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

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Date:   Wed, 7 Dec 2016 12:23:56 +0100
From:   Sebastian Frias <sf84@...oste.net>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
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 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>
-
 /*
  * 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?