linux-kernel - Re: [PATCH v3] add equivalent of BIT(x) for bitfields

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Message-ID: <20161207123800.6dd89eff@jkicinski-Precision-T1700>
Date:   Wed, 7 Dec 2016 12:38:00 +0000
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Sebastian Frias <sf84@...oste.net>
Cc:     Kalle Valo <kvalo@...eaurora.org>, zijun_hu <zijun_hu@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mason <slash.tmp@...e.fr>,
        Harvey Harrison <harvey.harrison@...il.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v3] add equivalent of BIT(x) for bitfields

On Wed, 7 Dec 2016 12:23:56 +0100, Sebastian Frias wrote:
> On 07/12/16 12:05, Jakub Kicinski wrote:
> > On Wed, 7 Dec 2016 11:00:57 +0100, Sebastian Frias wrote:  
> >> On 07/12/16 09:42, Kalle Valo wrote:  
> >>> Sebastian Frias <sf84@...oste.net> writes:
> >>>     
> >>>> Introduce GENVALUE(msb, lsb, value) macro to ease dealing with
> >>>> continuous bitfields, just as BIT(x) does for single bits.
> >>>>
> >>>> GENVALUE_ULL(msb, lsb, value) macro is also added.
> >>>>
> >>>> This is useful mostly for creating values to be packed together
> >>>> via OR operations, ex:
> >>>>
> >>>>    u32 val = 0x11110000;
> >>>>    val |= GENVALUE(19, 12, 0x5a);
> >>>>
> >>>> now 'val = 0x1115a000'
> >>>>
> >>>>
> >>>> Signed-off-by: Sebastian Frias <sf84@...oste.net>
> >>>> Link: https://marc.info/?l=linux-kernel&m=148094498711000&w=2
> >>>> ---
> >>>>
> >>>> Change in v2:
> >>>> - rename the macro to GENVALUE as proposed by Linus
> >>>> - longer comment attempts to show use case for the macro as
> >>>> proposed by Borislav
> >>>>
> >>>> Change in v3:
> >>>> - use BUILD_BUG_ON_ZERO() to break if some input parameters
> >>>> (essentially 'lsb' but also 'msb') are not constants as
> >>>> proposed by Linus.
> >>>> Indeed, 'lsb' is used twice so it cannot have side-effects;
> >>>> 'msb' is subjected to same constraints for consistency.    
> >>>
> >>> (I missed there was v3 already, but I'll repeat what I said in v1.)
> >>>
> >>> Please check FIELD_PREP() from include/linux/bitfield.h, doesn't it
> >>> already do the same?    
> >>
> >> Indeed, it appears to do the same :-)
> >> Any reason why "include/linux/bitfield.h" is not included by default in
> >> bitops.h?  
> > 
> > Hi!
> > 
> > The code is in a separate header because of circular dependencies
> > (coming from bug.h including bitops.h, IIRC).  You could possibly add an
> > include of bitfield.h in bitops.h if you're very careful, I haven't
> > tried TBH :)
> >   
> 
> Well, the following seems to work just fine:
> 
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index f6505d8..24c7480 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -15,8 +15,6 @@
>  #ifndef _LINUX_BITFIELD_H
>  #define _LINUX_BITFIELD_H
>  
> -#include <linux/bug.h>
> -

That would break users who include bitfield.h directly and don't
include bug.h, no?

>  /*
>   * Bitfield access macros
>   *
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index a83c822..7e5fab8 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -24,6 +24,8 @@
>  #define GENMASK_ULL(h, l) \
>         (((~0ULL) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
>  
> +#include "bitfield.h"
> +
>  extern unsigned int __sw_hweight8(unsigned int w);
>  extern unsigned int __sw_hweight16(unsigned int w);
>  extern unsigned int __sw_hweight32(unsigned int w);
> 
> 
> Is there a way to be sure it works in all cases? Otherwise
> I could just submit that, right?