media: v4l2-subdev: Add new ioctl for client capabilities

Add new ioctls to set and get subdev client capabilities. Client in this
context means the userspace application which opens the subdev device
node. The client capabilities are stored in the file handle of the
opened subdev device node, and the client must set the capabilities for
each opened subdev.

For now we only add a single flag, V4L2_SUBDEV_CLIENT_CAP_STREAMS, which
indicates that the client is streams-aware.

The reason for needing such a flag is as follows:

Many structs passed via ioctls, e.g. struct v4l2_subdev_format, contain
reserved fields (usually a single array field). These reserved fields
can be used to extend the ioctl. The userspace is required to zero the
reserved fields.

We recently added a new 'stream' field to many of these structs, and the
space for the field was taken from these reserved arrays. The assumption
was that these new 'stream' fields are always initialized to zero if the
userspace does not use them. This was a mistake, as, as mentioned above,
the userspace is required to zero the _reserved_ fields. In other words,
there is no requirement to zero this new stream field, and if the
userspace doesn't use the field (which is the case for all userspace
applications at the moment), the field may contain random data.

This shows that the way the reserved fields are defined in v4l2 is, in
my opinion, somewhat broken, but there is nothing to do about that.

To fix this issue we need a way for the userspace to tell the kernel
that the userspace has indeed set the 'stream' field, and it's fine for
the kernel to access it. This is achieved with the new ioctl, which the
userspace should usually use right after opening the subdev device node.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/include/uapi/linux/v4l2-subdev.h b/include/uapi/linux/v4l2-subdev.h
index 654d659de835..4a195b68f28f 100644
--- a/include/uapi/linux/v4l2-subdev.h
+++ b/include/uapi/linux/v4l2-subdev.h
@@ -233,6 +233,24 @@ struct v4l2_subdev_routing {
 	__u32 reserved[6];
 };
 
+/*
+ * The client is aware of streams. Setting this flag enables the use of 'stream'
+ * fields (referring to the stream number) with various ioctls. If this is not
+ * set (which is the default), the 'stream' fields will be forced to 0 by the
+ * kernel.
+ */
+ #define V4L2_SUBDEV_CLIENT_CAP_STREAMS		(1U << 0)
+
+/**
+ * struct v4l2_subdev_client_capability - Capabilities of the client accessing
+ *					  the subdev
+ *
+ * @capabilities: A bitmask of V4L2_SUBDEV_CLIENT_CAP_* flags.
+ */
+struct v4l2_subdev_client_capability {
+	__u64 capabilities;
+};
+
 /* Backwards compatibility define --- to be removed */
 #define v4l2_subdev_edid v4l2_edid
 
@@ -250,6 +268,9 @@ struct v4l2_subdev_routing {
 #define VIDIOC_SUBDEV_S_SELECTION		_IOWR('V', 62, struct v4l2_subdev_selection)
 #define VIDIOC_SUBDEV_G_ROUTING			_IOWR('V', 38, struct v4l2_subdev_routing)
 #define VIDIOC_SUBDEV_S_ROUTING			_IOWR('V', 39, struct v4l2_subdev_routing)
+#define VIDIOC_SUBDEV_G_CLIENT_CAP		_IOR('V',  101, struct v4l2_subdev_client_capability)
+#define VIDIOC_SUBDEV_S_CLIENT_CAP		_IOWR('V',  102, struct v4l2_subdev_client_capability)
+
 /* The following ioctls are identical to the ioctls in videodev2.h */
 #define VIDIOC_SUBDEV_G_STD			_IOR('V', 23, v4l2_std_id)
 #define VIDIOC_SUBDEV_S_STD			_IOW('V', 24, v4l2_std_id)