Re: [PATCH v12 01/11] bitops: Introduce the for_each_set_clump8 macro

Lukas Wunner
2019-03-26T09:43:50Z

On Tue, Mar 26, 2019 at 12:14:22PM +0900, William Breathitt Gray wrote:
> On Mon, Mar 25, 2019 at 10:38:54AM +0100, Lukas Wunner wrote:
> > On Mon, Mar 25, 2019 at 03:22:23PM +0900, William Breathitt Gray wrote:
> > > +/**
> > > + * find_next_clump8 - find next 8-bit clump with set bits in a memory region
> > > + * @clump: location to store copy of found clump
> > > + * @addr: address to base the search on
> > > + * @offset: bit offset at which to start searching
> > > + * @size: bitmap size in number of bits
> > > + *
> > > + * Returns the bit offset for the next set clump; the found clump value is
> > > + * copied to the location pointed by @clump. If no bits are set, returns @size.
> > > + */
> > > +unsigned int find_next_clump8(unsigned long *const clump,
> > > + const unsigned long *const addr,
> > > + unsigned int offset, const unsigned int size)
> > > +{
> > > + for (; offset < size; offset += 8) {
> > > + *clump = bitmap_get_value8(addr, size, offset);
> > > + if (!*clump)
> > > + continue;
> > > +
> > > + return offset;
> > > + }
> > > +
> > > + return size;
> > > +}
> > > +EXPORT_SYMBOL(find_next_clump8);
> >
> > Just use find_first_bit() / find_next_bit() to use optimized arch-specific
> > bitops instead of open-coding the iteration over the bitmap.
> >
> > See max3191x_get_multiple() for an example.
>
> Is this the sort of implementation you had in mind:
>
> offset = find_next_bit(addr, size, offset);
> if (offset == size)
> return size;
>
> offset -= offset % 8;
> *clump = bitmap_get_value8(addr, size, offset);
>
> return offset;

Almost. I'd use round_down() instead of "offset -= offset % 8".
Then it's just a single cheap logical and operation at runtime.

I'd try to avoid copying around the clump value and use a pointer
to u8 instead.

I don't understand the calculations in bitmap_get_value8() at all.
Why is it so complicated, does it allow passing in a start value
that's not a multiple of 8? Do you really need that? I imagine
a simplification is possible if that assumption can be made (and
is spelled out in the kerneldoc).

> Should the offset and size parameters be redefined as unsigned long to
> match the find_first_bit/find_next_bit function parameters?

Yes, probably. It's just the CPU's native length anyway.

Thanks,

Lukas