drm/amd/display: Add get_dig_frontend implementation for DCEx

Some old ASICs might not implement/require get_dig_frontend helper; in
this scenario, we can have a NULL pointer exception when we try to call
it inside vbios disable operation. For example, this situation might
happen when using Polaris12 with an eDP panel. This commit avoids this
situation by adding a specific get_dig_frontend implementation for DCEx.

Cc: Alex Deucher <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Harry Wentland <[e-mail]>
Cc: Nicholas Kazlauskas <[e-mail]>
Cc: Chiawen Huang <[e-mail]>
Reported-and-tested-by: Borislav Petkov <[e-mail]>
Acked-by: Alex Deucher <[e-mail]>
Signed-off-by: Rodrigo Siqueira <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Cc: [e-mail]

drivers/gpu/drm/amd/display/dc/dce/dce_link_encoder.c

···
 	.is_dig_enabled = dce110_is_dig_enabled,
 	.destroy = dce110_link_encoder_destroy,
 	.get_max_link_cap = dce110_link_encoder_get_max_link_cap,
+	.get_dig_frontend = dce110_get_dig_frontend,
 };
 
 static enum bp_result link_transmitter_control(
···
 
 	REG_UPDATE(DP_LINK_CNTL, DP_LINK_TRAINING_COMPLETE, complete);
 
+}
+
+unsigned int dce110_get_dig_frontend(struct link_encoder *enc)
+{
+	struct dce110_link_encoder *enc110 = TO_DCE110_LINK_ENC(enc);
+	u32 value;
+	enum engine_id result;
+
+	REG_GET(DIG_BE_CNTL, DIG_FE_SOURCE_SELECT, &value);
+
+	switch (value) {
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGA:
+		result = ENGINE_ID_DIGA;
+		break;
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGB:
+		result = ENGINE_ID_DIGB;
+		break;
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGC:
+		result = ENGINE_ID_DIGC;
+		break;
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGD:
+		result = ENGINE_ID_DIGD;
+		break;
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGE:
+		result = ENGINE_ID_DIGE;
+		break;
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGF:
+		result = ENGINE_ID_DIGF;
+		break;
+	case DCE110_DIG_FE_SOURCE_SELECT_DIGG:
+		result = ENGINE_ID_DIGG;
+		break;
+	default:
+		// invalid source select DIG
+		result = ENGINE_ID_UNKNOWN;
+	}
+
+	return result;
 }
 
 void dce110_link_encoder_set_dp_phy_pattern_training_pattern(
···
 	.disable_hpd = dce110_link_encoder_disable_hpd,
 	.is_dig_enabled = dce110_is_dig_enabled,
 	.destroy = dce110_link_encoder_destroy,
-	.get_max_link_cap = dce110_link_encoder_get_max_link_cap
+	.get_max_link_cap = dce110_link_encoder_get_max_link_cap,
+	.get_dig_frontend = dce110_get_dig_frontend
 };
 
 void dce60_link_encoder_construct(

drivers/gpu/drm/amd/display/dc/dce/dce_link_encoder.h

···
 	enum engine_id engine,
 	bool connect);
 
+unsigned int dce110_get_dig_frontend(struct link_encoder *enc);
+
 void dce110_link_encoder_set_dp_phy_pattern_training_pattern(
 	struct link_encoder *enc,
 	uint32_t index);