Date:   Mon, 27 Jul 2020 14:35:58 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Stanimir Varbanov <stanimir.varbanov@...aro.org>
Cc:     Helen Koike <helen.koike@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Hirokazu Honda <hiroh@...omium.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Brian Starkey <Brian.Starkey@....com>, kernel@...labora.com,
        Neil Armstrong <narmstrong@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        frkoenig@...omium.org, Maxime Jourdan <mjourdan@...libre.com>
Subject: Re: [PATCH v4 2/6] media: v4l2: Add extended buffer operations

Hi Stanimir,

On Fri, Jul 24, 2020 at 3:17 PM Stanimir Varbanov
<stanimir.varbanov@...aro.org> wrote:
>
>
>
> On 7/21/20 5:40 PM, Helen Koike wrote:
> >
> >
> > On 7/21/20 11:30 AM, Stanimir Varbanov wrote:
> >> Hi Helen,
> >>
> >> On 7/21/20 4:54 PM, Helen Koike wrote:
> >>> Hi,
> >>>
> >>> On 7/21/20 8:26 AM, Stanimir Varbanov wrote:
> >>>>
> >>>>
> >>>> 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?
> >>>
> >>> I see that in struct v4l2_buffer, we have __s32, I don't mind changing it
> >>> to keep the consistency, I just don't see where this value can be a negative
> >>> number.
> >>
> >> here
> >> https://elixir.bootlin.com/linux/v5.8-rc4/source/drivers/media/common/videobuf2/videobuf2-v4l2.c#L134
> >
> > I saw that -1 is used to signal an invalid value, but I was just wondering when request_fd = 0 is valid.
>
> The request_fd is valid system wide file descriptor and request_fd = 0
> is STDIN_FILENO thus IMO it is valid as far as we call it file descriptor.
>
> >
> >>
> >>>
> >>>>
> >>>>> + 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
> >>>> }
> >>>
> >>> Would it be better if, instead of adding a medatata_fd inside struct v4l2_ext_buffer,
> >>> we create a new ioctl that gets this structs for the controls and sync them using the
> >>> Request API ?
>
> New ioctl means new syscall. There are use-cases where encoding
> framerate is 480 fps (and more in near future, for example 960fps) this
> means 480 more syscalls per second. I don't think this is optimal and
> scalable solution at all.
>

Do you happen to have some data to confirm that it's indeed a problem?

Best regards,
Tomasz

> >>
> >> no, this solution has performance drawbacks when the metadata is big,
> >> think of 64K.
> >
> > Why? You could still use a dmabuf in this new ioctl, no?
> >
> >
> > Regards,
> > Helen
> >
> >>
> >>>
> >>> I'd like to avoid too much metadata in the buffer object.
> >>>
> >>> Regards,
> >>> Helen
> >>>
> >>>>
> >>>> 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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