iommu/exynos: Play nice in multi-platform builds

The Exynos System MMU driver unconditionally executes code and registers
a struct iommu_ops with the platform bus irrespective of whether it runs
on an Exynos SoC or not. This causes problems in multi-platform kernels
where drivers for other SoCs will no longer be able to register their
own struct iommu_ops or even try to use a struct iommu_ops for an IOMMU
that obviously isn't there.

The smallest fix I could think of is to check for the existence of any
Exynos System MMU devices in the device tree and skip initialization
otherwise.

This fixes a problem on Tegra20 where the DRM driver will try to use the
obviously non-existent Exynos System MMU.

Reported-by: Nicolas Chauvet <[email protected]>
Cc: Kukjin Kim <[email protected]>
Signed-off-by: Thierry Reding <[email protected]>
Signed-off-by: Joerg Roedel <[email protected]>

Author: thierryreding

drivers/iommu/exynos-iommu.c

@@ -1186,8 +1186,15 @@ static const struct iommu_ops exynos_iommu_ops = {
 
 static int __init exynos_iommu_init(void)
 {
+	struct device_node *np;
 	int ret;
 
+	np = of_find_matching_node(NULL, sysmmu_of_match);
+	if (!np)
+		return 0;
+
+	of_node_put(np);
+
 	lv2table_kmem_cache = kmem_cache_create("exynos-iommu-lv2table",
 				LV2TABLE_SIZE, LV2TABLE_SIZE, 0, NULL);
 	if (!lv2table_kmem_cache) {