spi: Suppress null pointer warning

[MaEtUgR]

Solved Problem

Depending on defines px4_spi_buses can be NULL but often it's not and in those cases the compiler correctly warns about it.
Reported in #23869 (comment) and #23963 (comment)

Solution

Suppress the warning.

Test coverage

Locally built fmu-v4 on Ubuntu 24.04 with the packaged GCC 13.2.1

[github-actions]

FLASH Analysis

px4_fmu-v5x

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +8  +0.0%      +8    .text
  +0.0%      +6  [ = ]       0    .debug_line
    +0.4%      +6  [ = ]       0    ../../platforms/common/spi.cpp
  -0.0%      -6  [ = ]       0    [Unmapped]
  +0.0%      +8  +0.0%      +8    TOTAL

px4_fmu-v6x

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +8  +0.0%      +8    .text
  +0.0%      +6  [ = ]       0    .debug_line
    +0.5%      +6  [ = ]       0    ../../platforms/common/spi.cpp
  -0.0%      -6  [ = ]       0    [Unmapped]
  +0.0%      +8  +0.0%      +8    TOTAL

[PetervdPerk-NXP]

We could avoid the use #pragma if we just check for the nullptr before looping.

if (px4_spi_buses == nullptr) {
   return -1;
}

[MaEtUgR]

@PetervdPerk-NXP I generally agree but your suggestion does the same check there already is just in a sperate condition so it produces the same error.

Another option would be to disable that warning for the entire compile unit but I'd try to avoid that more than the pragma because it might hide actual mistakes.

[github-actions]

FLASH Analysis

px4_fmu-v5x

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +8  +0.0%      +8    .text
  +0.0%      +7  [ = ]       0    .debug_line
    +0.5%      +7  [ = ]       0    ../../platforms/common/spi.cpp
  -0.0%      -7  [ = ]       0    [Unmapped]
  +0.0%      +8  +0.0%      +8    TOTAL

px4_fmu-v6x

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +8  +0.0%      +8    .text
  +0.0%      +7  [ = ]       0    .debug_line
    +0.5%      +7  [ = ]       0    ../../platforms/common/spi.cpp
  -0.0%      -7  [ = ]       0    [Unmapped]
  +0.0%      +8  +0.0%      +8    TOTAL

[slgrobotics]

Just FYI - Ubuntu 24.04, main branch:

BEFORE your changes
sudo apt install g++-arm-linux-gnueabi

• when running "make emlid_navio2", the build fails:

[437/1121] Building CXX object platforms/common/CMakeFiles/px4_platform.dir/spi.cpp.o
FAILED: platforms/common/CMakeFiles/px4_platform.dir/spi.cpp.o 
/usr/bin/arm-linux-gnueabihf-g++ -DCONFIG_ARCH_BOARD_EMLID_NAVIO2 -D__CUSTOM_FILE_IO__ -D__PX4_LINUX -D__PX4_POSIX -D__STDC_FORMAT_MACROS -Dnoreturn_function="__attribute__((noreturn))" -I/home/sergei/src0/PX4-Autopilot/boards/emlid/navio2/src -I/home/sergei/src0/PX4-Autopilot/platforms/posix/src/px4/common/include -I/home/sergei/src0/PX4-Autopilot/build/emlid_navio2_default -I/home/sergei/src0/PX4-Autopilot/build/emlid_navio2_default/src/lib -I/home/sergei/src0/PX4-Autopilot/platforms/posix/src/px4/generic/generic/include -I/home/sergei/src0/PX4-Autopilot/platforms/common -I/home/sergei/src0/PX4-Autopilot/platforms/common/include -I/home/sergei/src0/PX4-Autopilot/src -I/home/sergei/src0/PX4-Autopilot/src/include -I/home/sergei/src0/PX4-Autopilot/src/lib -I/home/sergei/src0/PX4-Autopilot/src/lib/matrix -I/home/sergei/src0/PX4-Autopilot/src/modules -I/home/sergei/src0/PX4-Autopilot/boards/emlid/navio2/include -I/home/sergei/src0/PX4-Autopilot/platforms/posix/include -I/home/sergei/src0/PX4-Autopilot/build/emlid_navio2_default/external/Install/include -O2 -g -DNDEBUG -std=gnu++14 -fPIC -g -fdata-sections -ffunction-sections -fomit-frame-pointer -fmerge-all-constants -fno-signed-zeros -fno-trapping-math -freciprocal-math -fno-math-errno -fno-strict-aliasing -fvisibility=hidden -include visibility.h -Wall -Wextra -Werror -Warray-bounds -Wcast-align -Wdisabled-optimization -Wdouble-promotion -Wfatal-errors -Wfloat-equal -Wformat-security -Winit-self -Wlogical-op -Wpointer-arith -Wshadow -Wuninitialized -Wunknown-pragmas -Wunused-variable -Wno-missing-field-initializers -Wno-missing-include-dirs -Wno-unused-parameter -fdiagnostics-color=always -Wno-stringop-truncation -fno-builtin-printf -fno-strength-reduce -Wformat=1 -Wunused-but-set-variable -Wno-format-truncation -fcheck-new -Wreorder -Wno-overloaded-virtual -fno-rtti -MD -MT platforms/common/CMakeFiles/px4_platform.dir/spi.cpp.o -MF platforms/common/CMakeFiles/px4_platform.dir/spi.cpp.o.d -o platforms/common/CMakeFiles/px4_platform.dir/spi.cpp.o -c /home/sergei/src0/PX4-Autopilot/platforms/common/spi.cpp
/home/sergei/src0/PX4-Autopilot/platforms/common/spi.cpp: In function ‘int px4_find_spi_bus(uint32_t)’:
/home/sergei/src0/PX4-Autopilot/platforms/common/spi.cpp:90:59: error: the address of ‘px4_spi_buses’ will never be NULL [-Werror=address]
   90 |         for (int i = 0; ((px4_spi_bus_t *) px4_spi_buses) != nullptr && i < SPI_BUS_MAX_BUS_ITEMS; ++i) {
      |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
compilation terminated due to -Wfatal-errors.
cc1plus: all warnings being treated as errors
[446/1121] Building CXX object platforms/posix/src/px4/common/px4_daemon/CMakeFiles/px4_daemon.dir/pxh.cpp.o
ninja: build stopped: subcommand failed.
make: *** [Makefile:232: emlid_navio2] Error 1

• same happens on "make emlid_navio2_arm64" - that needed some adjustments:
  sudo apt install g++-aarch64-linux-gnu
  add file boards/emlid/navio2/arm64.px4board containing a single line:

CONFIG_BOARD_TOOLCHAIN="aarch64-linux-gnu"

Both ARM and ARM64 compilers treat this line as fatal error

WITH YOUR CHANGES APPLIED

• both ARM and ARM64 builds complete fine

Thanks!

[dagar]

I'd also prefer to not carry the pragma, but perhaps we can revisit this once the updated toolchain(s) are in place and we can easily test everything at once.

[MaEtUgR]

we can easily test everything at once

@dagar I'm happy to test. Just give me a hint at what would be better.

both ARM and ARM64 builds complete fine

@slgrobotics Thanks for verifying. I ran these builds on my test setup as well now 👍 It's not a surprise since both compilers are GCC. Should we add the build 64Bit build? Is it in use anywhere?

[slgrobotics]

@MaEtUgR - the 64-bit ARM build is important for Raspberry Pi 4 and 5 under Ubuntu. I use it under modified navio2, there's another one too:

make emlid_navio2_arm64
make scumaker_pilotpi_arm64

I opened #24006 to make sure it is available for navio2