On 05/12/2016 02:17 AM, Laurent Pinchart wrote:
> Hello,
>
> This RFC patch series is a second attempt at adding support for passing
> statistics data to userspace using a standard API.
>
> The core requirements haven't changed. Statistics data capture requires
> zero-copy and decoupling statistics buffers from images buffers, in order to
> make statistics data available to userspace as soon as they're captured. For
> those reasons the early consensus we have reached is to use a video device
> node with a buffer queue to pass statistics buffers using the V4L2 API, and
> this new RFC version doesn't challenge that.
>
> The major change compared to the previous version is how the first patch has
> been split in two. Patch 1/4 now adds a new metadata buffer type and format
> (including their support in videobuf2-v4l2), usable with regular V4L2 video
> device nodes, while patch 2/4 adds the new metadata video device type.
> Metadata buffer queues are thus usable on both the regular V4L2 device nodes
> and the new metadata device nodes.
>
> This change was driven by the fact that an important category of use cases
> doesn't differentiate between metadata and image data in hardware at the DMA
> engine level. With such hardware (CSI-2 receivers in particular, but other bus
> types could also fall into this category) a stream containing both metadata
> and image data virtual streams is transmitted over a single physical link. The
> receiver demultiplexes, filters and routes the virtual streams to further
> hardware blocks, and in many cases, directly to DMA engines that are part of
> the receiver. Those DMA engines can capture a single virtual stream to memory,
> with as many DMA engines physically present in the device as the number of
> virtual streams that can be captured concurrently. All those DMA engines are
> usually identical and don't care about the type of data they receive and
> capture. For that reason limiting the metadata buffer type to metadata device
> nodes would require creating two device nodes for each DMA engine (and
> possibly more later if we need to capture other types of data). Not only would
> this make the API more complex to use for applications, it wouldn't bring any
> added value as the video and metadata device nodes associated with a DMA
> engine couldn't be used concurrently anyway, as they both correspond to the
> same hardware resource.
>
> For this reason the ability to capture metadata on a video device node is
> useful and desired, and is implemented patch 1/4 using a dedicated video
> buffers queue. In the CSI-2 case a driver will create two buffer queues
> internally for the same DMA engine, and can select which one to use based on
> the buffer type passed for instance to the REQBUFS ioctl (details still need
> to be discussed here).
Not quite. It still has only one vb2_queue, you just change the type depending
on what mode it is in (video or meta data). Similar to raw vs sliced VBI.
In the latter case it is the VIDIOC_S_FMT call that changes the vb2_queue type
depending on whether raw or sliced VBI is requested. That's probably where I
would do this for video vs meta as well.
There is one big thing missing here: how does userspace know in this case
whether
it will get metadata or video? Who decides which CSI virtual stream is routed
to which video node?
> A device that contains DMA engines dedicated to
> metadata would create a single buffer queue and implement metadata capture
> only.
>
> Patch 2/4 then adds a dedicated metadata device node type that is limited to
> metadata capture. Support for metadata on video device nodes isn't removed
> though, both device node types support metadata capture. I have included this
> patch as the code existed in the previous version of the series (and was
> explicitly requested during review of an earlier version), but I don't really
> see what value this would bring compared to just using video device nodes.
I'm inclined to agree with you.
> As before patch 3/4 defines a first metadata format for the R-Car VSP1 1-D
> statistics format as an example, and the new patch 4/4 adds support for the
> histogram engine to the VSP1 driver. The implementation uses a metadata device
> node, and switching to a video device node wouldn't require more than applying
> the following one-liner patch.
>
> - histo->queue.type = V4L2_BUF_TYPE_META_CAPTURE;
> + histo->queue.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
You probably mean replacing this:
histo->video.vfl_type = VFL_TYPE_META;
by this:
histo->video.vfl_type = VFL_TYPE_GRABBER;
Regards,
Hans
>
> Beside whether patch 2/4 should be included or not (I would prefer dropping
> it) and how to select the active queue on a multi-type video device node
> (through the REQBUFS ioctl or through a diffent mean), one point that remains
> to be discussed is what information to include in the metadata format. Patch
> 1/1 defines the new metadata format as
>
> struct v4l2_meta_format {
> __u32 dataformat;
> __u32 buffersize;
> __u8 reserved[24];
> } __attribute__ ((packed));
>
> but at least in the CSI-2 case metadata is, as image data, transmitted in
> lines and the receiver needs to be programmed with the line length and the
> number of lines for proper operation. We started discussing this on IRC but
> haven't reached a conclusion yet.
>
> Laurent Pinchart (4):
> v4l: Add metadata buffer type and format
> v4l: Add metadata video device type
> v4l: Define a pixel format for the R-Car VSP1 1-D histogram engine
> v4l: vsp1: Add HGO support
>
> Documentation/DocBook/media/v4l/dev-meta.xml | 97 ++++
> .../DocBook/media/v4l/pixfmt-meta-vsp1-hgo.xml | 307 +++++++++++++
> Documentation/DocBook/media/v4l/pixfmt.xml | 9 +
> Documentation/DocBook/media/v4l/v4l2.xml | 1 +
> drivers/media/platform/Kconfig | 1 +
> drivers/media/platform/vsp1/Makefile | 2 +
> drivers/media/platform/vsp1/vsp1.h | 3 +
> drivers/media/platform/vsp1/vsp1_drm.c | 2 +-
> drivers/media/platform/vsp1/vsp1_drv.c | 37 +-
> drivers/media/platform/vsp1/vsp1_entity.c | 131 +++++-
> drivers/media/platform/vsp1/vsp1_entity.h | 7 +-
> drivers/media/platform/vsp1/vsp1_hgo.c | 496
> +++++++++++++++++++++
> drivers/media/platform/vsp1/vsp1_hgo.h | 50 +++
> drivers/media/platform/vsp1/vsp1_histo.c | 307 +++++++++++++
> drivers/media/platform/vsp1/vsp1_histo.h | 68 +++
> drivers/media/platform/vsp1/vsp1_pipe.c | 30 +-
> drivers/media/platform/vsp1/vsp1_pipe.h | 2 +
> drivers/media/platform/vsp1/vsp1_regs.h | 24 +-
> drivers/media/platform/vsp1/vsp1_video.c | 22 +-
> drivers/media/v4l2-core/v4l2-compat-ioctl32.c | 19 +
> drivers/media/v4l2-core/v4l2-dev.c | 37 +-
> drivers/media/v4l2-core/v4l2-ioctl.c | 40 ++
> drivers/media/v4l2-core/videobuf2-v4l2.c | 3 +
> include/media/v4l2-dev.h | 3 +-
> include/media/v4l2-ioctl.h | 8 +
> include/uapi/linux/media.h | 2 +
> include/uapi/linux/videodev2.h | 17 +
> 27 files changed, 1678 insertions(+), 47 deletions(-)
> create mode 100644 Documentation/DocBook/media/v4l/dev-meta.xml
> create mode 100644 Documentation/DocBook/media/v4l/pixfmt-meta-vsp1-hgo.xml
> create mode 100644 drivers/media/platform/vsp1/vsp1_hgo.c
> create mode 100644 drivers/media/platform/vsp1/vsp1_hgo.h
> create mode 100644 drivers/media/platform/vsp1/vsp1_histo.c
> create mode 100644 drivers/media/platform/vsp1/vsp1_histo.h
>
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