On Monday, August 08, 2011 13:40:23 Laurent Pinchart wrote:
> On Monday 08 August 2011 11:16:41 Hans Verkuil wrote:
> > On Friday, August 05, 2011 09:47:13 Guennadi Liakhovetski wrote:
> > > A possibility to preallocate and initialise buffers of different sizes
> > > in V4L2 is required for an efficient implementation of asnapshot mode.
> > > This patch adds two new ioctl()s: VIDIOC_CREATE_BUFS and
> > > VIDIOC_PREPARE_BUF and defines respective data structures.
> > >
> > > Signed-off-by: Guennadi Liakhovetski <[email protected]>
> > > ---
> > >
> > > v4:
> > >
> > > 1. CREATE_BUFS now takes an array of plane sizes and a fourcc code in
its
> > >
> > > argument, instead of a frame format specification, including
> > > documentation update
> > >
> > > 2. documentation improvements, as suggested by Hans
> > > 3. increased reserved fields to 18, as suggested by Sakari
> > >
> > > Documentation/DocBook/media/v4l/io.xml | 17 ++
> > > Documentation/DocBook/media/v4l/v4l2.xml | 2 +
> > > .../DocBook/media/v4l/vidioc-create-bufs.xml | 161
> >
> > ++++++++++++++++++++
> >
> > > .../DocBook/media/v4l/vidioc-prepare-buf.xml | 96 ++++++++++++
> > > drivers/media/video/v4l2-compat-ioctl32.c | 6 +
> > > drivers/media/video/v4l2-ioctl.c | 26 +++
> > > include/linux/videodev2.h | 18 +++
> > > include/media/v4l2-ioctl.h | 2 +
> > > 8 files changed, 328 insertions(+), 0 deletions(-)
> > > create mode 100644
> > > Documentation/DocBook/media/v4l/vidioc-create-bufs.xml create mode
> > > 100644 Documentation/DocBook/media/v4l/vidioc-prepare-buf.xml
> >
> > <snip>
> >
> > > diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
> > > index fca24cc..3cd0cb3 100644
> > > --- a/include/linux/videodev2.h
> > > +++ b/include/linux/videodev2.h
> > > @@ -653,6 +653,9 @@ struct v4l2_buffer {
> > >
> > > #define V4L2_BUF_FLAG_ERROR 0x0040
> > > #define V4L2_BUF_FLAG_TIMECODE 0x0100 /* timecode field is valid
*/
> > > #define V4L2_BUF_FLAG_INPUT 0x0200 /* input field is valid */
> > >
> > > +/* Cache handling flags */
> > > +#define V4L2_BUF_FLAG_NO_CACHE_INVALIDATE 0x0400
> > > +#define V4L2_BUF_FLAG_NO_CACHE_CLEAN 0x0800
> > >
> > > /*
> > >
> > > * O V E R L A Y P R E V I E W
> > >
> > > @@ -2092,6 +2095,18 @@ struct v4l2_dbg_chip_ident {
> > >
> > > __u32 revision; /* chip revision, chip specific */
> > >
> > > } __attribute__ ((packed));
> > >
> > > +/* VIDIOC_CREATE_BUFS */
> > > +struct v4l2_create_buffers {
> > > + __u32 index; /* output: buffers index...index + count - 1 have
been
> >
> > created */
> >
> > > + __u32 count;
> > > + __u32 type;
> > > + __u32 memory;
> > > + __u32 fourcc;
> > > + __u32 num_planes;
> > > + __u32 sizes[VIDEO_MAX_PLANES];
> > > + __u32 reserved[18];
> > > +};
> >
> > I know you are going to hate me for this, but I've changed my mind: I
think
> > this should use a struct v4l2_format after all.
> >
> > This change of heart came out of discussions during the V4L2 brainstorm
> > meeting last week. The only way to be sure the buffers are allocated
> > optimally is if the driver has all the information. The easiest way to do
> > that is by passing struct v4l2_format. This is also consistent with
> > REQBUFS since that uses the information from the currently selected format
> > (i.e. what you get back from VIDIOC_G_FMT).
> >
> > There can be subtle behaviors such as allocating from different memory
back
> > based on the fourcc and the size of the image.
> >
> > One reason why I liked passing sizes directly is that it allows the caller
> > to ask for more memory than is strictly necessary.
> >
> > However, while brainstorming last week the suggestion was made that there
> > is no reason why the user can't set the sizeimage field in
> > v4l2_pix_format(_mplane) to something higher. The S/TRY_FMT spec
explicitly
> > mentions that the sizeimage field is set by the driver, but for the new
> > CREATEBUFS ioctl no such limitation has to be placed. The only thing
> > necessary is to ensure that sizeimage is not too small (and you probably
> > want some sanity check against crazy values as well).
>
> We need to decide on a policy here. What should be the maximum allowable
size
> for MMAP buffers ? How do we restrict the requested image size so that
> application won't be allowed to starve the system by requesting memory for
1GP
> images ?
Either just a arbitrary cap like 1 GB (mainly to prevent any weird calculation
problems around the 2 GB (signedness) and 4 GB (wrap-around) boundaries), or
something like 3 or 4 times the minimum buffer size.
I'm in favor of enforcing a 1 GB cap in vb2 and letting drivers enforce a
policy of their own if that makes sense for them.
I don't really see a problem with requesting large amounts of memory. What
constitutes 'large' is not something the kernel knows, that's dependent on the
use case.
Regards,
Hans
>
> > This way the decision on how to allocate memory is the same between
REQBUFS
> > and CREATEBUFS (i.e. both use v4l2_format information), but there is no
> > need for a union as we had in the initial proposal since apps can set the
> > sizeimage to something larger than strictly necessary (or just leave it to
> > 0 to get the smallest size).
>
> --
> Regards,
>
> Laurent Pinchart
>
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