Re: [RFC PATCH v2 3/7] media: v4l2: Add extended buffer operations

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu,  4 Apr 2019 10:16:56 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:


> +/**
> + * struct v4l2_ext_buffer - extended video buffer info
> + * @index: id number of the buffer
> + * @type: enum v4l2_buf_type; buffer type. _MPLANE and _OVERLAY formats are
> + *	  invalid
> + * @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
> + * @num_planes: number of plane buffers
> + * @request_fd: fd of the request that this buffer should use
> + * @reserved: some extra space reserved to add future fields (like timecode).
> + *	      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;
> +	struct v4l2_ext_plane planes[VIDEO_MAX_PLANES];

I had a discussion with Tomasz last week, and he made me realize I was
misunderstanding the concept of V4L2 planes. I thought it was encoding
pixel-component planes, but it's actually memory planes, and sometimes
those one memory planes might contain all component planes placed next
to each others (like the V4L2_PIX_FMT_NV12 format).

So, the question is, what do we want v4l2_ext_plane to encode (memory
planes or pixel component planes)?
If we go for the pixel component plane approach (which IMHO would be a
good thing), that means we'll have to convert V4L2_PIX_FMT_NV12-like
single-memory-plane buffers into v4l2_ext_buffer containing X planes,
each pointing to the same memory object but at a different offset.

> +	__u32 num_planes;
> +	__u32 request_fd;
> +	__u32 reserved[10];
> +};
> +
>  /**
>   * v4l2_timeval_to_ns - Convert timeval to nanoseconds
>   * @ts:		pointer to the timeval variable to be converted
> @@ -1062,6 +1139,35 @@ struct v4l2_exportbuffer {
>  	__u32		reserved[11];
>  };
>