linux-kernel - Re: drivers/net/wireless/intel/iwlwifi/queue/tx.c:310:18: warning: result of comparison of constant 262140 with expression of type 'u16' (aka 'unsigned short') is always false

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Date:   Mon, 4 Jan 2021 11:47:28 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     kernel test robot <lkp@...el.com>
Cc:     Mordechay Goodstein <mordechay.goodstein@...el.com>,
        kbuild-all@...ts.01.org, clang-built-linux@...glegroups.com,
        linux-kernel@...r.kernel.org,
        Luca Coelho <luciano.coelho@...el.com>
Subject: Re: drivers/net/wireless/intel/iwlwifi/queue/tx.c:310:18: warning:
 result of comparison of constant 262140 with expression of type 'u16' (aka
 'unsigned short') is always false

On Sun, Jan 03, 2021 at 09:51:17AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   3516bd729358a2a9b090c1905bd2a3fa926e24c6
> commit: 0cd1ad2d7fd41e0de4969fd1dd0aa846c99830d1 iwlwifi: move all bus-independent TX functions to common code
> date:   3 months ago
> config: powerpc-randconfig-r001-20210103 (attached as .config)
> compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project 20670ba44066df0aae536822b7f7834ee3198c0d)
> 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 powerpc cross compiling tool for clang build
>         # apt-get install binutils-powerpc-linux-gnu
>         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0cd1ad2d7fd41e0de4969fd1dd0aa846c99830d1
>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>         git fetch --no-tags linus master
>         git checkout 0cd1ad2d7fd41e0de4969fd1dd0aa846c99830d1
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=powerpc 
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
> 
> All warnings (new ones prefixed by >>):
> 
> >> drivers/net/wireless/intel/iwlwifi/queue/tx.c:310:18: warning: result of comparison of constant 262140 with expression of type 'u16' (aka 'unsigned short') is always false [-Wtautological-constant-out-of-range-compare]
>            if (WARN_ON(len > PAGE_SIZE - sizeof(void *))) {
>            ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    include/asm-generic/bug.h:188:25: note: expanded from macro 'WARN_ON'
>            int __ret_warn_on = !!(condition);                              \
>                                   ^~~~~~~~~
>    include/linux/compiler.h:56:47: note: expanded from macro 'if'
>    #define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) )
>                               ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
>    include/linux/compiler.h:58:52: note: expanded from macro '__trace_if_var'
>    #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
>                                                       ^~~~
> >> drivers/net/wireless/intel/iwlwifi/queue/tx.c:310:18: warning: result of comparison of constant 262140 with expression of type 'u16' (aka 'unsigned short') is always false [-Wtautological-constant-out-of-range-compare]
>            if (WARN_ON(len > PAGE_SIZE - sizeof(void *))) {
>            ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    include/asm-generic/bug.h:188:25: note: expanded from macro 'WARN_ON'
>            int __ret_warn_on = !!(condition);                              \
>                                   ^~~~~~~~~
>    include/linux/compiler.h:56:47: note: expanded from macro 'if'
>    #define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) )
>                               ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
>    include/linux/compiler.h:58:86: note: expanded from macro '__trace_if_var'
>    #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
>                                                                        ~~~~~~~~~~~~~~~~~^~~~~
>    include/linux/compiler.h:69:3: note: expanded from macro '__trace_if_value'
>            (cond) ?                                        \
>             ^~~~
>    2 warnings generated.

This warning happens due to PAGE_SIZE being 256K, which is larger than
u16.

$ rg "PPC_256K_PAGES|PPC_PAGE_SHIFT" .config
CONFIG_PPC_256K_PAGES=y
CONFIG_PPC_PAGE_SHIFT=18

On most configurations, which will use 64K or less, this check is valid.
I am not sure how it should be dealt with, aside from increasing len to
u32.

Cheers,
Nathan