linux-kernel - Re: [PATCH 01/17] bitfield: Add non-constant field

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Message-ID: <20211123153933.49ff8b72@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 23 Nov 2021 15:39:33 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Rajendra Nayak <rnayak@...eaurora.org>,
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        Michael Turquette <mturquette@...libre.com>,
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        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Tero Kristo <kristo@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Lorenzo Bianconi <lorenzo.bianconi83@...il.com>,
        Benoit Parrot <bparrot@...com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
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        Ping-Ke Shih <pkshih@...ltek.com>,
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}()
 helpers

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.