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@xxxxxxxxxxxxxxxx> > --- > 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@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
