Re: [PATCH 2/4] gpiolib: cdev: Ignore reconfiguration without direction

[Kent Gibson <warthog618@gmail.com>]

On Thu, Jun 27, 2024 at 04:06:21PM +0200, Bartosz Golaszewski wrote:
> On Wed, Jun 26, 2024 at 7:29 AM Kent Gibson <warthog618@gmail.com> wrote:
> >
> > linereq_set_config() behaves badly when direction is not set.
> > The configuration validation is borrowed from linereq_create(), where,
> > to verify the intent of the user, the direction must be set to in order to
> > effect a change to the electrical configuration of a line. But, when
> > applied to reconfiguration, that validation does not allow for the unset
> > direction case, making it possible to clear flags set previously without
> > specifying the line direction.
> >
> > Adding to the inconsistency, those changes are not immediately applied by
> > linereq_set_config(), but will take effect when the line value is next get
> > or set.
> >
> > For example, by requesting a configuration with no flags set, an output
> > line with GPIO_V2_LINE_FLAG_ACTIVE_LOW and GPIO_V2_LINE_FLAG_OPEN_DRAIN
> > set could have those flags cleared, inverting the sense of the line and
> > changing the line drive to push-pull on the next line value set.
> >
> > Skip the reconfiguration of lines for which the direction is not set, and
> > only reconfigure the lines for which direction is set.
> >
> > Fixes: a54756cb24ea ("gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG_IOCTL")
> > Signed-off-by: Kent Gibson <warthog618@gmail.com>
> > ---
> >  drivers/gpio/gpiolib-cdev.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> > index f7a129d67b7d..ef08b23a56e2 100644
> > --- a/drivers/gpio/gpiolib-cdev.c
> > +++ b/drivers/gpio/gpiolib-cdev.c
> > @@ -1534,12 +1534,14 @@ static long linereq_set_config(struct linereq *lr, void __user *ip)
> >                 line = &lr->lines[i];
> >                 desc = lr->lines[i].desc;
> >                 flags = gpio_v2_line_config_flags(&lc, i);
> > +               /*
> > +                * Lines not explicitly reconfigured as input or output
> > +                * are left unchanged.
> > +                */
> > +               if (!(flags & GPIO_V2_LINE_DIRECTION_FLAGS))
> > +                       continue;
>
> Series looks good, thanks. I'd say that this bit here calls for at
> least a debug-level message since we don't return an error unlike v1.
> What do you think?
>

The change to the libgpiod Python bindings makes use of this to support
reconfiguration of subsets, so on its own it isn't an abnormal path and
I'm not sure it warrants even a debug.

OTOH, I did consider if there should be a check that at least one line
in the reconfig has a direction, returning an error if there are none, but
was on the fence about it and left it out as it added complexity.

Would that make more sense?
Or do you have a problem with reconfiguring subsets?

Cheers,
Kent.