linux-kernel - Re: drivers/media/pci/intel/ipu3/ipu3-cio2.c:163:56: warning: implicit conversion from 'unsigned long' to 'u16' (aka 'unsigned short') changes value from 131072 to 0

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Message-ID: <20201123104018.GX4077@smile.fi.intel.com>
Date:   Mon, 23 Nov 2020 12:40:18 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     kernel test robot <lkp@...el.com>
Cc:     kbuild-all@...ts.01.org, clang-built-linux@...glegroups.com,
        linux-kernel@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: drivers/media/pci/intel/ipu3/ipu3-cio2.c:163:56: warning:
 implicit conversion from 'unsigned long' to 'u16' (aka 'unsigned short')
 changes value from 131072 to 0

On Sat, Nov 21, 2020 at 04:23:05PM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   27bba9c532a8d21050b94224ffd310ad0058c353
> commit: 7b285f41f7376dc37e7fad1e803995fd39f42848 media: ipu3-cio2: Introduce CIO2_LOP_ENTRIES constant
> date:   2 months ago
> config: arm64-randconfig-r031-20201121 (attached as .config)
> compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project bec968cbb367dd03439c89c1d4ef968ef662d7c0)
> 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/torvalds/linux.git/commit/?id=7b285f41f7376dc37e7fad1e803995fd39f42848
>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>         git fetch --no-tags linus master
>         git checkout 7b285f41f7376dc37e7fad1e803995fd39f42848
>         # 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@...el.com>
> 
> All warnings (new ones prefixed by >>):
> 
> >> drivers/media/pci/intel/ipu3/ipu3-cio2.c:163:56: warning: implicit conversion from 'unsigned long' to 'u16' (aka 'unsigned short') changes value from 131072 to 0 [-Wconstant-conversion]
>            entry[1].second_entry.num_of_pages = CIO2_LOP_ENTRIES * CIO2_MAX_LOPS;
>                                               ~ ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
>    1 warning generated.

Okay, now we have an interesting case. The IP is quite unlikely be used on
ARM64, but my patches made the clear picture about use of PAGE_SIZE here.

So, I see at least the following options to mitigate the above, i.e.:
 1/ reduce driver scope to X86
 2/ fix the variables to be wider type to be able to hold PAGE_SIZE > 4k
 3/ switch to custom PAGE_SIZE / _SHIFT / _MASK and accompanying macros

And I still consider 3/ is silly move because as we see the driver was
never assumed to work with big page sizes (besides unsigned short type
here, PAGE_SHIFT and PAGE_MASK in the original code was as is and on ARM64
they compiled to 0 values w/o warnings, effectively make the driver
improperly functioning anyway).


-- 
With Best Regards,
Andy Shevchenko