Re: [libgpiod] gpiod_line_get_value_bulk may be broken?

[Kent Gibson <warthog618@xxxxxxxxx>]

On Thu, Jul 27, 2023 at 10:17:05PM +0100, andy pugh wrote:
> On Thu, 27 Jul 2023 at 21:54, Kent Gibson <warthog618@xxxxxxxxx> wrote:
> 
> > That is not how the line_bulk API is used.
> > You don't request the lines separately and then add them to the bulk,
> > you add them to the bulk then request them with
> > gpiod_line_request_bulk_input(), or one of the other
> > gpiod_line_request_bulk_XXX() functions.
> 
> I did try that way first, but it didn't seem to be working for me.
> I am currently upgrading the system to Bookworm (gpiod v1.6) to try again.
> 

If you can repeat it, and ideally provide a failing test case, then we can
take a look at it.

> > Btw, the primary use case for the bulk is for when you need to perform
> > operations on a set of lines as simultaneously as possible.
> 
> I am trying to do things as quickly as possible on a predetermined set
> of lines.
> I am experimenting with gpiod as a replacement for an existing (and
> no-longer-working) driver that is part of LinuxCNC.
> 
> I suspect that gpiod won't be fast enough, ideally I would like to be
> able to write to 15 IO lines in 15µs. (because the code will run in a
> realtime thread which can't overrun)
> (There are other reasons that it might not work too, you can probably
> think of more than I can)
> 

Depends on what Pi you are on.  A Pi Zero would struggle, but on a Pi4
that is doable, of course depending on what else you are doing.
That is based on benchmarking libgpiod v2, but I would expect v1 to be
similar.

On a Pi is it significantly faster to go direct to hardware using
/dev/gpiomem, rather than going via the kernel as libgpiod does.
I do my best to avoid using gpiomem these days, but if you really need to
minimize CPU cycles or latency then that is another option.

Cheers,
Kent.