media: v4l2-ioctl.c: readbuffers depends on V4L2_CAP_READWRITE

If V4L2_CAP_READWRITE is not set, then readbuffers must be set to 0,
otherwise v4l2-compliance will complain.

A note on the Fixes tag below: this patch does not really fix that commit,
but it can be applied from that commit onwards. For older code there is no
guarantee that device_caps is set, so even though this patch would apply,
it will not work reliably.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 049e684f (media: v4l2-dev: fix WARN_ON(!vdev->device_caps))
Cc: <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/v4l2-core/v4l2-ioctl.c

@@ -2090,6 +2090,7 @@ static int v4l_prepare_buf(const struct v4l2_ioctl_ops *ops,
 static int v4l_g_parm(const struct v4l2_ioctl_ops *ops,
 				struct file *file, void *fh, void *arg)
 {
+	struct video_device *vfd = video_devdata(file);
 	struct v4l2_streamparm *p = arg;
 	v4l2_std_id std;
 	int ret = check_fmt(file, p->type);
@@ -2101,7 +2102,8 @@ static int v4l_g_parm(const struct v4l2_ioctl_ops *ops,
 	if (p->type != V4L2_BUF_TYPE_VIDEO_CAPTURE &&
 	    p->type != V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE)
 		return -EINVAL;
-	p->parm.capture.readbuffers = 2;
+	if (vfd->device_caps & V4L2_CAP_READWRITE)
+		p->parm.capture.readbuffers = 2;
 	ret = ops->vidioc_g_std(file, fh, &std);
 	if (ret == 0)
 		v4l2_video_std_frame_period(std, &p->parm.capture.timeperframe);