Re: [linux-next:master 1430/3917] drivers/platform/surface/surface_aggregator_registry.c:398:25: warning: implicit conversion from 'int' to '__u8' (aka 'unsigned char') changes value from 65535 to 255

[Maximilian Luz <luzmaximilian@gmail.com>]

On 3/11/21 2:06 PM, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> head:   98546348153dee5f8ced572fd6c4690461d20f51
> commit: 797e78564634275ed4fe6b3f586c4b96eb1d86bc [1430/3917] platform/surface: aggregator_registry: Add base device hub
> config: arm64-randconfig-r026-20210311 (attached as .config)
> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 574a9dabc63ba1e7a04c08d4bde2eacd61b44ce1)
> reproduce (this is a W=1 build):
>          wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>          chmod +x ~/bin/make.cross
>          # install arm64 cross compiling tool for clang build
>          # apt-get install binutils-aarch64-linux-gnu
>          # https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=797e78564634275ed4fe6b3f586c4b96eb1d86bc
>          git remote add linux-next https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
>          git fetch --no-tags linux-next master
>          git checkout 797e78564634275ed4fe6b3f586c4b96eb1d86bc
>          # save the attached .config to linux build tree
>          COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=arm64
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
> 
> All warnings (new ones prefixed by >>):
> 
>     In file included from drivers/platform/surface/surface_aggregator_registry.c:12:
>     In file included from include/linux/acpi.h:35:
>     In file included from include/acpi/acpi_io.h:7:
>     In file included from arch/arm64/include/asm/acpi.h:12:
>     include/linux/efi.h:1093:34: warning: passing 1-byte aligned argument to 4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer access [-Walign-mismatch]
>             status = get_var(L"SecureBoot", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size,
>                                             ^
>     include/linux/efi.h:1101:24: warning: passing 1-byte aligned argument to 4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer access [-Walign-mismatch]
>             get_var(L"SetupMode", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size, &setupmode);
>                                   ^
> >> drivers/platform/surface/surface_aggregator_registry.c:398:25: warning: implicit conversion from 'int' to '__u8' (aka 'unsigned char') changes value from 65535 to 255 [-Wconstant-conversion]
>             { SSAM_VDEV(HUB, 0x02, SSAM_ANY_IID, 0x00) },
>             ~                      ^~~~~~~~~~~~

This is a false positive:

>     include/linux/surface_aggregator/device.h:71:23: note: expanded from macro 'SSAM_ANY_IID'
>     #define SSAM_ANY_IID            0xffff
>                                     ^~~~~~

Here, clang complains that the input is SSAM_ANY_ID. That is a special
value which has special considerations below. The SSAM_DEVICE() and
thus SSAM_VDEV() macros are intended to only allow either __u8 or
SSAM_ANY_ID as input in this place.

>     include/linux/surface_aggregator/device.h:126:63: note: expanded from macro 'SSAM_VDEV'
>             SSAM_DEVICE(SSAM_DOMAIN_VIRTUAL, SSAM_VIRTUAL_TC_##cat, tid, iid, fun)
>                                                                          ^~~
>     include/linux/surface_aggregator/device.h:102:41: note: expanded from macro 'SSAM_DEVICE'
>             .instance = ((iid) != SSAM_ANY_IID) ? (iid) : 0,                        \
>                                                    ^~~

Here is the special handling for SSAM_ANY_ID. So there is an implicit
conversion from iid, which may be __u8, to int (i.e. the type of
SSAM_ANY_ID), but there is at no point any implicit conversion of
SSAM_ANY_ID to __u8, as clang alleges.

Is there any way to silence this warning in particular without
suppressing it (e.g. by explicit casting) when users of this macro
_actually_ input any non-u8 and non-SSAM_ANY_ID value as iid,
leading to an _actual_ implicit value-changing cast?

I believe GCC does get this right and only emits a warning if a
non-u8 _and_ non-SSAM_ANY_ID value is input.

Regards,
Max

>     3 warnings generated.
> 
> 
> vim +398 drivers/platform/surface/surface_aggregator_registry.c
> 
>     396	
>     397	static const struct ssam_device_id ssam_base_hub_match[] = {
>   > 398		{ SSAM_VDEV(HUB, 0x02, SSAM_ANY_IID, 0x00) },
>     399		{ },
>     400	};
>     401	
> 
> ---
> 0-DAY CI Kernel Test Service, Intel Corporation
> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
>