Re: [questions] : gpiolib and gpioset behaviour

[Kent Gibson <warthog618@xxxxxxxxx>]

On Fri, Apr 26, 2024 at 03:08:33PM +0200, Gilles BULOZ wrote:
> On Fri, Apr 26, 2024 at 04:07 AM, Kent Gibson wrote :
> > On Mon, Apr 22, 2024 at 06:49:05PM +0200, Gilles BULOZ wrote:
> >> On Mon, Apr 22, 2024 at 3:55 PM Bartosz Golaszewski wrote :
> >>> On Mon, Apr 22, 2024 at 2:44 PM Gilles BULOZ <gilles.buloz@xxxxxxxxxxx> wrote:
> >>>>

> >>
> >
> > I suspect you are referring to gpiolib here - the mask in gc->get_multiple()
> > being unsigned long*.
> >
> > The uAPI that libgpiod uses is limited to 64 lines per request, but that is
> > only relevant if you want to request more than 64 lines at once from userspace
> > (you would have to break that into two requests to access all 112 lines).
> >
> > Note that the mask in gc->get_multiple() is unsigned long*, so it is a
> > pointer to an array of unsigned long.  Its width is not limited by
> > unsigned long, but by the bits parameter.  In your case the mask you pass
> > should contain multiple unsigned longs to achieve 112 bits.
> > Refer to gpiod_get_array_value_complex() for an example of building bitmap
> > masks to pass to gc->get_multiple(), in that case via
> > gpio_chip_get_multiple().
> >
>
> I was refering the get_multiple/set_multiple callbacks in struct gpio_chip
> that are defined like this :
>  int (*get_multiple)(struct gpio_chip *gc, unsigned long *mask, unsigned long *bits);
>  void (*set_multiple)(struct gpio_chip *gc, unsigned long *mask, unsigned long *bits);
> With some debug in my GPIO chip driver implementing these functions, I saw that
> the bits set in "mask" and the ones used in "bits" are the ones whose bit
> numbers are directly matching the GPIO offset/line numbers of the chip. But I
> only used unsigned long, not arrays, so I thought I was limited to offset/line
> 31 on 32bit arch, and 63 on 64bit arch.
> As you suggested, I'm currently having a look to gpiod_get_array_value_complex()
> but I must admit I'm currently a little bit lost. I've never thought GPIO
> implementation could become so complex for my brain :-)
>

The bit of primary interest that I was referring to was the DECLARE_BITMAP()
as used for the fastpath mask:

		DECLARE_BITMAP(fastpath_mask, FASTPATH_NGPIO);

That does the sizing math for you.  In your case you would use 112 for
the NGPIO.  There are also examples of using __set_bit() to set bits in
the mask.  Take a look in linux/bitmaps.h and linux/bitops.h for the
relevant definitions if you want to dig deeper.

And, yeah, the amount of work that goes into just driving physical
lines up and down, fundamentally just toggling bits, frequently makes my brain
hurt too ;-).

Cheers,
Kent.