iommu/mediatek: Lookup phandle to retrieve syscon to infracfg

This driver will get support for more SoCs and the list of infracfg
compatibles is expected to grow: in order to prevent getting this
situation out of control and see a long list of compatible strings,
add support to retrieve a handle to infracfg's regmap through a
new "mediatek,infracfg" phandle.

In order to keep retrocompatibility with older devicetrees, the old
way is kept in place.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20220616110830.26037-3-angelogioacchino.delregno@collabora.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: AngeloGioacchino Del Regno

drivers/iommu/mtk_iommu.c

@@ -1140,22 +1140,32 @@ static int mtk_iommu_probe(struct platform_device *pdev)
 	data->protect_base = ALIGN(virt_to_phys(protect), MTK_PROTECT_PA_ALIGN);
 
 	if (MTK_IOMMU_HAS_FLAG(data->plat_data, HAS_4GB_MODE)) {
-		switch (data->plat_data->m4u_plat) {
-		case M4U_MT2712:
-			p = "mediatek,mt2712-infracfg";
-			break;
-		case M4U_MT8173:
-			p = "mediatek,mt8173-infracfg";
-			break;
-		default:
-			p = NULL;
+		infracfg = syscon_regmap_lookup_by_phandle(dev->of_node, "mediatek,infracfg");
+		if (IS_ERR(infracfg)) {
+			/*
+			 * Legacy devicetrees will not specify a phandle to
+			 * mediatek,infracfg: in that case, we use the older
+			 * way to retrieve a syscon to infra.
+			 *
+			 * This is for retrocompatibility purposes only, hence
+			 * no more compatibles shall be added to this.
+			 */
+			switch (data->plat_data->m4u_plat) {
+			case M4U_MT2712:
+				p = "mediatek,mt2712-infracfg";
+				break;
+			case M4U_MT8173:
+				p = "mediatek,mt8173-infracfg";
+				break;
+			default:
+				p = NULL;
+			}
+
+			infracfg = syscon_regmap_lookup_by_compatible(p);
+			if (IS_ERR(infracfg))
+				return PTR_ERR(infracfg);
 		}
 
-		infracfg = syscon_regmap_lookup_by_compatible(p);
-
-		if (IS_ERR(infracfg))
-			return PTR_ERR(infracfg);
-
 		ret = regmap_read(infracfg, REG_INFRA_MISC, &val);
 		if (ret)
 			return ret;