Re: [RFC PATCH v2 12/12] gpiolib: Add fast processing path to bitmap API functions

[Janusz Krzysztofik <jmkrzyszt@gmail.com>]

On Tuesday, August 7, 2018 1:43:56 AM CEST Linus Walleij wrote:
> On Tue, Aug 7, 2018 at 12:29 AM Janusz Krzysztofik <jmkrzyszt@gmail.com> 
wrote:
> 
> Hi Janusz!
> 
> > Certain GPIO descriptor arrays returned by gpio_get_array() may contain
> > information on a single GPIO chip driving array member pins in hardware
> > order.  In such cases, bitmaps of values can be passed directly to the
> > chip callback functions without wasting time on iterations.
> >
> > Add respective code to gpiod_get/set_array_bitmap_complex() functions.
> >
> > Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
> 
> I think it would be disappointing to leave all the users of the old
> array API without the performance improvement. I think we need to
> deal with this in a way such that everyone can benefit from it.

There are a few issues to be resolved:

1) array size limited by bitmap size:
    - are we ready to limit array size to a single bitmap for all users?
    - if not, how can we pass a bitmap of an arbitrary size?
    - if as an array of bitmaps, is that still clear enough and easy to use?
    - other ideas?

2) arbitrary array support:
     - are we ready to drop that?
     - if not, do we agree to require all users to pack their arbitrary arrays 
        inside the gpio_descs structure?

Maybe more.

> Also it is kludgy if users (consumers) would need to handle the case
> where all lines are on the same chip separately, through the bitmap
> API.

Not true as long as array size fits (arbitrary arrays can be packed by users), 
but I see your point.

> What we need is an API that:
> 
> - All drivers handling arrays can use (including current users).
> 
> - Enables speed-up if the lines are all on the same chip/register.
> 
> - Doesn't require consumers to know if they are all on the same
>   chip or not.
> 
> This means a deep API with a small surface.
> 
> How do we achieve this the best way?

I think widely accepted solutions to those two issues I've mentioned above can 
give the answer.

Thanks,
Janusz