linux-kernel - Re: [PATCH 1/2] bitreversal program

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Date:	Tue, 20 May 2008 13:01:27 +0200 (CEST)
From:	Soumyadip Das Mahapatra <kernelhacker@...ualserver.org>
To:	Harvey Harrison <harvey.harrison@...il.com>
cc:	linux-kernel@...r.kernel.org, akinobu.mita@...il.com
Subject: Re: [PATCH 1/2] bitreversal program



On Mon, 19 May 2008, Harvey Harrison wrote:

> On Mon, 2008-05-19 at 19:04 +0200, Soumyadip Das Mahapatra wrote:
>> --- a/include/linux/bitrev.h    2008-04-17 08:19:44.000000000 +0530
>> +++ b/include/linux/bitrev.h    2008-05-19 21:49:46.000000000 +0530
>> @@ -3,11 +3,32 @@
>>
>>   #include <linux/types.h>
>>
>> -extern u8 const byte_rev_table[256];
>> +/**
>> + * Here is a generalised bit reversal program
>> + * @x: word to get bits reversed
>> + * @k: key, explained below
>> + * for k = 31, it reverses the bits of word(32 bit)
>> + * for k = 24, it reverses the bytes in word
>> + * for k = 7, it reverses the bits in every byte without
>> + * changing the positions of bytes in a word
>> + * and for k = 16 it swaps the left and right halves of a
>> + * word
>> + */
>>
>> -static inline u8 bitrev8(u8 byte)
>
> What about anybody who currently uses bitrev8?
>
>> +static inline u32 gen_bit_rev(u32 x, u32 k)
>>   {
>> -       return byte_rev_table[byte];
>> +       if(k & 1)
>> +               x = (x & 0x55555555) << 1 | (x & 0xaaaaaaaa) >> 1;
>> +       if(k & 2)
>> +               x = (x & 0x33333333) << 2 | (x & 0xcccccccc) >> 2;
>> +       if(k & 4)
>> +               x = (x & 0x0f0f0f0f) << 4 | (x & 0xf0f0f0f0) >> 4;
>> +       if(k & 8)
>> +               x = (x & 0x00ff00ff) << 8 | (x & 0xff00ff00) >> 8;
>> +       if(k & 16)
>> +               x = (x & 0x0000ffff) << 16 | (x & 0xffff0000) >> 16;
>> +
>> +       return x;
>>   }
>
> Why is this better than a single 256 byte table?
>
> Harvey
>
>
> -- 
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>
>

Thanks for reviewing Harvey :-)
please look at the line below
>> -static inline u8 bitrev8(u8 byte)
It is a static function, so you cant use it from outside of this
file. So there should not be anyone using this function.

>Why is this better than a single 256 byte table?

Why store those things if stuffs can be done in smoother and cleaner
(using less memeory ofcourse) way!

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