linux-kernel - Re: [PATCH v2] blkdev: Replace blksize

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Date:   Fri, 29 May 2020 15:27:30 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Matthew Wilcox <willy@...radead.org>,
        Kaitao Cheng <pilgrimtao@...il.com>
Cc:     axboe@...nel.dk, hch@....de, sth@...ux.ibm.com,
        viro@...iv.linux.org.uk, clm@...com, jaegeuk@...nel.org,
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        chaitanya.kulkarni@....com, houtao1@...wei.com,
        asml.silence@...il.com, ajay.joshi@....com,
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        ocfs2-devel@....oracle.com, deepa.kernel@...il.com
Subject: Re: [PATCH v2] blkdev: Replace blksize_bits() with ilog2()

On 2020-05-29 13:27, Matthew Wilcox wrote:
> On Fri, May 29, 2020 at 10:11:00PM +0800, Kaitao Cheng wrote:
>> There is a function named ilog2() exist which can replace blksize.
>> The generated code will be shorter and more efficient on some
>> architecture, such as arm64. And ilog2() can be optimized according
>> to different architecture.
> 
> We'd get the same benefit from a smaller patch with just:
> 
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1502,15 +1502,9 @@ static inline int blk_rq_aligned(struct request_queue *q, unsigned long addr,
>  	return !(addr & alignment) && !(len & alignment);
>  }
>  
> -/* assumes size > 256 */
>  static inline unsigned int blksize_bits(unsigned int size)
>  {
> -	unsigned int bits = 8;
> -	do {
> -		bits++;
> -		size >>= 1;
> -	} while (size > 256);
> -	return bits;
> +	return ilog2(size);
>  }
>  
>  static inline unsigned int block_size(struct block_device *bdev)

Hi Matthew,

I had suggested to change all blksize_bits() calls into ilog2() calls
because I think that a single function to calculate the logarithm base 2
is sufficient.

Thanks,

Bart.