[Buildroot] [PATCH 1/1] protobuf: apply patch to compile for PowerPC

[Thomas Petazzoni]

Carlos,

On Thu, 28 Jan 2016 11:08:35 -0200, Carlos Santos wrote:
> From: Henrique Marks <henrique.marks at datacom.ind.br>
> 
> Signed-off-by: Henrique Marks <henrique.marks at datacom.ind.br>
> Signed-off-by: Carlos Santos <casantos at datacom.ind.br>
> ---
>  package/protobuf/0001-PowerPC-Support.patch | 54 +++++++++++++++++++++++++++++
>  package/protobuf/Config.in                  |  5 ++-
>  2 files changed, 56 insertions(+), 3 deletions(-)
>  create mode 100644 package/protobuf/0001-PowerPC-Support.patch

This patch doesn't actually work. First there are a number of problems:

 - The patch you apply has technically nothing to do with enabling the
   PowerPC architecture. It seems more related to supporting old
   compilers.

 - The patch you apply is already applied upstream, so in this case, we
   prefer to use the upstream patch directly.

 - You change the architecture dependencies in protobuf/Config.in, but
   forget to propagate this change to the reverse dependencies of
   protobuf, namely the mosh and ola packages. To make this easier,
   I've changed protobuf/Config.in to provide a
   BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS variable, and changed mosh and
   ola to use it. See commit
   https://git.busybox.net/buildroot/commit/?id=abdc56006bf253bec393066f96f69f0a6246b896.

But then, despite those issues, your patch still doesn't build on
PowerPC with the following defconfig for example:

BR2_powerpc=y
BR2_powerpc_8548=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_INIT_NONE=y
BR2_SYSTEM_BIN_SH_NONE=y
# BR2_PACKAGE_BUSYBOX is not set
BR2_PACKAGE_OLA=y
BR2_PACKAGE_MOSH=y
# BR2_TARGET_ROOTFS_TAR is not set

It fails with:

In file included from ./google/protobuf/stubs/once.h:81:0,
                 from google/protobuf/stubs/common.cc:34:
./google/protobuf/stubs/atomicops.h:209:2: error: #error GOOGLE_PROTOBUF_ATOMICOPS_ERROR
In file included from ./google/protobuf/stubs/once.h:81:0,
                 from google/protobuf/stubs/once.cc:38:
./google/protobuf/stubs/atomicops.h:209:2: error: #error GOOGLE_PROTOBUF_ATOMICOPS_ERROR
In file included from google/protobuf/stubs/atomicops_internals_x86_msvc.cc:37:0:
./google/protobuf/stubs/atomicops.h:209:2: error: #error GOOGLE_PROTOBUF_ATOMICOPS_ERROR
In file included from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:0:
./google/protobuf/stubs/atomicops.h:209:2: error: #error GOOGLE_PROTOBUF_ATOMICOPS_ERROR

Looking at atomicops.h, I can read:

#elif defined(__GNUC__)
#if defined(GOOGLE_PROTOBUF_ARCH_IA32) || defined(GOOGLE_PROTOBUF_ARCH_X64)
#include <google/protobuf/stubs/atomicops_internals_x86_gcc.h>
#elif defined(GOOGLE_PROTOBUF_ARCH_ARM) && defined(__linux__)
#include <google/protobuf/stubs/atomicops_internals_arm_gcc.h>
#elif defined(GOOGLE_PROTOBUF_ARCH_AARCH64)
#include <google/protobuf/stubs/atomicops_internals_arm64_gcc.h>
#elif defined(GOOGLE_PROTOBUF_ARCH_ARM_QNX)
#include <google/protobuf/stubs/atomicops_internals_arm_qnx.h>
#elif defined(GOOGLE_PROTOBUF_ARCH_MIPS) || defined(GOOGLE_PROTOBUF_ARCH_MIPS64)
#include <google/protobuf/stubs/atomicops_internals_mips_gcc.h>
#elif defined(__native_client__)
#include <google/protobuf/stubs/atomicops_internals_pnacl.h>
#elif (((__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)) || (__GNUC__ > 4))
#include <google/protobuf/stubs/atomicops_internals_generic_gcc.h>
#elif defined(__clang__)
#if __has_extension(c_atomic)
#include <google/protobuf/stubs/atomicops_internals_generic_gcc.h>
#else
#error GOOGLE_PROTOBUF_ATOMICOPS_ERROR
#endif
#else
#error GOOGLE_PROTOBUF_ATOMICOPS_ERROR
#endif

So it means that on i386, on x86-64, on ARM and on MIPS and MIPS64,
there are built-in implementation for the atomic operations.

For all other architectures, it relies on
atomicops_internals_generic_gcc.h, only when gcc >= 4.7. And indeed my
tested toolchain only has gcc 4.5.

In fact atomicops_internals_generic_gcc.h uses the __atomic_*()
built-ins of the compiler, which indeed are only introduced in gcc 4.7.
But on some architectures, they require linking with -latomic. See my
atomic patch series at
http://lists.busybox.net/pipermail/buildroot/2016-February/151672.html,
and especially patch
http://lists.busybox.net/pipermail/buildroot/2016-February/151674.html
which has all the gory details.

And in fact, with those atomics, not only PowerPC can be supported, but
any other architecture (except if protobuf has other
architecture-specific dependencies elsewhere).

So, once my atomic patch series is merged, we could do:

config BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS
        bool
        default y if BR2_arm
        default y if BR2_i386
        default y if BR2_mipsel
        default y if BR2_x86_64
	default y if BR2_TOOLCHAIN_HAS_ATOMIC
        depends on BR2_HOSTARCH = "x86_64" || BR2_HOSTARCH = "x86"

*and* ensure protobuf gets linked with -latomic.

What do you think ?

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com