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

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Date:   Tue, 6 Dec 2016 12:03:36 +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

On 06/12/16 11:42, Geert Uytterhoeven wrote:
> On Tue, Dec 6, 2016 at 11:31 AM, Sebastian Frias <sf84@...oste.net> wrote:
>> On 05/12/16 18:48, Geert Uytterhoeven wrote:
>>> On Mon, Dec 5, 2016 at 2:36 PM, Sebastian Frias <sf84@...oste.net> wrote:
>>>> Introduce SETBITFIELD(msb, lsb, value) macro to ease dealing with
>>>> continuous bitfields, just as BIT(x) does for single bits.
>>>
>>> If it's a bitfield, why not calling it that way?
>>
>> I don't know if you saw v2 (or v3 for that matter), but the name was changed
>> to GENVALUE.
> 
> ... 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?

There's already GENMASK, which "generates a mask".

>> Also a small use case was added to the commit message:
>>
>> "Introduce GENVALUE(msb, lsb, value) macro..."
>> "...This is useful mostly for creating values to be packed together
>> via OR operations, ex:
>>
>>    u32 val = 0x11110000;
>>    val |= GENVALUE(19, 12, 0x5a);
> 
> "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.

>> now 'val = 0x1115a000'"
>>
>>> So what about BITFIELD(start ,size), like arch/tile/kernel/tile-desc_32.c has?
>>>
>>>> SETBITFIELD_ULL(msb, lsb, value) macro is also added.
>>>
>>> Confused by the need for a "value" parameter...
>>
>> "value" is the value to be massaged (shifted, masked) into a [msb:lsb] bitfield.
> 
> 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.

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>