linux-kernel - Re: drivers/mtd/maps/physmap-bt1-rom.c:78:18: sparse: sparse: cast removes address space '_

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Date:   Thu, 12 Nov 2020 16:43:01 +0100
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Vignesh Raghavendra <vigneshr@...com>,
        Serge Semin <fancer.lancer@...il.com>,
        <linux-mtd@...ts.infradead.org>, <kbuild-all@...ts.01.org>,
        kernel test robot <lkp@...el.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: drivers/mtd/maps/physmap-bt1-rom.c:78:18: sparse: sparse: cast
 removes address space '__iomem' of expression

Hi Serge,

Serge Semin <Sergey.Semin@...kalelectronics.ru> wrote on Thu, 12 Nov
2020 18:27:39 +0300:

> Hello Vignesh
> 
> On Thu, Nov 12, 2020 at 08:30:42PM +0530, Vignesh Raghavendra wrote:
> > 
> > 
> > On 11/12/20 1:57 PM, Miquel Raynal wrote:  
> > > Hi Sergey,
> > > 
> > > Serge Semin <Sergey.Semin@...kalelectronics.ru> wrote on Wed, 11 Nov
> > > 2020 22:22:59 +0300:
> > >   
> > >> On Tue, Nov 10, 2020 at 04:35:56PM +0100, Miquel Raynal wrote:  
> > >>> Hi Serge,
> > >>>
> > >>> Serge Semin <Sergey.Semin@...kalelectronics.ru> wrote on Tue, 10 Nov
> > >>> 2020 14:38:27 +0300:
> > >>>     
> > >>>> Hello Miquel,
> > >>>>
> > >>>> A situation noted by the warning below won't cause any problem because
> > >>>> the casting is done to a non-dereferenced variable. It is utilized
> > >>>> as a pointer bias later in that function. Shall we just ignore the
> > >>>> warning or still fix it somehow?    
> > >>>     
> > >>  
> > >>> Do you think the cast to a !__iomem value is mandatory here?    
> > >>
> > >> It's not mandatory to have the casting with no __iomem, but wouldn't
> > >> doing like this:
> > >> + 	shift = (ssize_t __iomem)src & 0x3;
> > >> be looking weird? Really, is there a good way to somehow extract the first
> > >> two bits of a __iomem pointer without getting the sparse warning?  
> > > 
> > > I asked around me, what about trying uintptr_t?
> > >   
> >   
> 
> > One more way is to use __force to tell sparse that this casting is
> > intentional:
> > 
> >        shift = (__force ssize_t)src & 0x3;  
> 
> Oh, great! That solution is actually much better than using some
> currently unexplained sparse peculiarity! I was thinking about applying
> some other attribute, but __force just didn't come to my mind. Thank
> you very much for the suggestion. I'll post the fix with the solution
> suggested by you.

Is the ssize_t cast the right one btw? I would definitely prefer an
unsigned type here.

Thanks,
Miquèl