diff --git a/drivers/gpu/drm/ast/ast_drv.h b/drivers/gpu/drm/ast/ast_drv.h
index 7abda94..3bedcf7 100644
--- a/drivers/gpu/drm/ast/ast_drv.h
+++ b/drivers/gpu/drm/ast/ast_drv.h
@@ -113,7 +113,11 @@ struct ast_private {
 	struct ttm_bo_kmap_obj cache_kmap;
 	int next_cursor;
 	bool support_wide_screen;
-	bool DisableP2A;
+	enum {
+		ast_use_p2a,
+		ast_use_dt,
+		ast_use_defaults
+	} config_mode;
 
 	enum ast_tx_chip tx_chip_type;
 	u8 dp501_maxclk;
diff --git a/drivers/gpu/drm/ast/ast_main.c b/drivers/gpu/drm/ast/ast_main.c
index 533e762..fb99762 100644
--- a/drivers/gpu/drm/ast/ast_main.c
+++ b/drivers/gpu/drm/ast/ast_main.c
@@ -62,13 +62,84 @@ uint8_t ast_get_index_reg_mask(struct ast_private *ast,
 	return ret;
 }
 
+static void ast_detect_config_mode(struct drm_device *dev, u32 *scu_rev)
+{
+	struct device_node *np = dev->pdev->dev.of_node;
+	struct ast_private *ast = dev->dev_private;
+	uint32_t data, jregd0, jregd1;
+
+	/* Defaults */
+	ast->config_mode = ast_use_defaults;
+	*scu_rev = 0xffffffff;
+
+	/* Check if we have device-tree properties */
+	if (np && !of_property_read_u32(np, "aspeed,scu-revision-id",
+					scu_rev)) {
+		/* We do, disable P2A access */
+		ast->config_mode = ast_use_dt;
+		DRM_INFO("Using device-tree for configuration\n");
+		return;
+	}
+
+	/* Not all families have a P2A bridge */
+	if (dev->pdev->device != PCI_CHIP_AST2000)
+		return;
+
+	/*
+	 * The BMC will set SCU 0x40 D[12] to 1 if the P2 bridge
+	 * is disabled. We force using P2A if VGA only mode bit
+	 * is set D[7]
+	 */
+	jregd0 = ast_get_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xd0, 0xff);
+	jregd1 = ast_get_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xd1, 0xff);
+	if (!(jregd0 & 0x80) || !(jregd1 & 0x10)) {
+		/* Double check it's actually working */
+		data = ast_read32(ast, 0xf004);
+		if (data != 0xFFFFFFFF) {
+			/* P2A works, grab silicon revision */
+			ast->config_mode = ast_use_p2a;
+
+			DRM_INFO("Using P2A bridge for configuration\n");
+
+			/* Read SCU7c (silicon revision register) */
+			ast_write32(ast, 0xf004, 0x1e6e0000);
+			ast_write32(ast, 0xf000, 0x1);
+			*scu_rev = ast_read32(ast, 0x1207c);
+			return;
+		}
+	}
+
+	/* We have a P2A bridge but it's disabled */
+	DRM_INFO("P2A bridge disabled, using default configuration\n");
+}
 
 static int ast_detect_chip(struct drm_device *dev, bool *need_post)
 {
 	struct ast_private *ast = dev->dev_private;
-	uint32_t data, jreg;
+	uint32_t jreg, scu_rev;
+
+	/*
+	 * If VGA isn't enabled, we need to enable now or subsequent
+	 * access to the scratch registers will fail. We also inform
+	 * our caller that it needs to POST the chip
+	 * (Assumption: VGA not enabled -> need to POST)
+	 */
+	if (!ast_is_vga_enabled(dev)) {
+		ast_enable_vga(dev);
+		DRM_INFO("VGA not enabled on entry, requesting chip POST\n");
+		*need_post = true;
+	} else
+		*need_post = false;
+
+
+	/* Enable extended register access */
+	ast_enable_mmio(dev);
 	ast_open_key(ast);
 
+	/* Find out whether P2A works or whether to use device-tree */
+	ast_detect_config_mode(dev, &scu_rev);
+
+	/* Identify chipset */
 	if (dev->pdev->device == PCI_CHIP_AST1180) {
 		ast->chip = AST1100;
 		DRM_INFO("AST 1180 detected\n");
@@ -80,12 +151,7 @@ static int ast_detect_chip(struct drm_device *dev, bool *need_post)
 			ast->chip = AST2300;
 			DRM_INFO("AST 2300 detected\n");
 		} else if (dev->pdev->revision >= 0x10) {
-			uint32_t data;
-			ast_write32(ast, 0xf004, 0x1e6e0000);
-			ast_write32(ast, 0xf000, 0x1);
-
-			data = ast_read32(ast, 0x1207c);
-			switch (data & 0x0300) {
+			switch (scu_rev & 0x0300) {
 			case 0x0200:
 				ast->chip = AST1100;
 				DRM_INFO("AST 1100 detected\n");
@@ -110,26 +176,6 @@ static int ast_detect_chip(struct drm_device *dev, bool *need_post)
 		}
 	}
 
-	/*
-	 * If VGA isn't enabled, we need to enable now or subsequent
-	 * access to the scratch registers will fail. We also inform
-	 * our caller that it needs to POST the chip
-	 * (Assumption: VGA not enabled -> need to POST)
-	 */
-	if (!ast_is_vga_enabled(dev)) {
-		ast_enable_vga(dev);
-		ast_enable_mmio(dev);
-		DRM_INFO("VGA not enabled on entry, requesting chip POST\n");
-		*need_post = true;
-	} else
-		*need_post = false;
-
-	/* Check P2A Access */
-	ast->DisableP2A = true;
-	data = ast_read32(ast, 0xf004);
-	if (data != 0xFFFFFFFF)
-		ast->DisableP2A = false;
-
 	/* Check if we support wide screen */
 	switch (ast->chip) {
 	case AST1180:
@@ -146,17 +192,12 @@ static int ast_detect_chip(struct drm_device *dev, bool *need_post)
 			ast->support_wide_screen = true;
 		else {
 			ast->support_wide_screen = false;
-			if (ast->DisableP2A == false) {
-				/* Read SCU7c (silicon revision register) */
-				ast_write32(ast, 0xf004, 0x1e6e0000);
-				ast_write32(ast, 0xf000, 0x1);
-				data = ast_read32(ast, 0x1207c);
-				data &= 0x300;
-				if (ast->chip == AST2300 && data == 0x0) /* ast1300 */
-					ast->support_wide_screen = true;
-				if (ast->chip == AST2400 && data == 0x100) /* ast1400 */
-					ast->support_wide_screen = true;
-			}
+			if (ast->chip == AST2300 &&
+			    (scu_rev & 0x300) == 0x0) /* ast1300 */
+				ast->support_wide_screen = true;
+			if (ast->chip == AST2400 &&
+			    (scu_rev & 0x300) == 0x100) /* ast1400 */
+				ast->support_wide_screen = true;
 		}
 		break;
 	}
@@ -220,85 +261,102 @@ static int ast_detect_chip(struct drm_device *dev, bool *need_post)
 
 static int ast_get_dram_info(struct drm_device *dev)
 {
+	struct device_node *np = dev->pdev->dev.of_node;
 	struct ast_private *ast = dev->dev_private;
-	uint32_t data, data2;
-	uint32_t denum, num, div, ref_pll;
+	uint32_t mcr_cfg, mcr_scu_mpll, mcr_scu_strap;
+	uint32_t denum, num, div, ref_pll, dsel;
 
-	if (ast->DisableP2A)
-	{
+	switch (ast->config_mode) {
+	case ast_use_dt:
+		/*
+		 * If some properties are missing, use reasonable
+		 * defaults for AST2400
+		 */
+		if (of_property_read_u32(np, "aspeed,mcr-configuration",
+					 &mcr_cfg))
+			mcr_cfg = 0x00000577;
+		if (of_property_read_u32(np, "aspeed,mcr-scu-mpll",
+					 &mcr_scu_mpll))
+			mcr_scu_mpll = 0x000050C0;
+		if (of_property_read_u32(np, "aspeed,mcr-scu-strap",
+					 &mcr_scu_strap))
+			mcr_scu_strap = 0;
+		break;
+	case ast_use_p2a:
+		ast_write32(ast, 0xf004, 0x1e6e0000);
+		ast_write32(ast, 0xf000, 0x1);
+		mcr_cfg = ast_read32(ast, 0x10004);
+		mcr_scu_mpll = ast_read32(ast, 0x10120);
+		mcr_scu_strap = ast_read32(ast, 0x10170);
+		break;
+	case ast_use_defaults:
+	default:
 		ast->dram_bus_width = 16;
 		ast->dram_type = AST_DRAM_1Gx16;
 		ast->mclk = 396;
+		return 0;
 	}
-	else
-	{
-		ast_write32(ast, 0xf004, 0x1e6e0000);
-		ast_write32(ast, 0xf000, 0x1);
-		data = ast_read32(ast, 0x10004);
-
-		if (data & 0x40)
-			ast->dram_bus_width = 16;
-		else
-			ast->dram_bus_width = 32;
 
-		if (ast->chip == AST2300 || ast->chip == AST2400) {
-			switch (data & 0x03) {
-			case 0:
-				ast->dram_type = AST_DRAM_512Mx16;
-				break;
-			default:
-			case 1:
-				ast->dram_type = AST_DRAM_1Gx16;
-				break;
-			case 2:
-				ast->dram_type = AST_DRAM_2Gx16;
-				break;
-			case 3:
-				ast->dram_type = AST_DRAM_4Gx16;
-				break;
-			}
-		} else {
-			switch (data & 0x0c) {
-			case 0:
-			case 4:
-				ast->dram_type = AST_DRAM_512Mx16;
-				break;
-			case 8:
-				if (data & 0x40)
-					ast->dram_type = AST_DRAM_1Gx16;
-				else
-					ast->dram_type = AST_DRAM_512Mx32;
-				break;
-			case 0xc:
-				ast->dram_type = AST_DRAM_1Gx32;
-				break;
-			}
-		}
+	if (mcr_cfg & 0x40)
+		ast->dram_bus_width = 16;
+	else
+		ast->dram_bus_width = 32;
 
-		data = ast_read32(ast, 0x10120);
-		data2 = ast_read32(ast, 0x10170);
-		if (data2 & 0x2000)
-			ref_pll = 14318;
-		else
-			ref_pll = 12000;
-
-		denum = data & 0x1f;
-		num = (data & 0x3fe0) >> 5;
-		data = (data & 0xc000) >> 14;
-		switch (data) {
-		case 3:
-			div = 0x4;
+	if (ast->chip == AST2300 || ast->chip == AST2400) {
+		switch (mcr_cfg & 0x03) {
+		case 0:
+			ast->dram_type = AST_DRAM_512Mx16;
 			break;
-		case 2:
+		default:
 		case 1:
-			div = 0x2;
+			ast->dram_type = AST_DRAM_1Gx16;
 			break;
-		default:
-			div = 0x1;
+		case 2:
+			ast->dram_type = AST_DRAM_2Gx16;
+			break;
+		case 3:
+			ast->dram_type = AST_DRAM_4Gx16;
+			break;
+		}
+	} else {
+		switch (mcr_cfg & 0x0c) {
+		case 0:
+		case 4:
+			ast->dram_type = AST_DRAM_512Mx16;
+			break;
+		case 8:
+			if (mcr_cfg & 0x40)
+				ast->dram_type = AST_DRAM_1Gx16;
+			else
+				ast->dram_type = AST_DRAM_512Mx32;
+			break;
+		case 0xc:
+			ast->dram_type = AST_DRAM_1Gx32;
 			break;
 		}
-		ast->mclk = ref_pll * (num + 2) / (denum + 2) * (div * 1000);
 	}
+
+	if (mcr_scu_strap & 0x2000)
+		ref_pll = 14318;
+	else
+		ref_pll = 12000;
+
+	denum = mcr_scu_mpll & 0x1f;
+	num = (mcr_scu_mpll & 0x3fe0) >> 5;
+	dsel = (mcr_scu_mpll & 0xc000) >> 14;
+	switch (dsel) {
+	case 3:
+		div = 0x4;
+		break;
+	case 2:
+	case 1:
+		div = 0x2;
+		break;
+	default:
+		div = 0x1;
+		break;
+	}
+	ast->mclk = ref_pll * (num + 2) / (denum + 2) * (div * 1000);
 	return 0;
 }
 
diff --git a/drivers/gpu/drm/ast/ast_post.c b/drivers/gpu/drm/ast/ast_post.c
index 6c5391c..64549ce 100644
--- a/drivers/gpu/drm/ast/ast_post.c
+++ b/drivers/gpu/drm/ast/ast_post.c
@@ -379,17 +379,14 @@ void ast_post_gpu(struct drm_device *dev)
 	ast_open_key(ast);
 	ast_set_def_ext_reg(dev);
 
-	if (ast->DisableP2A == false)
-	{
+	if (ast->config_mode == ast_use_p2a) {
 		if (ast->chip == AST2300 || ast->chip == AST2400)
 			ast_init_dram_2300(dev);
 		else
 			ast_init_dram_reg(dev);
 
 		ast_init_3rdtx(dev);
-	}
-	else
-	{
+	} else {
 		if (ast->tx_chip_type != AST_TX_NONE)
 			ast_set_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xa3, 0xcf, 0x80);	/* Enable DVO */
 	}
