Re: [PATCH v2.1] v4l: Clearly document interactions between formats, controls and buffers

[Sakari Ailus]

Hi Laurent,

Thanks for the update.

On Sun, Mar 05, 2017 at 04:39:36PM +0200, Laurent Pinchart wrote:
> V4L2 exposes parameters that influence buffers sizes through the format
> ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT, VIDIOC_S_FMT, and possibly
> VIDIOC_G_SELECTION and VIDIOC_S_SELECTION). 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

V4L2_CID_ROTATE

> 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 and VIDIOC_S_SELECTION ioctls when buffers are allocated is
> also not fully specified.
> 
> This commit clearly defines and documents the interactions between

It's say it's a patch, and most kernel developers seem to agree:

$ git log v4.9..v4.10|grep -ci
'this patch'
1816
$ git log v4.9..v4.10|grep -ci
'this commit'
217

Up to you. :-)

> formats, selections, 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 
> ---
> Changes since v2:
> 
> - Document the interaction with ioctls that can affect formats
>   (VIDIOC_S_SELECTION, VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_STD and
>   VIDIOC_S_DV_TIMINGS)
> - Clarify the format/control change order
> ---
>  Documentation/media/uapi/v4l/buffer.rst | 108 
> 
>  1 file changed, 108 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/buffer.rst 
> b/Documentation/media/uapi/v4l/buffer.rst
> index ac58966ccb9b..ce46908bedfb 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -34,6 +34,114 @@ 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``

How about:

s/the format or control set ioctl/the ioctl setting the format or the
control/

> +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_EXT_CTRLS
> + #. VIDIOC_S_FMT
> + #. VIDIOC_QBUF
> + #. VIDIOC_STREAMON
> +
> +.. note::
> +
> +   The API doesn't mandate the above order for control (2.) and format (3.)
> +   

[PATCH v2.1] v4l: Clearly document interactions between formats, controls and buffers

[Laurent Pinchart]

V4L2 exposes parameters that influence buffers sizes through the format
ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT, VIDIOC_S_FMT, and possibly
VIDIOC_G_SELECTION and VIDIOC_S_SELECTION). 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 and VIDIOC_S_SELECTION ioctls when buffers are allocated is
also not fully specified.

This commit clearly defines and documents the interactions between
formats, selections, 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 
---
Changes since v2:

- Document the interaction with ioctls that can affect formats
  (VIDIOC_S_SELECTION, VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_STD and
  VIDIOC_S_DV_TIMINGS)
- Clarify the format/control change order
---
 Documentation/media/uapi/v4l/buffer.rst | 108 
 1 file changed, 108 insertions(+)

diff --git a/Documentation/media/uapi/v4l/buffer.rst 
b/Documentation/media/uapi/v4l/buffer.rst
index ac58966ccb9b..ce46908bedfb 100644
--- a/Documentation/media/uapi/v4l/buffer.rst
+++ b/Documentation/media/uapi/v4l/buffer.rst
@@ -34,6 +34,114 @@ 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_EXT_CTRLS
+ #. VIDIOC_S_FMT
+ #. VIDIOC_QBUF
+ #. VIDIOC_STREAMON
+
+.. note::
+
+   The API doesn't mandate the above order for control (2.) and format (3.)
+   changes. Format and controls can be set in a different order, or even
+   interleaved, depending on the device and use case. For instance some
+   controls might behave differently for different pixel formats, in which
+   case the format might need to be set first.
+
+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,