Re: [edk2-platforms][PATCH] Readme.md: Update cross compiling section

["Leif Lindholm" <leif@nuviainc.com>]

On Wed, Jul 15, 2020 at 14:05:03 +0800, Vin Xue wrote:
> 1. Update invalid URL link of Linaro GCC toolchains.
> 2. Add build command example for cross compiling.
> 
> Cc: Leif Lindholm <leif@nuviainc.com>
> Cc: Michael D Kinney <michael.d.kinney@intel.com>
> Signed-off-by: Vin Xue <vinxue@outlook.com>
> ---
>  Readme.md | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/Readme.md b/Readme.md
> index ea9492240a..2e30f672cc 100644
> --- a/Readme.md
> +++ b/Readme.md
> @@ -57,10 +57,13 @@ X64                 | x86_64-linux-gnu-
>  
>  ### GCC
>  Linaro provides GCC toolchains for
> -[aarch64-linux-gnu](https://releases.linaro.org/components/toolchain/binaries/latest/aarch64-linux-gnu/)
> -and [arm-linux-gnueabihf](https://releases.linaro.org/components/toolchain/binaries/latest/arm-linux-gnueabihf/)
> -compiled to run on x86_64/i686 Linux and i686 Windows. Some Linux distributions
> -provide their own packaged cross-toolchains.
> +[aarch64-linux-gnu](https://releases.linaro.org/components/toolchain/binaries/latest-7/aarch64-linux-gnu/)
> +and [arm-linux-gnueabihf](https://releases.linaro.org/components/toolchain/binaries/latest-7/arm-linux-gnueabihf/)
> +compiled to run on x86_64/i686 Linux and i686 Windows.

Yes, we should update the information about the Windows toolchains.
But gcc 7 is ancient. If we go to
https://releases.linaro.org/components/toolchain/binaries/, we see
they have added a gcc-8 "directory". I say "directory" with quotes
because clicking on that redirects you to
https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-a/downloads
This is the location we should now be pointing to - and these
toolchains are now gcc 9.2.

> +
> +Some Linux distributions provide their own packaged cross-toolchains. e.g. Install AArch64 GCC toolchain on Ubuntu (x86_64) with:
> +
> +`sudo apt install gcc-aarch64-linux-gnu`

We can't maintain a list of every possible package name with every
possible package install command (apt, yum, dnf, pacman, ?) for every
distribution (debian, ubuntu, redhat, fedora, centos, suse, arch,
gentoo, ?) out there. We'll also be adding RISC-V support in the near
future.

The abovementioned arm.com link includes prebuilt compilers for
Windows-x86_64, Linux-x86_64, and Linux-AArch64 (for AArch32).
Anyone unable to figure out how to find and install packages provided
with their distro can always grab a toolchain from that download page.

>  
>  ### clang
>  Clang does not require separate cross compilers, but it does need a
> @@ -160,6 +163,9 @@ variable `{TOOL_CHAIN_TAG}_{TARGET_ARCH}_PREFIX` - in the case above,
>  **GCC5_AARCH64_PREFIX**.
>  
>  So, referring to the cross compiler toolchain table above, we should prepend the `build` command line with `GCC5_AARCH64_PREFIX=aarch64-linux-gnu-`.
> +```
> +$ GCC5_AARCH64_PREFIX=aarch64-linux-gnu- build -n $NUM_CPUS -a AARCH64 -t GCC5 -p Platform/ARM/JunoPkg/ArmJuno.dsc
> +```

How to build a platform is already described in the section
immediately preceding this one. This paragraph tells you how to inform
the build system to apply a prefix for the compiler if cross compiling.

Seprate note (to self, really): we don't need the -n thing anymore,
the 'build' command has been updated to figure this out for itself.

/
    Leif

>  
>  ## Using uefi-tools helper scripts
>  uefi-tools is a completely unofficial set of helper-scripts developed by Linaro.
> -- 
> 2.27.0.windows.1
>