Hi Kamil, On Tuesday 30 March 2010 14:26:00 Kamil Debski wrote: > > From: Hans Verkuil [mailto:hverkuil@xxxxxxxxx] > > 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. > > > > > > > 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. In that case I have to agree with Hans, a private ioctl is better. > 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. I don't think it would. The format of the data is pretty much hardware specific. If you can make the format generic, then the private ioctl could become generic. > Do You have any better solution for such device? -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
