virtio-input: add multi-touch support

Without multi-touch slots allocated, ABS_MT_SLOT events will be lost by
input_handle_abs_event.

Implementation is based on uinput_create_device.

Signed-off-by: Mathias Crombez <mathias.crombez@faurecia.com>
Co-developed-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Signed-off-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Link: https://lore.kernel.org/r/20210115002623.8576-1-vasyl.vavrychuk@opensynergy.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/virtio/virtio_input.c

@@ -7,6 +7,7 @@
 
 #include <uapi/linux/virtio_ids.h>
 #include <uapi/linux/virtio_input.h>
+#include <linux/input/mt.h>
 
 struct virtio_input {
 	struct virtio_device       *vdev;
@@ -219,7 +220,7 @@ static int virtinput_probe(struct virtio_device *vdev)
 	struct virtio_input *vi;
 	unsigned long flags;
 	size_t size;
-	int abs, err;
+	int abs, err, nslots;
 
 	if (!virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
 		return -ENODEV;
@@ -304,6 +305,13 @@ static int virtinput_probe(struct virtio_device *vdev)
 				continue;
 			virtinput_cfg_abs(vi, abs);
 		}
+
+		if (test_bit(ABS_MT_SLOT, vi->idev->absbit)) {
+			nslots = input_abs_get_max(vi->idev, ABS_MT_SLOT) + 1;
+			err = input_mt_init_slots(vi->idev, nslots, 0);
+			if (err)
+				goto err_mt_init_slots;
+		}
 	}
 
 	virtio_device_ready(vdev);
@@ -319,6 +327,7 @@ err_input_register:
 	spin_lock_irqsave(&vi->lock, flags);
 	vi->ready = false;
 	spin_unlock_irqrestore(&vi->lock, flags);
+err_mt_init_slots:
 	input_free_device(vi->idev);
 err_input_alloc:
 	vdev->config->del_vqs(vdev);