Re: [RFC-UGLYPATCH] ata: small optimization in linux/libata.h

[Tejun Heo <htejun@gmail.com>]

Harvey Harrison wrote:
>>> I know it's ugly, but I had it done anyways.  The one real problem I have
>>> with it is that if link and ap->pmp_link ever get changed to different types
>>> the compiler will not even warn as we cast away to (char *).  To make it
>>> a bit more robust, a BUILD_BUG_ON checking the pointer types may be a
>>> good idea.
>> Sorry, but Nacked-by: Tejun Heo <htejun@gmail.com>
>>
> 
> Can't say I really blame you, other than this one error, drivers/ata
> builds almost sparse-clean, and I had it done anyways.  I wonder if
> a helper similar in spirit would be any better.  This doesn't have
> any typechecking, but perhaps that could be dealt with too.
> 
> Ran into a similar problem with mmzone.h, akpm has the patch, but
> maybe a helper (kernel.h?) would be cleaner.  Or maybe it's just
> better to live with the sparse warnings....
> 
> /*
>  * return the offset of the ptr from the base, in bytes.
>  */
> #define PTR_OFF(base, ptr) \
> ((char *)ptr - (char *)base)
> 
> if (PTR_OFF(ap->pmp_link, ++link) < ap->nr_pmp_links * sizeof(*link))
> 	return link;

Can you please explain why it warns against pointer substraction?  Is it
because it can generate division?

Thanks.

-- 
tejun