Re: [PATCH libgpiod] tests: uapi: add test-cases for open-drain and open-source emulation

[Kent Gibson <warthog618@gmail.com>]

On Thu, Apr 10, 2025 at 11:17:47AM +0200, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
>
> The kernel GPIO subsystem can emulate open-drain and open-source by not
> actively driving the line for active and inactive output values
> respectively. The kernel does it by setting the line to input in these
> cases but this still must be reported as output to user-space. Add new
> test-cases that verify this behavior.
>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
> ---
> Andy's comment on a GPIOLIB patch made me realize it's a good idea to
> add tests for open-drain and open-source emulation in the kernel where
> we don't actively drive the line for active and inactive values
> respectively.
> ---
>  tests/tests-kernel-uapi.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 87 insertions(+)
>
> diff --git a/tests/tests-kernel-uapi.c b/tests/tests-kernel-uapi.c
> index ff220fc..5955fac 100644
> --- a/tests/tests-kernel-uapi.c
> +++ b/tests/tests-kernel-uapi.c
> @@ -110,3 +110,90 @@ GPIOD_TEST_CASE(enable_debounce_then_edge_detection)
>
>  	g_assert_cmpuint(ts_falling, >, ts_rising);
>  }
> +
> +GPIOD_TEST_CASE(open_drain_emulation)
> +{
> +	static const guint offset = 2;
> +
> +	g_autoptr(GPIOSimChip) sim = g_gpiosim_chip_new("num-lines", 8, NULL);
> +	g_autoptr(struct_gpiod_chip) chip = NULL;
> +	g_autoptr(struct_gpiod_line_settings) settings = NULL;
> +	g_autoptr(struct_gpiod_line_config) line_cfg = NULL;
> +	g_autoptr(struct_gpiod_line_request) request = NULL;
> +	g_autoptr(struct_gpiod_line_info) info = NULL;
> +	gint ret;
> +
> +	chip = gpiod_test_open_chip_or_fail(g_gpiosim_chip_get_dev_path(sim));
> +	settings = gpiod_test_create_line_settings_or_fail();
> +	line_cfg = gpiod_test_create_line_config_or_fail();
> +
> +	gpiod_line_settings_set_direction(settings,
> +					  GPIOD_LINE_DIRECTION_OUTPUT);
> +	gpiod_line_settings_set_drive(settings, GPIOD_LINE_DRIVE_OPEN_DRAIN);
> +	gpiod_test_line_config_add_line_settings_or_fail(line_cfg, &offset, 1,
> +							 settings);
> +	request = gpiod_test_chip_request_lines_or_fail(chip, NULL, line_cfg);
> +
> +	ret = gpiod_line_request_set_value(request, offset,
> +					   GPIOD_LINE_VALUE_ACTIVE);
> +	g_assert_cmpint(ret, ==, 0);
> +	gpiod_test_return_if_failed();
> +
> +	/*
> +	 * The open-drain emulation in the kernel will set the line's direction
> +	 * to input but NOT set FLAG_IS_OUT. Let's verify the direction is
> +	 * still reported as output.
> +	 */

My understanding is that FLAG_IS_OUT is always set for output lines,
even if the direction is set to input for the emulation.

To quote gpiod_direction_output():

set_output_flag:
	/*
	 * When emulating open-source or open-drain functionalities by not
	 * actively driving the line (setting mode to input) we still need to
	 * set the IS_OUT flag or otherwise we won't be able to set the line
	 * value anymore.
	 */
	if (ret == 0)
		set_bit(FLAG_IS_OUT, &desc->flags);
	return ret;

Cheers,
Kent.