Hi Sakari,
On Thursday 01 September 2011 13:06:12 Sakari Ailus wrote:
> On Thu, Sep 01, 2011 at 12:51:58PM +0200, Guennadi Liakhovetski wrote:
> > On Thu, 1 Sep 2011, Sakari Ailus wrote:
> > > Guennadi Liakhovetski wrote:
> > > > On Thu, 1 Sep 2011, Sakari Ailus wrote:
> > > >> On Thu, Sep 01, 2011 at 09:03:52AM +0200, Guennadi Liakhovetski
wrote:
> > > >>> Hi Sakari
> > > >>
> > > >> Hi Guennadi,
> > > >>
> > > >>> On Thu, 1 Sep 2011, Sakari Ailus wrote:
> > > >> [clip]
> > > >>
> > > >>>>> diff --git a/include/linux/videodev2.h
> > > >>>>> b/include/linux/videodev2.h index fca24cc..988e1be 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 +2095,15 @@ 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;
> > > >>>>> + enum v4l2_memory memory;
> > > >>>>> + struct v4l2_format format; /* "type" is used
> > > >>>>> always, the
rest
> > > >>>>> if sizeimage == 0 */ + __u32 reserved[8];
> > > >>>>> +};
> > > >>>>
> > > >>>> How about splitting the above comments? These lines are really
> > > >>>> long. Kerneldoc could also be used, I think.
> > > >>>
> > > >>> Sure, how about this incremental patch:
> > > >>>
> > > >>> From: Guennadi Liakhovetski <g.liakhovet...@gmx.de>
> > > >>> Subject: V4L: improve struct v4l2_create_buffers documentation
> > > >>>
> > > >>> Signed-off-by: Guennadi Liakhovetski <g.liakhovet...@gmx.de>
> > > >>> ---
> > > >>> diff --git a/include/linux/videodev2.h b/include/linux/videodev2.h
> > > >>> index 988e1be..64e0bf2 100644
> > > >>> --- a/include/linux/videodev2.h
> > > >>> +++ b/include/linux/videodev2.h
> > > >>> @@ -2095,12 +2095,20 @@ struct v4l2_dbg_chip_ident {
> > > >>>
> > > >>> __u32 revision; /* chip revision, chip specific */
> > > >>>
> > > >>> } __attribute__ ((packed));
> > > >>>
> > > >>> -/* VIDIOC_CREATE_BUFS */
> > > >>> +/**
> > > >>> + * struct v4l2_create_buffers - VIDIOC_CREATE_BUFS argument
> > > >>> + * @index: on return, index of the first created buffer
> > > >>> + * @count: entry: number of requested buffers,
> > > >>> + * return: number of created buffers
> > > >>> + * @memory: buffer memory type
> > > >>> + * @format: frame format, for which buffers are requested
> > > >>> + * @reserved: future extensions
> > > >>> + */
> > > >>>
> > > >>> struct v4l2_create_buffers {
> > > >>>
> > > >>> - __u32 index; /* output: buffers
> > > >>> index...index + count - 1
have
> > > >>> been created */ + __u32 index;
> > > >>>
> > > >>> __u32 count;
> > > >>> enum v4l2_memory memory;
> > > >>>
> > > >>> - struct v4l2_format format; /* "type" is used
> > > >>> always, the
rest if
> > > >>> sizeimage == 0 */ + struct v4l2_format format;
> > > >>>
> > > >>> __u32 reserved[8];
> > > >>>
> > > >>> };
> > > >>
> > > >> Thanks! This looks good to me. Could you do a similar change to the
> > > >> compat-IOCTL version of this struct (v4l2_create_buffers32)?
> > > >
> > > > Of course, I'll submit an incremental patch as soon as this is
> > > > accepted for upstream, unless there are other important changes to
> > > > this patch and a new revision is anyway unavoidable.
> > >
> > > Ok. I'll send a small patch to the documentation as well then.
> >
> > Good, thanks!
> >
> > > >>>>> +
> > > >>>>>
> > > >>>>> /*
> > > >>>>>
> > > >>>>> * 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
> > > >>>>> *
> > > >>>>>
> > > >>>>> @@ -2182,6 +2194,9 @@ 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_PREPARE_BUF _IOW('V', 93,
> > > >>>>> struct v4l2_buffer)
> > > >>>>
> > > >>>> Does prepare_buf ever do anything that would need to return
> > > >>>> anything to the user? I guess the answer is "no"?
> > > >>>
> > > >>> Exactly, that's why it's an "_IOW" ioctl(), not an "_IOWR", or have
> > > >>> I misunderstood you?
> > > >>
> > > >> I was thinking if this will be the case now and in the foreseeable
> > > >> future as this can't be changed after once defined. I just wanted
> > > >> to bring this up even though I don't see myself that any of the
> > > >> fields would need to be returned to the user. But there are
> > > >> reserved fields...
> > > >>
> > > >> So unless someone comes up with something quick, I think this should
> > > >> stay as-is.
> > > >
> > > > Agree. I understand, it is important to try to design the user-space
> > > > API as clever as possible, so, I'm relying on our combined wisdom
> > > > for it. But even that is probably limited, so, mistakes are still
> > > > possible. Therefore, unless someone comes up with a realistic
> > > > reason, why this has to be _IOWR, we shall keep it _IOW and be
> > > > prepared to delight our user-space colleagues with more shiny new
> > > > ioctl()s in the somewhat near future;-)
> > >
> > > What we could also do is to mark the new IOCTLs experimental, and
> > > remove the note after one or two more kernel releases. This would
> > > allow postponing the decision.
> >
> > Is there a standard way to do this, or is it just a free-form note in the
> > documentation / in the header?
>
> I think a free form note saying this out loud in a visible place should be
> enough. A note should also be added to
> Documentation/DocBook/media/v4l/compat.xml to the section "Experimental API
> Elements".
>
> Speaking of this, we seem to have quite a few of these that probably
> shouldn't be experimental anymore, such as VIDIOC_ENUM_FRAMESIZES.
> Interestingly enough, VIDIOC_ENUM_FRAMEINTERVALS no longer is experimental.
> I think I'll send a patch for this.
>
> <URL:http://hverkuil.home.xs4all.nl/spec/media.html#vidioc-enum-framesizes>
>
> > > We also don't have anyone using these ioctls from user space as far as
> > > I understand, so we might get important input later on as well.
> >
> > Does either of these allow us to actually _change_ ioctl()s after their
> > appearance in the mainline?
>
> That's my understanding, but of course someone could just say "no" when we
> try to do that. I think that if something is marked experimental at least
> the argument that it can't be changed is a little bit moot since the users
> have been notified of this beforehand.
>
> There are a few examples of this. At least the V4L2 subdev and MC
> interfaces are marked experimental. However, we haven't actually tried to
> use that to make changes which might break user space since we haven't got
> a need to.
>
> Hans, Laurent: do you have an opinion on this?
We should of course try to keep the API and ABI compatible across kernel
versions, but experimental APIs can be changed. It also depends on how widely
the API has been picked up by userspace and how much the changes would break
it. Being experimental isn't an excuse for making userspace's life a
nightmare.
--
Regards,
Laurent Pinchart
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