linux-kernel - Re: [PATCH v5 0/4] Introduce the for_each_set

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Date:   Mon, 4 May 2020 14:41:09 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     akpm@...ux-foundation.org, vilhelm.gray@...il.com,
        michal.simek@...inx.com, arnd@...db.de, rrichter@...vell.com,
        linus.walleij@...aro.org, bgolaszewski@...libre.com,
        yamada.masahiro@...ionext.com, rui.zhang@...el.com,
        daniel.lezcano@...aro.org, amit.kucheria@...durent.com,
        linux-arch@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v5 0/4] Introduce the for_each_set_clump macro

On Sun, May 03, 2020 at 04:38:36AM +0530, Syed Nayyar Waris wrote:
> This patchset introduces a new generic version of for_each_set_clump. 
> The previous version of for_each_set_clump8 used a fixed size 8-bit
> clump, but the new generic version can work with clump of any size but
> less than or equal to BITS_PER_LONG. The patchset utilizes the new macro 
> in several GPIO drivers.
> 
> The earlier 8-bit for_each_set_clump8 facilitated a
> for-loop syntax that iterates over a memory region entire groups of set
> bits at a time.
> 
> For example, suppose you would like to iterate over a 32-bit integer 8
> bits at a time, skipping over 8-bit groups with no set bit, where
> XXXXXXXX represents the current 8-bit group:
> 
>     Example:        10111110 00000000 11111111 00110011
>     First loop:     10111110 00000000 11111111 XXXXXXXX
>     Second loop:    10111110 00000000 XXXXXXXX 00110011
>     Third loop:     XXXXXXXX 00000000 11111111 00110011
> 
> Each iteration of the loop returns the next 8-bit group that has at
> least one set bit.
> 
> But with the new for_each_set_clump the clump size can be different from 8 bits.
> Moreover, the clump can be split at word boundary in situations where word 
> size is not multiple of clump size. Following are examples showing the working 
> of new macro for clump sizes of 24 bits and 6 bits.
> 
> Example 1:
> clump size: 24 bits, Number of clumps (or ports): 10
> bitmap stores the bit information from where successive clumps are retrieved.
> 
>      /* bitmap memory region */
>         0x00aa0000ff000000;  /* Most significant bits */
>         0xaaaaaa0000ff0000;
>         0x000000aa000000aa;
>         0xbbbbabcdeffedcba;  /* Least significant bits */
> 
> Different iterations of for_each_set_clump:-
> 'offset' is the bit position and 'clump' is the 24 bit clump from the
> above bitmap.
> Iteration first:        offset: 0 clump: 0xfedcba
> Iteration second:       offset: 24 clump: 0xabcdef
> Iteration third:        offset: 48 clump: 0xaabbbb
> Iteration fourth:       offset: 96 clump: 0xaa
> Iteration fifth:        offset: 144 clump: 0xff
> Iteration sixth:        offset: 168 clump: 0xaaaaaa
> Iteration seventh:      offset: 216 clump: 0xff
> Loop breaks because in the end the remaining bits (0x00aa) size was less
> than clump size of 24 bits.
> 
> In above example it can be seen that in iteration third, the 24 bit clump
> that was retrieved was split between bitmap[0] and bitmap[1]. This example 
> also shows that 24 bit zeroes if present in between, were skipped (preserving
> the previous for_each_set_macro8 behaviour). 
> 
> Example 2:
> clump size = 6 bits, Number of clumps (or ports) = 3.
> 
>      /* bitmap memory region */
>         0x00aa0000ff000000;  /* Most significant bits */
>         0xaaaaaa0000ff0000;
>         0x0f00000000000000;
>         0x0000000000000ac0;  /* Least significant bits */
> 
> Different iterations of for_each_set_clump:
> 'offset' is the bit position and 'clump' is the 6 bit clump from the
> above bitmap.
> Iteration first:        offset: 6 clump: 0x2b
> Loop breaks because 6 * 3 = 18 bits traversed in bitmap.
> Here 6 * 3 is clump size * no. of clumps.

Looking into the last patches where we have examples I still do not see a
benefit of variadic clump sizes. power of 2 sizes would make sense (and be
optimized accordingly (64-bit, 32-bit).

-- 
With Best Regards,
Andy Shevchenko