On 04/03/17 15:37, Sakari Ailus wrote:
Hi Hans (and Laurent).
On Sat, Mar 04, 2017 at 11:53:45AM +0100, Hans Verkuil wrote:
Hi Laurent,
Here is my review:
On 28/02/17 16:03, 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
S_SELECTION should be mentioned here as well (more about that later).
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.
This is not what happens today: it's not the streaming part that causes EBUSY to
be returned but whether or not buffers are allocated.
Today we do not support changing buffer sizes on the fly, so any attempt to
call an ioctl that would change the buffer size is blocked and EBUSY is returned.
To be precise: drivers call vb2_is_busy() to determine this.
It certainly shouldn't be like that. Not allowing S_FMT() while there are
buffers allocated makes CREATE_BUFS entirely useless.
Well, CREATE_BUFS can still be used to make large buffers in which the image
is composed. But yes, CREATE_BUFS has limited usefulness today, but the simple
fact is that nobody wrote drivers that really can do on the fly format changes.
To my knowledge all vb2-using drivers behave like this. There may be old drivers
that do not do this (and these have a high likelyhood of being wrong).
What's really needed is that the driver verifies that the buffer is large
enough to be used for a given format. vb2_is_busy() shouldn't be used to
check whether setting format is allowed.
Again, there are no drivers that support this. Using vb2_is_busy() prevents
userspace from trying this, since without proper driver support this *will*
fail.
Of course, drivers that do support this will not test vb2_is_busy but the
vb2_is_streaming instead (and with the request API even that can be dropped).
+
+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
+ #. 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.
Actually, today qbuf will return -EINVAL (from __verify_length in videobuf2-v4l2.c)
in these cases. I am pretty sure you can't change that to ENOSPC since this has
always been -EINVAL, and so changing this would break the ABI.
*If* today drivers return -EBUSY on S_FMT if there are buffers allocated,
you can't have this happening in the first place: it is a new condition so
using a new error code is possible. I would not expect applications to try
that either for the same reason.
Yes, this can happen when you pass in USERPTR or DMABUF buffers. It indeed can't
happen with MMAP buffers.
It is rather that applications designed for a particular device are more
likely to attempt this (e.g. capturing a still image after streaming
viewfinder first).
I realised -EBUSY is not even documented for S_FMT; I'll post a patch to fix
that.
Trying to change the format while buffers are allocated will return EBUSY today.
If you are trying to document what will happen when drivers allow format changes
on the fly then this is not at all clear from what you write here.
So:
If the driver does not support changing the format while buffers are queued, then
it will return EBUSY (true for almost (?) all drivers today). If it does support
this, then it will behave as described above, except for the ENOSPC error in QBUF.
Note that the meaning of ENOSPC should also be explicitly documented in the ioctls
that can return this.
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
+ #. 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.
Ah, here you describe the 99% situation. This should come first. You've been working
on the 1% that allows changing formats while buffers are allocated so that feels
all-important to you, but in reality almost all drivers use the 'simplified'
implementation. Describe that first, then go on describing what will happen for
your driver. That will make much more sense to me (as you can tell from the preceding
comments) and I'm sure to the end-user as well.
What should be mentioned here as well is that S_SELECTION can also implicitly change
the format, specifically if there is no scaler or if the scaler has limitations.
I think it'd be fine to mention that, but the effect is indeed implicit
through the change in format.
Also there are a few ioctls that can reset selections and formats when called:
S_INPUT/S_OUTPUT, S_STD and S_DV_TIMINGS.
I think this should be mentioned here. It can be just in passing, no need to go
in-depth on that. As long as people are aware of it.
The documentation of S_STD and S_DV_TIMINGS should also be updated saying that if
the new std or timings differ from the existing std or timings, then the format
will also change and selection rectangles will be reset to the defaults. Setting
the std/dv_timings with the current std or timings will not do anything: there
are applications that do this even when streaming so this should be allowed.
Unfortunately, this is not documented but it really should.
If you don't have time to update the S_STD/DV_TIMINGS ioctls, then let me know and
I will do that.
Regards,
Hans
