linux-kernel - Re: [PATCH v3 6/6] gpiolib: notify user-space about in-kernel line state changes

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Message-ID: <20241016085719.GA195243@rigel>
Date: Wed, 16 Oct 2024 16:57:19 +0800
From: Kent Gibson <warthog618@...il.com>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3 6/6] gpiolib: notify user-space about in-kernel line
 state changes

On Wed, Oct 16, 2024 at 09:50:58AM +0200, Bartosz Golaszewski wrote:
> On Wed, Oct 16, 2024 at 9:27 AM Kent Gibson <warthog618@...il.com> wrote:
> >
> > On Wed, Oct 16, 2024 at 01:19:44PM +0800, Kent Gibson wrote:
> > > On Tue, Oct 15, 2024 at 12:56:18PM +0200, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > >
> > > > -   return gpio_do_set_config(guard.gc, gpio_chip_hwgpio(desc), config);
> > > > +   ret = gpio_do_set_config(guard.gc, gpio_chip_hwgpio(desc), config);
> > > > +   if (ret == 0) {
> > > > +           /* These are the only options we notify the userspace about. */
> > > > +           switch (pinconf_to_config_param(config)) {
> > > > +           case PIN_CONFIG_BIAS_DISABLE:
> > > > +           case PIN_CONFIG_BIAS_PULL_DOWN:
> > > > +           case PIN_CONFIG_BIAS_PULL_UP:
> > > > +           case PIN_CONFIG_DRIVE_OPEN_DRAIN:
> > > > +           case PIN_CONFIG_DRIVE_OPEN_SOURCE:
> > > > +           case PIN_CONFIG_DRIVE_PUSH_PULL:
> > > > +           case PIN_CONFIG_INPUT_DEBOUNCE:
> > > > +                   gpiod_line_state_notify(desc,
> > > > +                                           GPIO_V2_LINE_CHANGED_CONFIG);
> > > > +                   break;
> > > > +           default:
> > > > +                   break;
> > > > +           }
> > > > +   }
> > > > +
> > > > +   return ret;
> > > >  }
> > >
> > > Ah, the debounce - I forgot about that, and other features that cdev
> > > might emulate.
> > >
> > > What happens if userspace requests a line with debounce that is
> > > supported by hardware?  Seems to me we'll see both a LINE_REQUESTED and a
> > > LINE_CONFIG_CHANGED when the line is requested.
> > >
> >
> > This is problematic for me to test at the moment, as gpiosim doesn't support
> > debounce. Any chance we could make that configurable?  Similarly drive.
> >
> > > Conversely, what if a config change impacts features that don't result in a
> > > notification from gpiod_set_config(), like active low, or emulated
> > > drive or debounce?
> > >
> >
> > Bah, drive is emulated in gpiolib itself, so that should be fine.
> >
> > When changing config cdev always calls gpiod_direction_input/output(), so I
> > think that covers the active low case.
> >
> > But I have a test taking a line from input to output|open_drain and I
> > get two change events.  The first is the most interesting as it reports
> > input|open_drain, the second then reports output|open_drain.
> > That is due to gpiod_direction_output() calling gpiod_set_config() to
>
> No, it never calls gpiod_set_config() but gpio_set_config() which
> never emits an event.
>

Depends whether the driver supports drive or not.

If it does then
line 2858:

		ret = gpio_set_config(desc, PIN_CONFIG_DRIVE_OPEN_DRAIN);

will emit.

So your _nonotify function is calling a bunch of functions that can
emit.

Cheers,
Kent.