Re: [PATCH v1 5/5] gpiolib: cdev: Utilize more bitmap APIs

[Kent Gibson <warthog618@xxxxxxxxx>]

On Tue, Sep 26, 2023 at 05:46:07PM -0700, Yury Norov wrote:
> On Tue, Sep 26, 2023 at 08:20:07AM +0300, Andy Shevchenko wrote:
> > Currently we have a few bitmap calls that are open coded in the library
> > module. Let's convert them to use generic bitmap APIs instead.
> > 
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

> > +	bitmap_from_arr64(mask, &lv->mask, GPIO_V2_LINES_MAX);
> > +	bitmap_from_arr64(bits, &lv->bits, GPIO_V2_LINES_MAX);
> > +
> > +	num_set = bitmap_gather(vals, bits, mask, lr->num_lines);
> 
> It looks like GPIO_V2_LINES_MAX is always 64, and so I wonder: is
> my understanding correct that all bits in ->mask and ->bits beyond
> lr->num_lines are clear?
> 

The lv fields come from userspace and so cannot be guaranteed to be
zeroed beyond lr->num_lines.  Any set bits beyond that must be ignored,
one way or another.

> If so, you can seemingly pass the GPIO_V2_LINES_MAX instead of
> lr->num_lines, and that way it will be small_cons_nbits()-optimized.
> 

But that would be decidedly non-optimal for the most common case where
lr->num_lines == 1.

Cheers,
Kent.