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Date:   Tue, 23 Nov 2021 15:39:33 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}()
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On Tue, 23 Nov 2021 09:36:22 +0100 Geert Uytterhoeven wrote:
> > > Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit
> > > architectures (afaict), so that seems a bit awkward.
> > >
> > > Maybe we can make {u32,...}_get_bits() be doing compile-time only checks
> > > if it is indeed a constant? The __field_overflow() usage is already only
> > > done if __builtin_constant_p(v), so I guess we can do the same with
> > > __bad_mask()?  
> >
> > Either that or add decomposition macros. Are compilers still really bad
> > at passing small structs by value?  
> 
> Sorry, I don't get what you mean by adding decomposition macros.
> Can you please elaborate?

#define DECOMPOSE(_mask) \
  (struct bf){ .mask = _mask, .shf = __bf_shf(_mask), }

Then drivers can save or pass around the mask and shift params 
broken apart as a small struct.