gpu: host1x: Allow loading tegra-drm without enabled engines

Add support to register host1x devices without requiring subdevices.
This ensures syncpoint functionality remains available even when engine
subdevices are not present.

Add softdep for tegra-drm to make userspace interface available
without module autoloading through device tree entries.

Signed-off-by: Vamsee Vardhan Thummala <[email protected]>
[[email protected]: some rewording]
Signed-off-by: Mikko Perttunen <[email protected]>
Signed-off-by: Thierry Reding <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

drivers/gpu/host1x/bus.c

@@ -471,6 +471,18 @@ static int host1x_device_add(struct host1x *host1x,
 
 	mutex_unlock(&clients_lock);
 
+	/*
+	 * Add device even if there are no subdevs to ensure syncpoint functionality
+	 * is available regardless of whether any engine subdevices are present
+	 */
+	if (list_empty(&device->subdevs)) {
+		err = device_add(&device->dev);
+		if (err < 0)
+			dev_err(&device->dev, "failed to add device: %d\n", err);
+		else
+			device->registered = true;
+	}
+
 	return 0;
 }
 

drivers/gpu/host1x/dev.c

@@ -821,6 +821,7 @@ u64 host1x_get_dma_mask(struct host1x *host1x)
 }
 EXPORT_SYMBOL(host1x_get_dma_mask);
 
+MODULE_SOFTDEP("post: tegra-drm");
 MODULE_AUTHOR("Thierry Reding <[email protected]>");
 MODULE_AUTHOR("Terje Bergstrom <[email protected]>");
 MODULE_DESCRIPTION("Host1x driver for Tegra products");