of: restricted dma: Don't fail device probe on rmem init failure

If CONFIG_DMA_RESTRICTED_POOL=n then probing a device with a reference
to a "restricted-dma-pool" will fail with a reasonably cryptic error:

  | pci-host-generic: probe of 10000.pci failed with error -22

Rework of_dma_set_restricted_buffer() so that it does not cause probing
failure and instead either returns early if CONFIG_DMA_RESTRICTED_POOL=n
or emits a diagnostic if the reserved DMA pool fails to initialise.

Cc: Claire Chang <tientzu@chromium.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/of/device.c

@@ -53,12 +53,15 @@ int of_device_add(struct platform_device *ofdev)
 	return device_add(&ofdev->dev);
 }
 
-static int
+static void
 of_dma_set_restricted_buffer(struct device *dev, struct device_node *np)
 {
 	struct device_node *node, *of_node = dev->of_node;
 	int count, i;
 
+	if (!IS_ENABLED(CONFIG_DMA_RESTRICTED_POOL))
+		return;
+
 	count = of_property_count_elems_of_size(of_node, "memory-region",
 						sizeof(u32));
 	/*
@@ -79,11 +82,11 @@ of_dma_set_restricted_buffer(struct device *dev, struct device_node *np)
 		 */
 		if (of_device_is_compatible(node, "restricted-dma-pool") &&
 		    of_device_is_available(node))
-			return of_reserved_mem_device_init_by_idx(dev, of_node,
-								  i);
+			break;
 	}
 
-	return 0;
+	if (i != count && of_reserved_mem_device_init_by_idx(dev, of_node, i))
+		dev_warn(dev, "failed to initialise \"restricted-dma-pool\" memory node\n");
 }
 
 /**
@@ -200,7 +203,7 @@ int of_dma_configure_id(struct device *dev, struct device_node *np,
 	arch_setup_dma_ops(dev, dma_start, size, iommu, coherent);
 
 	if (!iommu)
-		return of_dma_set_restricted_buffer(dev, np);
+		of_dma_set_restricted_buffer(dev, np);
 
 	return 0;
 }