Re: [PATCH v5 0/4] Introduce the for_each_set_clump macro

Andy Shevchenko
2020-05-04T11:41:12Z

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