drm/amd/display: Avoid unnecessary pixel rate divider programming

[Why]
Programming pixel rate divider when FIFO is enabled can cause FIFO error.

[How]
Skip divider programming when divider values are the same to prevent FIFO
error.
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Taimur Hassan <Syed.Hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn32/dcn32_dccg.c

@@ -42,6 +42,48 @@
 #define DC_LOGGER \
 	dccg->ctx->logger
 
+static void dccg32_get_pixel_rate_div(
+		struct dccg *dccg,
+		uint32_t otg_inst,
+		enum pixel_rate_div *k1,
+		enum pixel_rate_div *k2)
+{
+	struct dcn_dccg *dccg_dcn = TO_DCN_DCCG(dccg);
+	uint32_t val_k1 = PIXEL_RATE_DIV_NA, val_k2 = PIXEL_RATE_DIV_NA;
+
+	*k1 = PIXEL_RATE_DIV_NA;
+	*k2 = PIXEL_RATE_DIV_NA;
+
+	switch (otg_inst) {
+	case 0:
+		REG_GET_2(OTG_PIXEL_RATE_DIV,
+			OTG0_PIXEL_RATE_DIVK1, &val_k1,
+			OTG0_PIXEL_RATE_DIVK2, &val_k2);
+		break;
+	case 1:
+		REG_GET_2(OTG_PIXEL_RATE_DIV,
+			OTG1_PIXEL_RATE_DIVK1, &val_k1,
+			OTG1_PIXEL_RATE_DIVK2, &val_k2);
+		break;
+	case 2:
+		REG_GET_2(OTG_PIXEL_RATE_DIV,
+			OTG2_PIXEL_RATE_DIVK1, &val_k1,
+			OTG2_PIXEL_RATE_DIVK2, &val_k2);
+		break;
+	case 3:
+		REG_GET_2(OTG_PIXEL_RATE_DIV,
+			OTG3_PIXEL_RATE_DIVK1, &val_k1,
+			OTG3_PIXEL_RATE_DIVK2, &val_k2);
+		break;
+	default:
+		BREAK_TO_DEBUGGER();
+		return;
+	}
+
+	*k1 = (enum pixel_rate_div)val_k1;
+	*k2 = (enum pixel_rate_div)val_k2;
+}
+
 static void dccg32_set_pixel_rate_div(
 		struct dccg *dccg,
 		uint32_t otg_inst,
@@ -50,6 +92,17 @@ static void dccg32_set_pixel_rate_div(
 {
 	struct dcn_dccg *dccg_dcn = TO_DCN_DCCG(dccg);
 
+	enum pixel_rate_div cur_k1 = PIXEL_RATE_DIV_NA, cur_k2 = PIXEL_RATE_DIV_NA;
+
+	// Don't program 0xF into the register field. Not valid since
+	// K1 / K2 field is only 1 / 2 bits wide
+	if (k1 == PIXEL_RATE_DIV_NA || k2 == PIXEL_RATE_DIV_NA)
+		return;
+
+	dccg32_get_pixel_rate_div(dccg, otg_inst, &cur_k1, &cur_k2);
+	if (k1 == cur_k1 && k2 == cur_k2)
+		return;
+
 	switch (otg_inst) {
 	case 0:
 		REG_UPDATE_2(OTG_PIXEL_RATE_DIV,