined
to specify the name prefix of the RISC-V compiler toolchain executables, e.g.
*riscv64-unknown-elf-* if the gcc executable used is *riscv64-unknown-elf-gcc*.
*riscv64-linux-gnu-* if the gcc executable used is *riscv64-linux-gnu-gcc*.
To build *libsbi.a* simply execute:
```
@@ -192,16 +200,16 @@ Building 32-bit / 64-bit OpenSBI Images
---------------------------------------
By default, building OpenSBI generates 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
to *riscv64-linux-gnu-*, 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-*.
pre-configured to generate 32-bit RISC-V codes, like *riscv32-linux-gnu-*.
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-
export CROSS_COMPILE=riscv64-linux-gnu-
export PLATFORM_RISCV_XLEN=32
```
@@ -235,7 +243,7 @@ valid triple.
These can also be mixed; for example using a GCC cross-compiler but LLVM
binutils would be:
```
make CC=riscv64-unknown-elf-gcc LLVM=1
make CC=riscv64-linux-gnu-gcc LLVM=1
```
These variables must be passed for all the make invocations described in this