[libcamera-devel] [PATCH v2 01/10] [DNI] include: linux: Extend VIDIOC_ENUM_FMT to support MC-centric devices

[Laurent Pinchart]

This is an experimental change that hasn't been accepted in mainline
yet. Its commit message for the Linux kernel is as follows.

media: v4l2: Extend VIDIOC_ENUM_FMT to support MC-centric devices

The VIDIOC_ENUM_FMT ioctl enumerates all formats supported by a video
node. For MC-centric devices, its behaviour has always been ill-defined,
with drivers implementing one of the following behaviours:

- No support for VIDIOC_ENUM_FMT at all
- Enumerating all formats supported by the video node, regardless of the
  configuration of the pipeline
- Enumerating formats supported by the video node for the active
  configuration of the connected subdevice

The first behaviour is obviously useless for applications. The second
behaviour provides the most information, but doesn't offer a way to find
what formats are compatible with a given pipeline configuration. The
third behaviour fixes that, but with the drawback that applications
can't enumerate all supported formats anymore, and have to modify the
active configuration of the pipeline to enumerate formats.

The situation is messy as none of the implemented behaviours are ideal,
and userspace can't predict what will happen as the behaviour is
driver-specific.

To fix this, let's extend the VIDIOC_ENUM_FMT with a missing capability:
enumerating pixel formats for a given media bus code. The media bus code
is passed through the v4l2_fmtdesc structure in a new mbus_code field
(repurposed from the reserved fields), and an additional flag is added
to report if the driver supports this API extension. With this
capability in place, applications can enumerate pixel formats for a
given media bus code without modifying the active configuration of the
device.

The current behaviour of the ioctl is preserved when the new mbus_code
field is set to 0, ensuring compatibility with existing userspace. This
behaviour is now documented as mandatory for MC-centric devices as well
as the traditional video node-centric devices. This allows applications
to query MC-centric devices for all the supported pixel formats, as well
as for the pixel formats corresponding to a given media bus code.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 include/linux/videodev2.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
index ab40b3272ed2..120130c4aaae 100644
--- a/include/linux/videodev2.h
+++ b/include/linux/videodev2.h
@@ -769,13 +769,15 @@ struct v4l2_fmtdesc {
 	__u32               flags;
 	__u8		    description[32];   /* Description string */
 	__u32		    pixelformat;       /* Format fourcc      */
-	__u32		    reserved[4];
+	__u32		    mbus_code;		/* Media bus code    */
+	__u32		    reserved[3];
 };
 
 #define V4L2_FMT_FLAG_COMPRESSED		0x0001
 #define V4L2_FMT_FLAG_EMULATED			0x0002
 #define V4L2_FMT_FLAG_CONTINUOUS_BYTESTREAM	0x0004
 #define V4L2_FMT_FLAG_DYN_RESOLUTION		0x0008
+#define V4L2_FMT_FLAG_MBUS_CODE			0x0010
 
 	/* Frame Size and frame rate enumeration */
 /*
-- 
Regards,

Laurent Pinchart