Re: drivers/mtd/maps/physmap-bt1-rom.c:78:18: sparse: sparse: cast removes address space '__iomem' of expression

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Serge,

Serge Semin <Sergey.Semin@baikalelectronics.ru> wrote on Thu, 12 Nov
2020 19:10:43 +0300:

> On Thu, Nov 12, 2020 at 04:43:01PM +0100, Miquel Raynal wrote:
> > Hi Serge,
> > 
> > Serge Semin <Sergey.Semin@baikalelectronics.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@baikalelectronics.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@baikalelectronics.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.  
> 
> The reason of me deciding to use the ssize_t type here was to prevent
> the types casting across the "shift", "chunk" and "len" variables
> within this method. It seemed a bit better than having a standard type
> like "unsigned int" here seeing the ssize_t type width won't exceed
> the long type size anyway. Moreover since the "len" variable has got
> the ssize_t type and I couldn't change it (the method is the map_info
> callback), I've decided to stick with what is available and defined
> "shift" and "chunk" as ssize_t-es. Another callback method
> bt1_rom_map_read() in his module has been designed in the same way.
> 
> Do you think it's better to change it in favor of using a different
> type like "unsigned int" here anyway?

I would say yes.

> If so for unification I'd need to
> change bt1_rom_map_read() (though the "shift" variable has been
> defined as "unsigned long" there in the first place because the offs
> argument has got that type).

Fine.

> 
> What to do with the __force attribute here? It does seem appropriate
> even if for some mystical reasons we haven't got the sparse warning
> for the unsigned types.

Yeah this is strange. I would, however, suggest not to add this keyword
if we don't need it.

Thanks,
Miquèl

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