drm/vc4: kms: Warn if we have an incompatible muxing setup

The documentation explicitly states we must prevent the output
2 and 3 from feeding from the same HVS channel.

Let's add a warning to make some noise if we ever find ourselves in such
a case.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220328153659.2382206-6-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_kms.c

@@ -290,6 +290,10 @@ static void vc5_hvs_pv_muxing_commit(struct vc4_dev *vc4,
 
 		switch (vc4_crtc->data->hvs_output) {
 		case 2:
+			drm_WARN_ON(&vc4->base,
+				    VC4_GET_FIELD(HVS_READ(SCALER_DISPCTRL),
+						  SCALER_DISPCTRL_DSP3_MUX) == channel);
+
 			mux = (channel == 2) ? 0 : 1;
 			reg = HVS_READ(SCALER_DISPECTRL);
 			HVS_WRITE(SCALER_DISPECTRL,