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

Lukas Wunner
2019-03-26T13:03:24Z

On Tue, Mar 26, 2019 at 07:08:18PM +0900, William Breathitt Gray wrote:
> On Tue, Mar 26, 2019 at 10:43:45AM +0100, Lukas Wunner wrote:
> > I'd try to avoid copying around the clump value and use a pointer
> > to u8 instead.
>
> Although in this case we are handling 8-bit clumps, I anticipate device
> drivers in the future which may benefit from larger size clumps (e.g.
> GPIO devices with 24-bit ports). It'll be better to define clumps
> similar to how we're defining bitmaps now (unsigned long *) so that we
> can support these sizes if need be in the future without requiring data
> type changes.

It's just that copying around data in memory may be more expensive
than, say, returning from a function, in which case it's usually
stored in a register.

> In this case, bitmap_get_value8 could be simplified to something like
> this:
>
> index = BIT_WORD(start);
> offset = start % BITS_PER_LONG;
> return (bitmap[index] >> offset) & 0xFF;

Hm, shouldn't that be "offset = round_down(start, 8)" ?

(I prefer the multi-line version FWIW.)

> Would it be better to define bitmap_get_value8 as a macro then?

Or a static inline.

Thanks,

Lukas