Re: [PATCH v3 7/7] perf docs: Remove the Android cross building document

[Leo Yan]

Hi Namhyung,

On 7/12/24 05:58, Namhyung Kim wrote:
> On Sat, Jul 06, 2024 at 07:29:12PM +0100, Leo Yan wrote:

[...]

> > The Android NDK (as time being the latest LTS version is r26d) changes
> > toolchain to LLVM / Clang, so GCC compilers is not included in the NDK
> > anymore. Therefore, the Android document contains obsolete info for
> > building perf binary with NDK.
>
> Do you know if the version prior to the change is still used?

Based on the Android NDK wiki claims [1], since NDK r15 (backed to 26th
July, 2017) "GCC is no longer supported. It will not be removed from the NDK
just yet, but is no longer receiving backports".

> > Furthermore, the Clang included in the Android NDK is problematic for
> > cross compilation Aarch64 target. The building reports multiple errors
> > with the compiler aarch64-linux-android34-clang.
> >
> > Thus, delete Documentation/android.txt to avoid confusion.
>
> If so, maybe we can keep the document little more and add a note that
> this works only for some old versions.
>
> I'm also curious if it's still broken after your fixes.

When I tried Android NDK for cross building perf, it is still broken after
applied this series.

From my testing, now using LLVM for native building works well:

  make LLVM=-15 VF=1 DEBUG=1 -C tools/perf

But it failed for cross compilation. Since Android NDK system root does not
contain some dependency libs, we must use extra options to disable some
features (e.g. NO_LIBELF=1 NO_LIBTRACEEVENT=1). The commands I tried for
cross building but failed:

  make ARCH=arm64 CC=$NDK_TOOLCHAIN/aarch64-linux-android-clang \
    LD=$NDK_TOOLCHAIN/ld.lld AR=$NDK_TOOLCHAIN/llvm-ar \
    NM=$NDK_TOOLCHAIN/llvm-nm STRIP=$NDK_TOOLCHAIN/llvm-strip \
    OBJCOPY=$NDK_TOOLCHAIN/llvm-objcopy \
    OBJDUMP=$NDK_TOOLCHAIN/llvm-objdump \
    READELF=$NDK_TOOLCHAIN/llvm-readelf \
    HOSTCC=$NDK_TOOLCHAIN/clang \
    HOSTCXX=$NDK_TOOLCHAIN/clang++ HOSTAR=$NDK_TOOLCHAIN/llvm-ar \
    HOSTLD=$NDK_TOOLCHAIN/ld.lld VF=1 DEBUG=1 NO_LIBELF=1 \
    NO_LIBTRACEEVENT=1 EXTRA_CFLAGS=-fgnuc-version=0 \
    -C tools/perf

Given the test result, my conclusion is the doc for Android cross building
is not useful for a long while. If we really want to support it, I would
like to suggest to take a separate task for fixing LLVM / Clang cross
compilation.

Thanks,
Leo

[1] https://github.com/android/ndk/wiki/Changelog-r15