drm/tegra: Obtain head number from DT

The head number of a given display controller is fixed in hardware and
required to program outputs appropriately. Relying on the driver probe
order to determine this number will not work, since that could yield a
situation where the second head was probed first and would be assigned
head number 0 instead of 1.

By explicitly specifying the head number in the device tree, it is no
longer necessary to rely on these assumptions. As a fallback, if the
property isn't available, derive the head number from the display
controller node's position in the device tree. That's somewhat more
reliable than the previous default but not a proper solution.

Tested-by: Stephen Warren <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

Documentation/devicetree/bindings/gpu/nvidia,tegra20-host1x.txt

···
     See ../reset/reset.txt for details.
   - reset-names: Must include the following entries:
     - dc
+  - nvidia,head: The number of the display controller head. This is used to
+    setup the various types of output to receive video data from the given
+    head.
 
   Each display controller node has a child node, named "rgb", that represents
   the RGB output associated with the controller. It can take the following

drivers/gpu/drm/tegra/dc.c

···
 	struct tegra_dc *dc = host1x_client_to_dc(client);
 	int err;
 
-	dc->pipe = tegra->drm->mode_config.num_crtc;
-
 	drm_crtc_init(tegra->drm, &dc->base, &tegra_crtc_funcs);
 	drm_mode_crtc_set_gamma_size(&dc->base, 256);
 	drm_crtc_helper_add(&dc->base, &tegra_crtc_helper_funcs);
···
 	}
 };
 
+static int tegra_dc_parse_dt(struct tegra_dc *dc)
+{
+	struct device_node *np;
+	u32 value = 0;
+	int err;
+
+	err = of_property_read_u32(dc->dev->of_node, "nvidia,head", &value);
+	if (err < 0) {
+		dev_err(dc->dev, "missing \"nvidia,head\" property\n");
+
+		/*
+		 * If the nvidia,head property isn't present, try to find the
+		 * correct head number by looking up the position of this
+		 * display controller's node within the device tree. Assuming
+		 * that the nodes are ordered properly in the DTS file and
+		 * that the translation into a flattened device tree blob
+		 * preserves that ordering this will actually yield the right
+		 * head number.
+		 *
+		 * If those assumptions don't hold, this will still work for
+		 * cases where only a single display controller is used.
+		 */
+		for_each_matching_node(np, tegra_dc_of_match) {
+			if (np == dc->dev->of_node)
+				break;
+
+			value++;
+		}
+	}
+
+	dc->pipe = value;
+
+	return 0;
+}
+
 static int tegra_dc_probe(struct platform_device *pdev)
 {
 	const struct of_device_id *id;
···
 	INIT_LIST_HEAD(&dc->list);
 	dc->dev = &pdev->dev;
 	dc->soc = id->data;
+
+	err = tegra_dc_parse_dt(dc);
+	if (err < 0)
+		return err;
 
 	dc->clk = devm_clk_get(&pdev->dev, NULL);
 	if (IS_ERR(dc->clk)) {