Date:   Tue, 21 Jul 2020 14:26:18 +0300
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     Helen Koike <helen.koike@...labora.com>, mchehab@...nel.org,
        hans.verkuil@...co.com, laurent.pinchart@...asonboard.com,
        sakari.ailus@....fi, linux-media@...r.kernel.org
Cc:     Boris Brezillon <boris.brezillon@...labora.com>,
        tfiga@...omium.org, hiroh@...omium.org, nicolas@...fresne.ca,
        Brian.Starkey@....com, kernel@...labora.com,
        narmstrong@...libre.com, linux-kernel@...r.kernel.org,
        frkoenig@...omium.org, mjourdan@...libre.com
Subject: Re: [PATCH v4 2/6] media: v4l2: Add extended buffer operations



On 7/17/20 2:54 PM, Helen Koike wrote:
> From: Hans Verkuil <hans.verkuil@...co.com>
> 
> Those extended buffer ops have several purpose:
> 1/ Fix y2038 issues by converting the timestamp into an u64 counting
>    the number of ns elapsed since 1970
> 2/ Unify single/multiplanar handling
> 3/ Add a new start offset field to each v4l2 plane buffer info struct
>    to support the case where a single buffer object is storing all
>    planes data, each one being placed at a different offset
> 
> New hooks are created in v4l2_ioctl_ops so that drivers can start using
> these new objects.
> 
> The core takes care of converting new ioctls requests to old ones
> if the driver does not support the new hooks, and vice versa.
> 
> Note that the timecode field is gone, since there doesn't seem to be
> in-kernel users. We can be added back in the reserved area if needed or
> use the Request API to collect more metadata information from the
> frame.
> 
> Signed-off-by: Hans Verkuil <hans.verkuil@...co.com>
> Signed-off-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Helen Koike <helen.koike@...labora.com>
> ---
> Changes in v4:
> - Use v4l2_ext_pix_format directly in the ioctl, drop v4l2_ext_format,
> making V4L2_BUF_TYPE_VIDEO_[OUTPUT,CAPTURE] the only valid types.
> - Drop VIDIOC_EXT_EXPBUF, since the only difference from VIDIOC_EXPBUF
> was that with VIDIOC_EXT_EXPBUF we could export multiple planes at once.
> I think we can add this later, so I removed it from this RFC to simplify it.
> - Remove num_planes field from struct v4l2_ext_buffer
> - Add flags field to struct v4l2_ext_create_buffers
> - Reformulate struct v4l2_ext_plane
> - Fix some bugs caught by v4l2-compliance
> - Rebased on top of media/master (post 5.8-rc1)
> 
> Changes in v3:
> - Rebased on top of media/master (post 5.4-rc1)
> 
> Changes in v2:
> - Add reserved space to v4l2_ext_buffer so that new fields can be added
>   later on
> ---
>  drivers/media/v4l2-core/v4l2-dev.c   |  29 ++-
>  drivers/media/v4l2-core/v4l2-ioctl.c | 349 +++++++++++++++++++++++++--
>  include/media/v4l2-ioctl.h           |  26 ++
>  include/uapi/linux/videodev2.h       |  89 +++++++
>  4 files changed, 471 insertions(+), 22 deletions(-)
> 

<cut>

> +/**
> + * struct v4l2_ext_plane - extended plane buffer info
> + * @buffer_length:	size of the entire buffer in bytes, should fit
> + *			@offset + @plane_length
> + * @plane_length:	size of the plane in bytes.
> + * @userptr:		when memory is V4L2_MEMORY_USERPTR, a userspace pointer pointing
> + *			to this plane.
> + * @dmabuf_fd:		when memory is V4L2_MEMORY_DMABUF, a userspace file descriptor
> + *			associated with this plane.
> + * @offset:		offset in the memory buffer where the plane starts. If
> + *			V4L2_MEMORY_MMAP is used, then it can be a "cookie" that
> + *			should be passed to mmap() called on the video node.
> + * @reserved:		extra space reserved for future fields, must be set to 0.
> + *
> + *
> + * Buffers consist of one or more planes, e.g. an YCbCr buffer with two planes
> + * can have one plane for Y, and another for interleaved CbCr components.
> + * Each plane can reside in a separate memory buffer, or even in
> + * a completely separate memory node (e.g. in embedded devices).
> + */
> +struct v4l2_ext_plane {
> +	__u32 buffer_length;
> +	__u32 plane_length;
> +	union {
> +		__u64 userptr;
> +		__s32 dmabuf_fd;
> +	} m;
> +	__u32 offset;
> +	__u32 reserved[4];
> +};
> +
>  /**
>   * struct v4l2_buffer - video buffer info
>   * @index:	id number of the buffer
> @@ -1055,6 +1086,36 @@ struct v4l2_buffer {
>  	};
>  };
>  
> +/**
> + * struct v4l2_ext_buffer - extended video buffer info
> + * @index:	id number of the buffer
> + * @type:	V4L2_BUF_TYPE_VIDEO_CAPTURE or V4L2_BUF_TYPE_VIDEO_OUTPUT
> + * @flags:	buffer informational flags
> + * @field:	enum v4l2_field; field order of the image in the buffer
> + * @timestamp:	frame timestamp
> + * @sequence:	sequence count of this frame
> + * @memory:	enum v4l2_memory; the method, in which the actual video data is
> + *		passed
> + * @planes:	per-plane buffer information
> + * @request_fd:	fd of the request that this buffer should use
> + * @reserved:	extra space reserved for future fields, must be set to 0
> + *
> + * Contains data exchanged by application and driver using one of the Streaming
> + * I/O methods.
> + */
> +struct v4l2_ext_buffer {
> +	__u32 index;
> +	__u32 type;
> +	__u32 flags;
> +	__u32 field;
> +	__u64 timestamp;
> +	__u32 sequence;
> +	__u32 memory;
> +	__u32 request_fd;

This should be __s32, at least for consistency with dmabuf_fd?

> +	struct v4l2_ext_plane planes[VIDEO_MAX_PLANES];
> +	__u32 reserved[4];

I think we have to reserve more words here for future extensions.

I'd like also to propose to add here __s32 metadata_fd. The idea behind
this is to have a way to pass per-frame metadata dmabuf buffers for
synchronous type of metadata where the metadata is coming at the same
time with data buffers. What would be the format of the metadata buffer
is TBD.

One option for metadata buffer format could be:

header {
	num_ctrls
	array_of_ctrls [0..N]
		ctrl_id
		ctrl_size
		ctrl_offset
}

data {
	cid0	//offset of cid0 in dmabuf buffer
	cid1
	cidN
}

This will make easy to get concrete ctrl id without a need to parse the
whole metadata buffer. Also using dmabuf we don't need to copy data
between userspace <-> kernelspace (just cache syncs through
begin/end_cpu_access).

The open question is who will validate the metadata buffer when it comes
from userspace. The obvious answer is v4l2-core but looking into DRM
subsytem they give more freedom to the drivers, and just provide generic
helpers which are not mandatory.

I guess this will be a voice in the wilderness but I wanted to know your
opinion.

> +};
> +
>  #ifndef __KERNEL__
>  /**
>   * v4l2_timeval_to_ns - Convert timeval to nanoseconds
> @@ -2520,6 +2581,29 @@ struct v4l2_create_buffers {
>  	__u32			reserved[6];
>  };
>  
> +/**
> + * struct v4l2_ext_create_buffers - VIDIOC_EXT_CREATE_BUFS argument
> + * @index:	on return, index of the first created buffer
> + * @count:	entry: number of requested buffers,
> + *		return: number of created buffers
> + * @memory:	enum v4l2_memory; buffer memory type
> + * @capabilities: capabilities of this buffer type.
> + * @format:	frame format, for which buffers are requested
> + * @flags:	additional buffer management attributes (ignored unless the
> + *		queue has V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS capability
> + *		and configured for MMAP streaming I/O).
> + * @reserved:	extra space reserved for future fields, must be set to 0
> + */
> +struct v4l2_ext_create_buffers {
> +	__u32				index;
> +	__u32				count;
> +	__u32				memory;
> +	struct v4l2_ext_pix_format	format;
> +	__u32				capabilities;
> +	__u32				flags;
> +	__u32 reserved[4];
> +};
> +
>  /*
>   *	I O C T L   C O D E S   F O R   V I D E O   D E V I C E S
>   *
> @@ -2623,6 +2707,11 @@ struct v4l2_create_buffers {
>  #define VIDIOC_G_EXT_PIX_FMT	_IOWR('V', 104, struct v4l2_ext_pix_format)
>  #define VIDIOC_S_EXT_PIX_FMT	_IOWR('V', 105, struct v4l2_ext_pix_format)
>  #define VIDIOC_TRY_EXT_PIX_FMT	_IOWR('V', 106, struct v4l2_ext_pix_format)
> +#define VIDIOC_EXT_CREATE_BUFS	_IOWR('V', 107, struct v4l2_ext_create_buffers)
> +#define VIDIOC_EXT_QUERYBUF	_IOWR('V', 108, struct v4l2_ext_buffer)
> +#define VIDIOC_EXT_QBUF		_IOWR('V', 109, struct v4l2_ext_buffer)
> +#define VIDIOC_EXT_DQBUF	_IOWR('V', 110, struct v4l2_ext_buffer)
> +#define VIDIOC_EXT_PREPARE_BUF	_IOWR('V', 111, struct v4l2_ext_buffer)
>  
>  /* Reminder: when adding new ioctls please add support for them to
>     drivers/media/v4l2-core/v4l2-compat-ioctl32.c as well! */
> 

-- 
regards,
Stan

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