Re: [RFC] gpio: expose pull-up/pull-down line flags to userspace

[Drew Fustini <pdp7pdp7@gmail.com>]

Hi Kent, that makes sense to me. Hopefully, Bartosz and/or Linus will
advise if this is an issue.

thanks,
drew

On Thu, Oct 10, 2019 at 1:17 PM Kent Gibson <warthog618@gmail.com> wrote:
>
> On Thu, Oct 10, 2019 at 06:14:21PM +0800, Kent Gibson wrote:
> > On Thu, Oct 10, 2019 at 09:47:35AM +0200, Drew Fustini wrote:
> > > On Thu, Oct 10, 2019 at 1:59 AM Kent Gibson <warthog618@gmail.com> wrote:
> > > > It is basically working for me on my Pi4:
> > > >
> > > > pi@quoll:~ $ ./gpiodctl get gpiochip0 7
> > > > 0
> > > > pi@quoll:~ $ ./gpiodctl get -u gpiochip0 7
> > > > 1
> > > > pi@quoll:~ $ ./gpiodctl get gpiochip0 7
> > > > 1
> > > > pi@quoll:~ $ ./gpiodctl get -d gpiochip0 7
> > > > 0
> > > > pi@quoll:~ $ ./gpiodctl get gpiochip0 7
> > > > 0
> > > >
> > > > That is using the gpiodctl tool from my gpiod library.
> > > > My gpiod test suite also passes, but it doesn't do much to
> > > > exercise the UAPI.
> > > > I was intending to run my uapi test suite, which is more thorough,
> > > > but it turns out that only targets gpio-mockup, whereas my gpiod
> > > > test suite can target either.
> > > > Something else for the todo list.
> > > >
> > > > Hopefully it is obvious what gpiodctl is doing. (-u sets the
> > > > pull-up flag, -d sets the pull-down flag)
> > > > Looks like the pulls stick when the line is released, and the
> > > > subsequent get, without pull-up set, either doesn't clear the
> > > > pull-up/down or the line stays floating at the old pull level.
> > > > More investigation required, but that will have to wait til
> > > > I get back to this later in the day.
> > > >
> > > > Oh, and that is running on the rpi-5.3.3 kernel patched with everything
> > > > on my topic/gpio-uapi-config branch from 5.4-rc2 onward.
> > >
> > > Thanks for sharing your results.
> > >
> > > My Pi 3 had been running 5.3.0-v7+ from September 20th with my
> > > pull-up/down patch (82fc38f6ab59).
> > >
> > > I removed that patch and just cross-compiled 5.4-rc2 with
> > > multi_v7_defconfig for the Pi3.
> > >
> > > Are these the commits that I should apply from your topic branch?
> > >
> > > bdc9696a27ed pull up/down requires explicit input mode in lineevent_create
> > > 14ee636232d4 disallow pull up/down on outputs
> > > ce03bf5af1ec implement SET_CONFIG_IOCTL
> > > f38b7554eb52 pull common validation code into linehandle_validate_flags
> > > 31c0aa53ffc3 Add default values for setting output
> > > 3c7ec03efcd9 add support for pull up/down to lineevent_create
> > > 99b85d1c26ea gpio: add new ioctl() to gpio chardev
> > > 82fc38f6ab59 gpio: expose pull-up/pull-down line flags to userspace
> > > f6cfbbe2950b gpiolib: sanitize flags before allocating memory in
> > > lineevent_create()
> > >
> > Those are the ones.
> >
> > Plus there are now another 3:
> > 625cd0a0df3ad9 actively disable bias on outputs
> > 9d1f9db81b4dc4 actively disable bias on inputs when pull up/down not set
> > c6d4bf32c05189 add set_config to support pull up/down
> >
> > Those add pull up/down support to gpio-mockup, and fix the stuck pulls
> > I noted earlier - though they still remain applied until the next time
> > the line is requested.
> >
> > 9d1f9db81b4dc4 doesn't like being applied to the rpi kernel, I assume as
> > there have been conflicting changes between 5.3.3 and 5.4-rc2, so I
> > patched that one manually.
> >
> The disable bias changes I mentioned above are problematic - they break
> ABI backward compatibility by explicitly disabling bias when neither
> PULL_UP nor PULL_DOWN are set - which will be the case for legacy apps.
>
> We really need to handle 4 possible bias states - None, Pull Up, Pull
> Down and As Is, with the zero default being As Is.
> So I intend to reinterpret the pull up and down flags as a two bit field
> encoding the following:
>
> 0 - AsIs
> 1 - PullUp
> 2 - PullDown
> 3 - PullNone
>
> Any objections?
>
> Cheers,
> Kent.