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

From: William Breathitt Gray <[email-masked]>
Date: Mon, 4 May 2020 10:36:38 -0400

On Mon, May 04, 2020 at 02:41:09PM +0300, Andy Shevchenko wrote:
> 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

There is of course benefit in defining for_each_set_clump with clump
sizes of powers of 2 (we can optimize for 32 and 64 bit sizes and avoid
boundary checks that we know will not occur), but at the very least the
variable size bitmap_set_value and bitmap_get_value provide significant
benefit for the readability of the gpio-xilinx code:

	bitmap_set_value(old, state[0], 0, width[0]);
	bitmap_set_value(old, state[1], width[0], width[1]);
	...
	state[0] = bitmap_get_value(new, 0, width[0]);
	state[1] = bitmap_get_value(new, width[0], width[1]);

These lines are simple and clear to read: we know immediately what they
do. But if we did not have bitmap_set_value/bitmap_get_value, we'd have
to use several bitwise operations for each line; the obfuscation of the
code would be an obvious hinderance here.

William Breathitt Gray
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