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

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Message-ID: <20241016083747.GB120095@rigel>
Date: Wed, 16 Oct 2024 16:37:47 +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 03:27:30PM +0800, Kent Gibson 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
> set the drive, and later to set the direction, in that order.
> Given it will be setting the direction, it should inhibit the event from
> the drive setting?
>

Ok, I oversimplified, it was actually

input -> output|active_low|open_drain

and

input -> output|open_source

fails the same way.

> Still haven't tested any debounce changes...
>

Have now.

input -> input|debounce

does not report the debounce.  Only get the one event and it does not
contain any debounce.

Similarly

input -> input|edge|debounce

which exercises a different path through cdev.

Cheers,
Kent.