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

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Date:   Tue, 6 Dec 2016 15:56:53 +0100
From:   Sebastian Frias <sf84@...oste.net>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     zijun_hu <zijun_hu@....com>, Sasha Levin <sasha.levin@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mason <slash.tmp@...e.fr>,
        Maxime Coquelin <maxime.coquelin@...com>,
        Harvey Harrison <harvey.harrison@...il.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] bitops: add equivalent of BIT(x) for bitfields

Hi Geert,

On 06/12/16 12:12, Geert Uytterhoeven wrote:
>>> ... which means "generate a value"??
>>>
>>
>> Yes.
>> Although I'm not sure if I understood the essence of your point.
>> Are you suggesting that the name should be GENERATE_A_VALUE?
> 
> No. I mean that "value" is a way too generic name.
> Hence "GENVALUE" may be suitable for a macro local to a driver, but is way
> too generic and fuzzy for a global function.

IMHO GENMASK presents the same situation, but actually I don't really mind
about the name.

> 
>> There's already GENMASK, which "generates a mask".
> 
> Yes. And it generates a (bit)mask, which is clear from its name.
> But a "value" is just too generic for a global function, and make me think of
> a pseudo-random number generator ;-)

:-)

>>> "val |= 0x5a << 12;" looks much more readable to me...
>>>
>>
>> Well, the idea behind this is that one can use it like:
>>
>> (see https://marc.info/?l=linux-kernel&m=148095872915717&w=2)
>>
>> ...
>> #define TIMEOUT_CLK_UNIT_MHZ       BIT(6)
>> #define BUS_CLK_FREQ_FOR_SD_CLK(x) GENVALUE(14,7,x)
>> ...
>>     val = 0;
>>     val |= TIMEOUT_CLK_UNIT_MHZ;         /* unit: MHz */
>>     val |= BUS_CLK_FREQ_FOR_SD_CLK(200); /* SDIO clock: 200MHz */
>> ...
>>
>> which makes it very practical for writing macros for associated HW
>> documentation.
> 
> Actually I more like the SETBITFIELD name...
> 

Well, Linus did not like it for example.
Since the start I was open to suggestions because I felt the name
was not great either.

https://marc.info/?l=linux-kernel&m=148095805515487&w=2


>>> OK. So it inserts a value into a bitfield.
>>>
>>> Yes, that can be useful. Now let's find a sensible name for this.
>>> Perhaps inspired by a PowerPC mnemonic? At least that would be more
>>> obvious than "GENVALUE", IMHO...
>>
>> I'm open to suggestions.
> 
> BITFIELD_INSERT()?

The problem is that right now it does not inserts into anything,
it just generates a value. The user can then OR that with something else
essentially "inserting" the value.

(see my reply to Borislav here:
https://marc.info/?l=linux-kernel&m=148095872915717&w=2 )

This allows a use case where BIT() and GENVALUE() can be used in a
similar way:

#define TIMEOUT_CLK_UNIT_MHZ       BIT(6)
#define BUS_CLK_FREQ_FOR_SD_CLK(x) GENVALUE(14,7,x)
...
    val = 0;
    val |= TIMEOUT_CLK_UNIT_MHZ;         /* unit: MHz */
    val |= BUS_CLK_FREQ_FOR_SD_CLK(200); /* SDIO clock: 200MHz */
...

I can write BITFIELD_INSERT as well, but I would not want to have
to choose between BITFIELD_INSERT and GENVALUE, because that would
mean that the snippet above would become:

#define BITFIELD_INSERT(target, msb, lsb, val)				\
	(target = ((target & ~GENMASK(msb, lsb)) | GENVALUE(msb, lsb, val)))
...
#define TIMEOUT_CLK_UNIT_MHZ          BIT(6)
#define BUS_CLK_FREQ_FOR_SD_CLK(y, x) BITFIELD_INSERT(y,14,7,x)
...
    val = 0;
    val |= TIMEOUT_CLK_UNIT_MHZ;         /* unit: MHz */
    BUS_CLK_FREQ_FOR_SD_CLK(val, 200);   /* SDIO clock: 200MHz */
...


NOTES:
- Going for BITFIELD_INSERT() means that we probably
need to decide its behaviour w.r.t existing bits, does it OR
(thus preserving bits set) or does it overwrite? (most likely
the later option)
- Going for BITFIELD_INSERT() calls for its counter-part
BITFIELD_EXTRACT(), so that we can do:

...
   val = 0x1115a000;
   if (BITFIELD_EXTRACT(val, 19, 12) == 0x5a)
...


Best regards,

Sebastian