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Date:   Mon, 12 Feb 2018 09:34:35 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Scott Wood <oss@buserror.net>, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v4 2/5] powerpc/mm/slice: Enhance for supporting PPC32
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On Sun, 11 Feb 2018 21:04:42 +0530
"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> On 02/11/2018 07:29 PM, Nicholas Piggin wrote:
> > On Sat, 10 Feb 2018 13:54:27 +0100 (CET)
> > Christophe Leroy <christophe.leroy@c-s.fr> wrote:
> >   
> >> In preparation for the following patch which will fix an issue on
> >> the 8xx by re-using the 'slices', this patch enhances the
> >> 'slices' implementation to support 32 bits CPUs.
> >>
> >> On PPC32, the address space is limited to 4Gbytes, hence only the low
> >> slices will be used.
> >>
> >> This patch moves "slices" functions prototypes from page64.h to slice.h
> >>
> >> The high slices use bitmaps. As bitmap functions are not prepared to
> >> handling bitmaps of size 0, the bitmap_xxx() calls are wrapped into
> >> slice_bitmap_xxx() functions which will void on PPC32  
> > 
> > On this last point, I think it would be better to put these with the
> > existing slice bitmap functions in slice.c and just have a few #ifdefs
> > for SLICE_NUM_HIGH == 0.
> >   
> 
> We went back and forth with that. IMHO, we should avoid as much #ifdef 
> as possible across platforms. It helps to understand the platform 
> restrictions better as we have less and less access to these platforms. 
> The above change indicates that nohash 32 wants to use the slice code 
> and they have different restrictions. With that we now know that 
> book3s64 and nohash 32 are the two different configs using slice code.

I don't think it's the right place to put it. It's not platform dependent
so much as it just depends on whether or not you have 0 high slices as
a workaround for bitmap API not accepting 0 length.

Another platform that uses the slice code would just have to copy and
paste either the nop or the bitmap implementation depending if it has
high slices. So I don't think it's the right abstraction. And it
implies a bitmap operation but it very specifically only works for
struct slice_mask.high_slices bitmap, which is not clear. Better to
just work with struct slice_mask.

Some ifdefs inside .c code for small helper functions like this IMO isn't
really a big deal -- it's not worse than having it in headers. You just
want to avoid ifdef mess when looking at non-trivial logic.

static inline void slice_or_mask(struct slice_mask *dst, struct slice_mask *src)
{
    dst->low_slices |= src->low_slices;
#if SLICE_NUM_HIGH > 0
    bitmap_or(result, dst->high_slices, src->high_slices, SLICE_NUM_HIGH);
#endif
}

I think that's pretty fine. If you have a singular hatred for ifdef in .c,
then if() works just as well.

Thanks,
Nick