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: <20201214152955.GH4077@smile.fi.intel.com>
Date:   Mon, 14 Dec 2020 17:29:55 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     kernel test robot <lkp@...el.com>, 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
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 Fri, Dec 11, 2020 at 06:56:14PM +0200, Sakari Ailus wrote:
> On Mon, Nov 23, 2020 at 12:40:18PM +0200, Andy Shevchenko wrote:
> > On Sat, Nov 21, 2020 at 04:23:05PM +0800, kernel test robot wrote:

...

> > > 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).
> 
> Apologies for the late answer.
> 
> I think I'd favour the first option. It's not really useful to be able to
> compile this elsewhere; as such the driver doesn't do anything special that
> would make it prone to breakage through changes elsewhere.
> 
> Would you like to send a patch? :-)

Done.

-- 
With Best Regards,
Andy Shevchenko