linux-kernel - Re: [PATCH 1/4] bitops: Add single_bit

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Message-ID: <e2675600-7b04-19b0-79ce-28a4e1d1f225@fi.rohmeurope.com>
Date:   Mon, 22 Nov 2021 13:18:42 +0000
From:   "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:     Matti Vaittinen <mazziesaccount@...il.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jiri Kosina <trivial@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yury Norov <yury.norov@...il.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] bitops: Add single_bit_set()

On 11/22/21 14:57, Andy Shevchenko wrote:
> On Mon, Nov 22, 2021 at 12:42:21PM +0000, Vaittinen, Matti wrote:
>> On 11/22/21 13:28, Andy Shevchenko wrote:
>>> On Mon, Nov 22, 2021 at 01:03:25PM +0200, Matti Vaittinen wrote:
> 
> What do you mean by this?
> 
> hweight() will return you the number of the non-zero elements in the set.

Exactly. The function I added did only check if given set of bits had 
only one bit set.

> In application to boolean based arrays it means the number of bits that
> are set. Obviously, the condition `hweight() == 1` is what you are looking
> for.

Yes. (hweight() == 1) will result exactly same result - which does not 
mean the single_bit_set() was same as hweight(). Say, we have a 32bit 
wide value, which is likely to be '11' (binary). The single_bit_set() 
would return 0 after checking the 2.nd bit. So, this is not equal to 
hamming weight. But as I said, none of this is really needed for my 
use-case and does not warrant adding new function.

>> I think I actually tried using hweight() at some point but don't really
>> remember why I rolled the single_bit_set. (I remember the hweight()
>> usage because I had to do some googling as I had never heard term
>> hamming weight before).
> 
> Oh, it should be a very good reason not to use hweight() since on some
> architectures it might become just one assembly instruction.

I didn't know of this. Thanks for sharing.

> (Side note: all your sentences but last in all of the paragraphs are ending
>   with trailing space. For example, "series " and "thanks." for the comparison
>   in the previous paragraph. Can you fix this, please?)

I changed from evolution to Thunderbird when my old laptop broke. It 
seems Thunderbird is doing this by default, don't know if it can be 
changed :( I'll see if I find a solution.

Best Regards
	--Matti

-- 
The Linux Kernel guy at ROHM Semiconductors

Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~ this year is the year of a signature writers block ~~