Re: [PATCH v3 15/15] drm/tests: hdmi: Add test for unsuccessful forced fallback to YUV420

[Maxime Ripard <mripard@kernel.org>]

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Hi,

On Wed, Mar 26, 2025 at 12:20:04PM +0200, Cristian Ciocaltea wrote:
> Provide test to verify a forced fallback to YUV420 output cannot succeed
> when driver doesn't advertise YUV420 support.
> 
> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
> ---
>  drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c | 46 ++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
> 
> diff --git a/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c b/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c
> index 99bedb2d6f555b3b140256000dfa7491d2a8f515..c2976b42aa2aacd2a68a871bffe97e795ca713d4 100644
> --- a/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c
> +++ b/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c
> @@ -1493,6 +1493,51 @@ static void drm_test_check_max_tmds_rate_format_fallback_yuv420(struct kunit *te
>  	drm_modeset_acquire_fini(&ctx);
>  }
>  
> +/*
> + * Test that if a driver supports only RGB, but the chosen mode can be
> + * supported by the screen only in YUV420 output format, we end up with
> + * an unsuccessful forced fallback attempt.

What do you mean by "forced"?

> + */
> +static void drm_test_check_driver_unsupported_fallback_yuv420(struct kunit *test)
> +{
> +	struct drm_atomic_helper_connector_hdmi_priv *priv;
> +	struct drm_modeset_acquire_ctx ctx;
> +	struct drm_display_info *info;
> +	struct drm_display_mode *yuv420_only_mode;
> +	struct drm_connector *conn;
> +	struct drm_device *drm;
> +	struct drm_crtc *crtc;
> +	int ret;
> +
> +	priv = drm_kunit_helper_connector_hdmi_init_with_edid(test,
> +				BIT(HDMI_COLORSPACE_RGB),
> +				12,
> +				test_edid_hdmi_1080p_rgb_yuv_4k_yuv420_dc_max_200mhz);
> +	KUNIT_ASSERT_NOT_NULL(test, priv);
> +
> +	drm = &priv->drm;
> +	crtc = priv->crtc;
> +	conn = &priv->connector;
> +	info = &conn->display_info;
> +	KUNIT_ASSERT_TRUE(test, info->is_hdmi);
> +	KUNIT_ASSERT_FALSE(test, conn->ycbcr_420_allowed);
> +
> +	yuv420_only_mode = drm_kunit_display_mode_from_cea_vic(test, drm, 95);
> +	KUNIT_ASSERT_NOT_NULL(test, yuv420_only_mode);
> +	KUNIT_ASSERT_TRUE(test, drm_mode_is_420_only(info, yuv420_only_mode));
> +
> +	drm_modeset_acquire_init(&ctx, 0);
> +
> +	ret = drm_kunit_helper_enable_crtc_connector(test, drm,
> +						     crtc, conn,
> +						     yuv420_only_mode,
> +						     &ctx);
> +	KUNIT_EXPECT_LT(test, ret, 0);

I think that's where your current approach falls a bit short. You should
really craft the state yourself and check the returned value of
drm_atomic_check_only(), not rely on
drm_kunit_helper_enable_crtc_connector() doing the right thing, when it
doesn't really tell you :)

> +	drm_modeset_drop_locks(&ctx);
> +	drm_modeset_acquire_fini(&ctx);
> +}
> +

We still need to do the same with a driver that supports both, but the
monitor doesn't.

Maxime

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