Re: [PATCH v2 2/3] v4l: Clearly document interactions between formats, controls and buffers

[Sakari Ailus <sakari.ailus@iki.fi>]

Hi Laurent,

On Tue, Feb 28, 2017 at 05:03:19PM +0200, Laurent Pinchart wrote:
> V4L2 exposes parameters that influence buffers sizes through the format
> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT and VIDIO_S_FMT). Other parameters
> not part of the format structure may also influence buffer sizes or
> buffer layout in general. One existing such parameter is rotation, which
> is implemented by the VIDIOC_ROTATE control and thus exposed through the
> V4L2 control ioctls.
> 
> The interaction between those parameters and buffers is currently only
> partially specified by the V4L2 API. In particular interactions between
> controls and buffers isn't specified at all. The behaviour of the
> VIDIOC_S_FMT ioctl when buffers are allocated is also not fully
> specified.
> 
> This commit clearly defines and documents the interactions between
> formats, controls and buffers.
> 
> The preparatory discussions for the documentation change considered
> completely disallowing controls that change the buffer size or layout,
> in favour of extending the format API with a new ioctl that would bundle
> those controls with format information. The idea has been rejected, as
> this would essentially be a restricted version of the upcoming request
> API that wouldn't bring any additional value.
> 
> Another option we have considered was to mandate the use of the request
> API to modify controls that influence buffer size or layout. This has
> also been rejected on the grounds that requiring the request API to
> change rotation even when streaming is stopped would significantly
> complicate implementation of drivers and usage of the V4L2 API for
> applications.
> 
> Applications will however be required to use the upcoming request API to
> change at runtime formats or controls that influence the buffer size or
> layout, because of the need to synchronize buffers with the formats and
> controls. Otherwise there would be no way to interpret the content of a
> buffer correctly.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> ---
>  Documentation/media/uapi/v4l/buffer.rst | 88 +++++++++++++++++++++++++++++++++
>  1 file changed, 88 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index ac58966ccb9b..5c58db98ab7a 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -34,6 +34,94 @@ flags are copied from the OUTPUT video buffer to the CAPTURE video
>  buffer.
>  
>  
> +Interactions between formats, controls and buffers
> +==================================================
> +
> +V4L2 exposes parameters that influence the buffer size, or the way data is
> +laid out in the buffer. Those parameters are exposed through both formats and
> +controls. One example of such a control is the ``V4L2_CID_ROTATE`` control
> +that modifies the direction in which pixels are stored in the buffer, as well
> +as the buffer size when the selected format includes padding at the end of
> +lines.
> +
> +The set of information needed to interpret the content of a buffer (e.g. the
> +pixel format, the line stride, the tiling orientation or the rotation) is
> +collectively referred to in the rest of this section as the buffer layout.
> +
> +Modifying formats or controls that influence the buffer size or layout require
> +the stream to be stopped. Any attempt at such a modification while the stream
> +is active shall cause the format or control set ioctl to return the ``EBUSY``
> +error code.
> +
> +Controls that only influence the buffer layout can be modified at any time
> +when the stream is stopped. As they don't influence the buffer size, no
> +special handling is needed to synchronize those controls with buffer
> +allocation.
> +
> +Formats and controls that influence the buffer size interact with buffer
> +allocation. As buffer allocation is an expensive operation, drivers should
> +allow format or controls that influence the buffer size to be changed with
> +buffers allocated. A typical ioctl sequence to modify format and controls is
> +
> + #. VIDIOC_STREAMOFF
> + #. VIDIOC_S_FMT
> + #. VIDIOC_S_EXT_CTRLS

Which one do you set first, the format or the controls? Supposedly the user
would have to get the format again after setting the ROTATE control.

> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +Queued buffers must be large enough for the new format or controls.
> +
> +Drivers shall return a ``ENOSPC`` error in response to format change
> +(:c:func:`VIDIOC_S_FMT`) or control changes (:c:func:`VIDIOC_S_CTRL` or
> +:c:func:`VIDIOC_S_EXT_CTRLS`) if buffers too small for the new format are
> +currently queued. As a simplification, drivers are allowed to return an error
> +from these ioctls if any buffer is currently queued, without checking the
> +queued buffers sizes. Drivers shall also return a ``ENOSPC`` error from the
> +:c:func:`VIDIOC_QBUF` ioctl if the buffer being queued is too small for the
> +current format or controls. Together, these requirements ensure that queued
> +buffers will always be large enough for the configured format and controls.
> +
> +Userspace applications can query the buffer size required for a given format
> +and controls by first setting the desired control values and then trying the
> +desired format. The :c:func:`VIDIOC_TRY_FMT` ioctl will return the required
> +buffer size.
> +
> + #. VIDIOC_S_EXT_CTRLS(x)
> + #. VIDIOC_TRY_FMT()
> + #. VIDIOC_S_EXT_CTRLS(y)
> + #. VIDIOC_TRY_FMT()
> +
> +The :c:func:`VIDIOC_CREATE_BUFS` ioctl can then be used to allocate buffers
> +based on the queried sizes (for instance by allocating a set of buffers large
> +enough for all the desired formats and controls, or by allocating separate set
> +of appropriately sized buffers for each use case).
> +
> +To simplify their implementation, drivers may also require buffers to be
> +reallocated in order to change formats or controls that influence the buffer
> +size. In that case, to perform such changes, userspace applications shall
> +first stop the video stream with the :c:func:`VIDIOC_STREAMOFF` ioctl if it
> +is running and free all buffers with the :c:func:`VIDIOC_REQBUFS` ioctl if
> +they are allocated. The format or controls can then be modified, and buffers
> +shall then be reallocated and the stream restarted. A typical ioctl sequence
> +is
> +
> + #. VIDIOC_STREAMOFF
> + #. VIDIOC_REQBUFS(0)
> + #. VIDIOC_S_FMT
> + #. VIDIOC_S_EXT_CTRLS

Same here.

Would it be safe to say that controls are changed first? I wonder if there
could be special cases where this wouldn't apply though. It could ultimately
come down to hardware features: rotation might be only available for certain
formats so you'd need to change the format first to enable rotation.

What you're documenting above is a typical sequence so it doesn't have to be
applicable to all potential hardware. I might mention there could be such
dependencies. I wonder if one exists at the moment. No?

> + #. VIDIOC_REQBUFS(n)
> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +The second :c:func:`VIDIOC_REQBUFS` call will take the new format and control
> +value into account to compute the buffer size to allocate. Applications can
> +also retrieve the size by calling the :c:func:`VIDIOC_G_FMT` ioctl if needed.
> +
> +When reallocation is required, any attempt to modify format or controls that
> +influences the buffer size while buffers are allocated shall cause the format
> +or control set ioctl to return the ``EBUSY`` error code.
> +
> +
>  .. c:type:: v4l2_buffer
>  
>  struct v4l2_buffer

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi	XMPP: sailus@retiisi.org.uk