[Linux-uvc-devel] Accessing Extension Units
laurent.pinchart at skynet.be
Sat Aug 18 23:47:45 CEST 2007
On Friday 17 August 2007, Nathanael Galpin wrote:
> I'm testing some cameras right now, and I'd like to be able to access
> the controls in the camera's extension unit. I've spent some time this
> last week looking at the driver and attempting to add the controls in,
> but I haven't been successful. What steps do I need to take to get the
> driver to recognize these controls?
You will have to modify the uvc_ctrls and uvc_ctrl_mappings arrays in
Add an entry to uvc_ctrls with the following fields:
- entity entity GUID
- selector UVC control selector (the identifier to be used in requests)
- index UVC control index in extension unit descriptor bmControls bitfield
- size UVC control size in bytes
- flags UVC control flags (see existing controls for examples)
Then add one or more entries to uvc_ctrl_mappings to map the UVC control to
V4L2 control(s). A single UVC control can be mapped to several V4L2 controls.
The fields are:
- id V4L2 control ID
- name V4L2 control name
- entity associated UVC control entity GUID
- selector associated UVC control selector
- size V4L2 control size in bits
- offset V4L2 control offset in bits from the UVC control base (*)
- v4l2_type V4L2 control type
- data_type V4L2 control field type in the UVC control (**)
(*) A single UVC control can be mapped to several V4L2 controls. For instance,
a pan/tilt 32-bit (4 bytes) UVC control made of a 16-bit pan value followed
by a 16-bit tilt value could be mapped to a 16-bit V4L2 pan control and a
16-bit V4L2 tilt control. The first V4L2 control's offset would be equal to 0
and the second V4L2 control's offset would be equal to 16.
(**) The currently supported values are UVC_CTRL_DATA_TYPE_SIGNED and
UVC_CTRL_DATA_TYPE_UNSIGNED. They are used to perform sign extension when
mapping from signed UVC controls to signed V4L2 controls if the control size
are not identical.
Finally, modify uvcvideo.h and set V4L2_CID_PRIVATE_LAST equal to your last
private V4L2 control identifier.
> So far I've added a GUID for the camera's extension unit, as well I've
> added entries to the uvc_ctrl and uvc_ctrl_mappings lists. I've written
> a test application using the V4L2 API to enumerate the controls on the
> camera, and it does see a new control,
Sounds good so far.
> but I'm not able to use that control. It seems like I'm missing something to
> associate the control with the camera it belongs to.
What are the symptoms ? Do you get an error when you try to access the
control ? If so, what error (errno) do you get ?
If you can enumerate the control on your device instance, there should be
no "association" problem. Try to raise the trace level to 255 (modprobe
uvcvideo trace=255 or echo 255 > /sys/modules/uvcvideo/parameters/trace) and
check the kernel log (dmesg) for error messages after you try to access the
control from your userspace application.
> Hopefully that makes sense, at this point I'm not looking for a really
> elegant solution, I'd just like to hard code it into the driver to do
> some testing.
The elegant solution is in the dynctrls branch. The idea is to let a userspace
application configure the driver to add new control mappings at runtime. I
have to bring the branch up to date, as I haven't worked on it for some time.
More information about the Linux-uvc-devel