RE: [PATCH/RFC 0/1] v4l: Add support for binary controls

[Kamil Debski <k.debski@samsung.com>]

> From: Hans Verkuil [mailto:hverkuil@xs4all.nl]
> 
> Hi Kamil!

Hi Hans,

> 
> On Monday 29 March 2010 11:53:05 Kamil Debski wrote:
> >
> > Hello,
> >
> > This patch introduces new type of v4l2 control - the binary control.
> It
> > will be useful for exchanging raw binary data between the user space
> and
> > the driver/hardware.
> >
> > The patch is pretty small – basically it adds a new control type.
> >
> > 1.  Reasons to include this new type
> > - Some devices require data which are not part of the stream, but
> there
> > are necessary for the device to work e.g. coefficients for
> transformation
> > matrices.
> > - String control is not suitable as it suggests that the data is a
> null
> > terminated string. This might be important when printing debug
> information -
> > one might output strings as they are and binary data in hex.
> >
> > 2. How does the binary control work
> > The binary control has been based on the string control. The
> principle of
> > use is the same. It uses v4l2_ext_control structure to pass the
> pointer and
> > size of the data. It is left for the driver to call the
> copy_from_user/
> > copy_to_user function to copy the data.
> >
> > 3. About the patch
> > The patch is pretty small – it basically adds a new control type.
> >
> > Best wishes,
> >
> 
> I don't think this is a good idea. Controls are not really meant to be
> used
> as an ioctl replacement.
> 
> Controls can be used to control the hardware via a GUI (e.g. qv4l2).
> Obviously,
> this will fail for binary controls. Controls can also be used in cases
> where
> it is not known up front which controls are needed. This typically
> happens for
> bridge drivers that can use numerous combinations of i2c sub-devices.
> Each
> subdev can have its own controls.
> 
> There is a grey area where you want to give the application access to
> low-level
> parameters but without showing them to the end-user. This is currently
> not
> possible, but it will be once the control framework is finished and
> once we
> have the possibility to create device nodes for subdevs.
> 
> But what you want is to basically pass whole structs as a control.
> That's
> something that ioctls where invented for. Especially once we have
> subdev nodes
> this shouldn't be a problem.
> 
> Just the fact that it is easy to implement doesn't mean it should be
> done :-)
> 
> Do you have specific use-cases for your proposed binary control?

Yes, I have. I am working on a driver for a video codec which is using 
the mem2mem framework. I have to admit it's a pretty difficult 
hardware to work with. It was one of the reasons for Pawel Osciak 
to add multiplane support to videobuf.

Before decoding, the hardware has to parse the header of the video
stream to get all necessary parameters such as the number of buffers,
width, height and some internal, codec specific stuff. The video stream
is then demultiplexed and divided into encoded frames in software and
the hardware can only process one, separated frame at a time.

The whole codec setup cannot be achieved by using VIDIOC_S_FMT call, 
because hardware requires access to the header data. I wanted to use
this binary control to pass the header to the codec after setting the
right format with VIDIOC_S_FMT. Then video frames can be easily decoded
as a standard calls to QBUF/DQBUF pairs.

It is similar for encoding - the basic parameters are set with
VIDIOC_S_FMT, then some codec specific/advanced are accessible as
standard v4l2 controls. Then the encoding engine is initialized and the
hardware returns an header of the output video stream. The header can
be acquired by getting the value of the binary control. After that the
frame to be encoded is provided and hw returns a single frame of
encoded stream. 

Using custom ioctls seems appropriate for a hardware specific driver.
Whereas the proposed binary control for getting and setting the video
stream header could be generic solution and can be used in many drivers
for hardware codecs.

Do You have any better solution for such device?

Best regards,
--
Kamil Debski
Linux Platform Group
Samsung Poland R&D Center