USB Video (UVC) extension unit

Extension unit property IDs correspond to similarly numbered extension unit control IDs exposed by the UVC hardware (or more accurately firmware). Since the extension unit control IDs are 1-based, Property ID=0 (KSPROPERTY_EXTENSION_UNIT_INFO) is used to return part of the extension unit descriptor, the syntax for which is defined by the UVC specification. The data returned by KSPROPERTY_EXTENSION_UNIT_INFO starts at the GUID and lists the number of controls, number of input pins and the IDs of the units connected as input to the extension unit. The input pin IDs are hardware IDs not DShow Pin IDs. In the above case it seems that bNumControls=1, bNrInPins=1 and baSourceID[0]=3. Now, the error value of 0x80070490 (ERROR_NOT_FOUND) would normally mean that the hardware doesn't support any control request with control ID=1 which seems odd given that bNumControls = 1. Is it possible there is a firmware bug here ? It is also odd that KSPROPERTY_EXTENSION_UNIT_INFO returns six bytes after the GUID whereas, given that bNrInPins=1, one would expect that there are only three. The latter fact suggests that the Length field of the extension unit descriptor may be incorrect.

AnandGanesh at 2007-10-8 >

The data for KSPROPERTY_EXTENSION_UNIT_INFO (PropertyId = 0) is copied over from the device's extension unit descriptor which is cached during device startup. During startup, the driver issues a number of control requests to the device like GET_INFO, GET_LEN, GET_MIN and GET_MAX. The value returned by GET_INFO tells the driver which of GET and SET requests are valid for a given control, whether the control is asynchronous, etc. If any of these initial requests fail, the driver disables the particular control. This may explain why the driver returns ERROR_NOT_FOUND on a subsequent property request. You could monitor extension unit traffic during startup to debug this problem.

AnandGanesh at 2007-10-8 >