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Subject: Re: [PATCH v2] blkdev: Replace blksize_bits() with ilog2()
To:     Bart Van Assche <bvanassche@acm.org>,
        Matthew Wilcox <willy@infradead.org>,
        Kaitao Cheng <pilgrimtao@gmail.com>
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References: <20200529141100.37519-1-pilgrimtao@gmail.com>
 <20200529202713.GC19604@bombadil.infradead.org>
 <c7a5bbc4-ffc2-6a63-fed3-9874969afc31@acm.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <fe0fc36e-fa67-a6b9-cc7a-d86d3f21cf2c@kernel.dk>
Date:   Fri, 29 May 2020 16:39:26 -0600
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On 5/29/20 4:27 PM, Bart Van Assche wrote:
> 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.

I think this should be a two-parter:

1) Use the inode blkbits where appropriate
2) Then do this change

I'm leaning towards just doing it in blksize_bits() instead of updating
every caller, unless there aren't that many left once we've gone
through patch 1.

-- 
Jens Axboe