On Friday 13 May 2011 09:45:51 Guennadi Liakhovetski wrote:
> I've found some more time to get back to this. Let me try to recap, what
> has been discussed. I've looked through all replies again (thanks to
> all!), so, I'll present a summary. Any mistakes and misinterpretations are
> mine;) If I misunderstand someone or forget anything - please, shout!
>
> On Fri, 1 Apr 2011, 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 three new ioctl()s: VIDIOC_CREATE_BUFS,
> > VIDIOC_DESTROY_BUFS, and VIDIOC_SUBMIT_BUF and defines respective data
> > structures.
> >
> > Signed-off-by: Guennadi Liakhovetski <[email protected]>
> > ---
> >
> > drivers/media/video/v4l2-compat-ioctl32.c | 3 ++
> > drivers/media/video/v4l2-ioctl.c | 43
> > +++++++++++++++++++++++++++++ include/linux/videodev2.h
> > | 24 ++++++++++++++++ include/media/v4l2-ioctl.h |
> > 3 ++
> > 4 files changed, 73 insertions(+), 0 deletions(-)
> >
> > diff --git a/drivers/media/video/v4l2-compat-ioctl32.c
> > b/drivers/media/video/v4l2-compat-ioctl32.c index 7c26947..d71b289
> > 100644
> > --- a/drivers/media/video/v4l2-compat-ioctl32.c
> > +++ b/drivers/media/video/v4l2-compat-ioctl32.c
> > @@ -922,6 +922,9 @@ long v4l2_compat_ioctl32(struct file *file, unsigned
> > int cmd, unsigned long arg)
> >
> > case VIDIOC_DQEVENT:
> > case VIDIOC_SUBSCRIBE_EVENT:
> >
> > case VIDIOC_UNSUBSCRIBE_EVENT:
> > + case VIDIOC_CREATE_BUFS:
> > + case VIDIOC_DESTROY_BUFS:
> >
> > + case VIDIOC_SUBMIT_BUF:
> > ret = do_video_ioctl(file, cmd, arg);
> > break;
> >
> > diff --git a/drivers/media/video/v4l2-ioctl.c
> > b/drivers/media/video/v4l2-ioctl.c index a01ed39..b80a211 100644
> > --- a/drivers/media/video/v4l2-ioctl.c
> > +++ b/drivers/media/video/v4l2-ioctl.c
> > @@ -259,6 +259,9 @@ static const char *v4l2_ioctls[] = {
> >
> > [_IOC_NR(VIDIOC_DQEVENT)] = "VIDIOC_DQEVENT",
> > [_IOC_NR(VIDIOC_SUBSCRIBE_EVENT)] = "VIDIOC_SUBSCRIBE_EVENT",
> > [_IOC_NR(VIDIOC_UNSUBSCRIBE_EVENT)] = "VIDIOC_UNSUBSCRIBE_EVENT",
> >
> > + [_IOC_NR(VIDIOC_CREATE_BUFS)] = "VIDIOC_CREATE_BUFS",
> > + [_IOC_NR(VIDIOC_DESTROY_BUFS)] = "VIDIOC_DESTROY_BUFS",
> > + [_IOC_NR(VIDIOC_SUBMIT_BUF)] = "VIDIOC_SUBMIT_BUF",
> >
> > };
> > #define V4L2_IOCTLS ARRAY_SIZE(v4l2_ioctls)
> >
> > @@ -2184,6 +2187,46 @@ static long __video_do_ioctl(struct file *file,
> >
> > dbgarg(cmd, "type=0x%8.8x", sub->type);
> > break;
> >
> > }
> >
> > + case VIDIOC_CREATE_BUFS:
> > + {
> > + struct v4l2_create_buffers *create = arg;
> > +
> > + if (!ops->vidioc_create_bufs)
> > + break;
> > + ret = check_fmt(ops, create->format.type);
> > + if (ret)
> > + break;
> > +
> > + if (create->size)
> > + CLEAR_AFTER_FIELD(create, count);
> > +
> > + ret = ops->vidioc_create_bufs(file, fh, create);
> > +
> > + dbgarg(cmd, "count=%d\n", create->count);
> > + break;
> > + }
> > + case VIDIOC_DESTROY_BUFS:
> > + {
> > + struct v4l2_buffer_span *span = arg;
> > +
> > + if (!ops->vidioc_destroy_bufs)
> > + break;
> > +
> > + ret = ops->vidioc_destroy_bufs(file, fh, span);
> > +
> > + dbgarg(cmd, "count=%d", span->count);
> > + break;
> > + }
> > + case VIDIOC_SUBMIT_BUF:
> > + {
> > + unsigned int *i = arg;
> > +
> > + if (!ops->vidioc_submit_buf)
> > + break;
> > + ret = ops->vidioc_submit_buf(file, fh, *i);
> > + dbgarg(cmd, "index=%d", *i);
> > + break;
> > + }
> >
> > default:
> > {
> >
> > bool valid_prio = true;
> >
> > diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
> > index aa6c393..b6ef46e 100644
> > --- a/include/linux/videodev2.h
> > +++ b/include/linux/videodev2.h
> > @@ -1847,6 +1847,26 @@ struct v4l2_dbg_chip_ident {
> >
> > __u32 revision; /* chip revision, chip specific */
> >
> > } __attribute__ ((packed));
> >
> > +/* VIDIOC_DESTROY_BUFS */
> > +struct v4l2_buffer_span {
> > + __u32 index; /* output: buffers index...index +
> > count - 1 have been
> > created */ + __u32 count;
> > + __u32 reserved[2];
> > +};
> > +
> > +/* struct v4l2_createbuffers::flags */
> > +#define V4L2_BUFFER_FLAG_NO_CACHE_INVALIDATE (1 << 0)
>
> 1. An additional flag FLAG_NO_CACHE_FLUSH is needed for output devices.
Shouldn't it be NO_CACHE_CLEAN ?
> 2. Both these flags should not be passed with CREATE, but with SUBMIT
> (which will be renamed to PREPARE or something similar). It should be
> possible to prepare the same buffer with different cacheing attributes
> during a running operation. Shall these flags be added to values, taken by
> struct v4l2_buffer::flags, since that is the struct, that will be used as
> the argument for the new version of the SUBMIT ioctl()?
Do you have a use case for per-submit cache handling ?
> > +
> > +/* VIDIOC_CREATE_BUFS */
> > +struct v4l2_create_buffers {
> > + __u32 index; /* output: buffers
> > index...index + count - 1 have
been
> > created */ + __u32 count;
> > + __u32 flags; /* V4L2_BUFFER_FLAG_* */
> > + enum v4l2_memory memory;
> > + __u32 size; /* Explicit size, e.g., for
> > compressed streams */
> > + struct v4l2_format format; /* "type" is used always, the
> > rest if
size
> > == 0 */ +};
>
> 1. Care must be taken to keep index <= V4L2_MAX_FRAME
Does that requirement still make sense ?
> 2. A reserved field is needed.
>
> > +
> >
> > /*
> >
> > * I O C T L C O D E S F O R V I D E O D E V I C E S
> > *
> >
> > @@ -1937,6 +1957,10 @@ struct v4l2_dbg_chip_ident {
> >
> > #define VIDIOC_SUBSCRIBE_EVENT _IOW('V', 90, struct
> > v4l2_event_subscription) #define VIDIOC_UNSUBSCRIBE_EVENT _IOW('V', 91,
> > struct v4l2_event_subscription)
> >
> > +#define VIDIOC_CREATE_BUFS _IOWR('V', 92, struct v4l2_create_buffers)
> > +#define VIDIOC_DESTROY_BUFS _IOWR('V', 93, struct v4l2_buffer_span)
> > +#define VIDIOC_SUBMIT_BUF _IOW('V', 94, int)
>
> This has become the hottest point for discussion.
>
> 1. VIDIOC_CREATE_BUFS: should the REQBUFS and CREATE/DESTROY APIs be
> allowed to be mixed? REQBUFS is compulsory, CREATE/DESTROY will be
> optional. But shall applications be allowed to mix them? No consensus has
> been riched. This will also depend on whether DESTROY will be implemented
> at all (see below).
Would mixing them help in any known use case ? The API and implementation
would be cleaner if we didn't allow mixing them.
> 2. VIDIOC_DESTROY_BUFS: has been discussed a lot
>
> (a) shall it be allowed to create holes in indices? agreement was: not at
> this stage, but in the future this might be needed.
>
> (b) ioctl() argument: shall it take a span or an array of indices? I don't
> think arrays make any sense here: on CREATE you always get contiguous
> index sequences, and you are only allowed to DESTROY the same index sets.
>
> (c) shall it be implemented at all, now that we don't know, how to handle
> holes, or shall we just continue using REQBUFS(0) or close() to release
> all buffers at once? Not implementing DESTROY now has the disadvantage,
> that if you allocate 2 buffer sets of sizes A (small) and B (big), and
> then don't need B any more, but instead need C != B (big), you cannot do
> this. But this is just one of hypothetical use-cases. No consensus
> reached.
We could go with (c) as a first step, but it might be temporary only. I feel a
bit uneasy implementing an API that we know will be temporary.
> 3. VIDIOC_SUBMIT_BUF:
>
> (a) shall be renamed to something with prepare or pre-queue in the name
> and call the .buf_prepare() videobuf2 method. This hasn't raised any
> objections, has been implemented in v2 (has not been posted yet). Name
> will be changed to VIDIOC_PREPARE_BUF
>
> (b) Proposed to use struct v4l2_buffer as the argument. Applications
> anyway need those structs for other ioctl()s and the information is needed
> for .buf_prepare(). This is done in v2.
>
> 4. It has been proposed to create wrappers to allow drivers to only
> implement CREATE/DESTROY and have those wrappers also provide REQBUFS,
> using them.
>
> > +
> >
> > /* Reminder: when adding new ioctls please add support for them to
> >
> > drivers/media/video/v4l2-compat-ioctl32.c as well! */
>
> 1. 'enum memory' and struct v4l2_format need special handling. Fixed in
> v2, untested.
>
> > diff --git a/include/media/v4l2-ioctl.h b/include/media/v4l2-ioctl.h
> > index dd9f1e7..00962c6 100644
> > --- a/include/media/v4l2-ioctl.h
> > +++ b/include/media/v4l2-ioctl.h
> > @@ -122,6 +122,9 @@ struct v4l2_ioctl_ops {
> >
> > int (*vidioc_qbuf) (struct file *file, void *fh, struct v4l2_buffer
> > *b); int (*vidioc_dqbuf) (struct file *file, void *fh, struct
> > v4l2_buffer *b);
> >
> > + int (*vidioc_create_bufs) (struct file *file, void *fh, struct
> > v4l2_create_buffers *b); + int (*vidioc_destroy_bufs)(struct file *file,
> > void *fh, struct v4l2_buffer_span *b); + int (*vidioc_submit_buf)
> > (struct file *file, void *fh, unsigned int i);
> >
> > int (*vidioc_overlay) (struct file *file, void *fh, unsigned int i);
> > int (*vidioc_g_fbuf) (struct file *file, void *fh,
>
> Personally, I think, one of viable solutions for now would be to implement
>
> > +#define VIDIOC_CREATE_BUFS _IOWR('V', 92, struct v4l2_create_buffers)
> > +#define VIDIOC_PREPARE_BUF _IOW('V', 93, struct v4l2_buffer)
--
Regards,
Laurent Pinchart
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