drm/tegra: dc: Do not register DC without primary plane

Tegra194 contains a fourth display controller that does not own any
windows. Therefore, we cannot currently assign a primary plane to it
which causes KMS to eventually crash. Do not register the display
controller if it owns no windows to work around this.

Note that we still have to enable and probe the display controller
because for some reason all display controllers need to be powered
(and/or clocked) before any registers can be accessed in any of the
display controllers.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/dc.c

@@ -1988,6 +1988,28 @@ static int tegra_dc_init(struct host1x_client *client)
 	struct drm_plane *cursor = NULL;
 	int err;
 
+	/*
+	 * XXX do not register DCs with no window groups because we cannot
+	 * assign a primary plane to them, which in turn will cause KMS to
+	 * crash.
+	 */
+	if (dc->soc->wgrps) {
+		bool has_wgrps = false;
+		unsigned int i;
+
+		for (i = 0; i < dc->soc->num_wgrps; i++) {
+			const struct tegra_windowgroup_soc *wgrp = &dc->soc->wgrps[i];
+
+			if (wgrp->dc == dc->pipe && wgrp->num_windows > 0) {
+				has_wgrps = true;
+				break;
+			}
+		}
+
+		if (!has_wgrps)
+			return 0;
+	}
+
 	dc->syncpt = host1x_syncpt_request(client, flags);
 	if (!dc->syncpt)
 		dev_warn(dc->dev, "failed to allocate syncpoint\n");