media: v4l2-subdev: add VIDIOC_SUBDEV_QUERYCAP ioctl

While normal video/radio/vbi/swradio nodes have a proper QUERYCAP ioctl
that apps can call to determine that it is indeed a V4L2 device, there
is currently no equivalent for v4l-subdev nodes. Adding this ioctl will
solve that, and it will allow utilities like v4l2-compliance to be used
with these devices as well.

SUBDEV_QUERYCAP currently returns the version and capabilities of the
subdevice. Define a capability flag to report if the subdevice is
registered in read-only mode.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

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

@@ -15,6 +15,7 @@
 #include <linux/types.h>
 #include <linux/videodev2.h>
 #include <linux/export.h>
+#include <linux/version.h>
 
 #include <media/v4l2-ctrls.h>
 #include <media/v4l2-device.h>
@@ -344,6 +345,16 @@ static long subdev_do_ioctl(struct file *file, unsigned int cmd, void *arg)
 	int rval;
 
 	switch (cmd) {
+	case VIDIOC_SUBDEV_QUERYCAP: {
+		struct v4l2_subdev_capability *cap = arg;
+
+		memset(cap->reserved, 0, sizeof(cap->reserved));
+		cap->version = LINUX_VERSION_CODE;
+		cap->capabilities = ro_subdev ? V4L2_SUBDEV_CAP_RO_SUBDEV : 0;
+
+		return 0;
+	}
+
 	case VIDIOC_QUERYCTRL:
 		/*
 		 * TODO: this really should be folded into v4l2_queryctrl (this

include/uapi/linux/v4l2-subdev.h

@@ -155,9 +155,25 @@ struct v4l2_subdev_selection {
 	__u32 reserved[8];
 };
 
+/**
+ * struct v4l2_subdev_capability - subdev capabilities
+ * @version: the driver versioning number
+ * @capabilities: the subdev capabilities, see V4L2_SUBDEV_CAP_*
+ * @reserved: for future use, set to zero for now
+ */
+struct v4l2_subdev_capability {
+	__u32 version;
+	__u32 capabilities;
+	__u32 reserved[14];
+};
+
+/* The v4l2 sub-device video device node is registered in read-only mode. */
+#define V4L2_SUBDEV_CAP_RO_SUBDEV		BIT(0)
+
 /* Backwards compatibility define --- to be removed */
 #define v4l2_subdev_edid v4l2_edid
 
+#define VIDIOC_SUBDEV_QUERYCAP			_IOR('V',  0, struct v4l2_subdev_capability)
 #define VIDIOC_SUBDEV_G_FMT			_IOWR('V',  4, struct v4l2_subdev_format)
 #define VIDIOC_SUBDEV_S_FMT			_IOWR('V',  5, struct v4l2_subdev_format)
 #define VIDIOC_SUBDEV_G_FRAME_INTERVAL		_IOWR('V', 21, struct v4l2_subdev_frame_interval)