drm/vc4: hdmi: Force modeset when bpc or format changes

Whenever the maximum BPC is changed, vc4_hdmi_encoder_compute_config()
might pick up a different BPC or format depending on the display
capabilities.

That change will have a number of side effects, including the clock
rates and whether the scrambling is enabled.

However, only drm_crtc_state.connectors_changed will be set to true,
since that properly only affects the connector.

This means that while drm_atomic_crtc_needs_modeset() will return true,
and thus drm_atomic_helper_commit_modeset_enables() will call our
encoder atomic_enable() hook, mode_changed will be false.

So crtc_set_mode() will not call our encoder .atomic_mode_set() hook. We
use this hook in vc4 to set the vc4_hdmi_connector_state.output_bpc (and
output_format), and will then reuse the value in .atomic_enable() to select
whether or not scrambling should be enabled.

However, since our clock rate is pre-computed during .atomic_check(), we
end up with the clocks properly configured, but the scrambling disabled,
leading to a blank screen.

Let's set mode_changed to true in our HDMI driver to force the update of
output_bpc, and thus prevent the issue entirely.

Fixes: ba8c0fae ("drm/vc4: hdmi: Enable 10/12 bpc output")
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Link: https://lore.kernel.org/r/20220613144800.326124-32-maxime@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -1605,9 +1605,14 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 					 struct drm_crtc_state *crtc_state,
 					 struct drm_connector_state *conn_state)
 {
+	struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
+	struct drm_connector *connector = &vc4_hdmi->connector;
+	struct drm_connector_state *old_conn_state =
+		drm_atomic_get_old_connector_state(conn_state->state, connector);
+	struct vc4_hdmi_connector_state *old_vc4_state =
+		conn_state_to_vc4_hdmi_conn_state(old_conn_state);
 	struct vc4_hdmi_connector_state *vc4_state = conn_state_to_vc4_hdmi_conn_state(conn_state);
 	struct drm_display_mode *mode = &crtc_state->adjusted_mode;
-	struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
 	unsigned long long tmds_char_rate = mode->clock * 1000;
 	unsigned long long tmds_bit_rate;
 	int ret;
@@ -1636,6 +1641,11 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 	if (ret)
 		return ret;
 
+	/* vc4_hdmi_encoder_compute_config may have changed output_bpc and/or output_format */
+	if (vc4_state->output_bpc != old_vc4_state->output_bpc ||
+	    vc4_state->output_format != old_vc4_state->output_format)
+		crtc_state->mode_changed = true;
+
 	return 0;
 }