Medium GUIDs

Minidrivers must be able to support multiple devices. In addition, because the TV/radio tuner, TV audio, crossbar, and video capture components are separated into different kernel streaming filters, a method is necessary to correctly describe the topological hardware connections between these components on the device, as well as on multiple devices. For example, two built but not running filter graphs that support FM radio and TV capture must be able to coexist. Minidrivers use mediums to address these scenarios. Also, filter graph building applications, such as Graph Edit, use mediums during filter graph construction to ensure that filters for one device connect correctly to filters of another device. For example, the tuner filter of one device should not connect to the crossbar filter of another device.

A minidriver describes mediums with the KSPIN_MEDIUM structure that consists of a GUID data type member (Set) followed by two ULONG members (Id and Flags):

  • The Set member should be assigned the GUID representing the topological hardware connection.

  • The minidriver must set the Id member to a unique value for the device instance.

  • The Flags member is reserved for system use and should be set to zero.

To ensure proper construction of filter graphs with multiple devices present in the system, the Set member of the KSPIN_MEDIUM structure remains the same for each device instance. However, the minidriver must assign a unique value to the Id member of the KSPIN_MEDIUM structure per device instance. Failure to set the Id member to a unique value causes problems when multiple devices are present in the system. If two devices are installed in the system, then the minidriver must set the Id member to a different value for each device instance. Note that the Id member for filters of devices that reside on the same piece of hardware, such as tuners and crossbars, must be the same. One technique to ensure that the Id member differs between device instances is to have a global counter in the minidriver and increment that counter during device Plug and Play start time, before setting Id to its value.

Depending on the kernel streaming interface that the minidriver follows (AVStream or the Stream class), the minidriver must specify the value of the Id member differently:

  • Stream class minidrivers specify the value when processing SRB_GET_STREAM_INFO.

  • AVStream minidrivers specify the value in the KSPIN_DESCRIPTOR_EX structure. There are two separate ways for AVStream minidrivers to specify the value in the KSPIN_DESCRIPTOR_EX structure:

    1. Provide static descriptors with a global Id counter and call _KsEdit during Add or Start dispatch handlers to change the Id member to a unique value.
    2. Call KsCreateFilterFactory to dynamically build the filter/pin descriptors during Add or Start dispatch handlers.

Minidrivers must also call a special function to register themselves with Microsoft DirectShow to allow applications to automatically construct filter graphs with TV/radio tuner, TV audio, and crossbar filters because they do not actually create kernel-streaming pins for their inputs and outputs. When the minidriver registers these filters, it should set the Id member of the KSPIN_MEDIUM structure to a unique value. If the minidriver does not set the Id member of the KSPIN_MEDIUM structure to a unique value, then automatic graph building applications can fail to load the necessary adjacent filters. However, manual filter-graph building, in Graph Edit for example, may still work.

To register a minidriver with DirectShow:

The KSPIN_MEDIUM structure returned from either SRB_GET_STREAM_INFO (for a Stream class minidriver), or KSPIN_DESCRIPTOR_EX (for an AVStream minidriver) must match the KSPIN_MEDIUM member returned in the following properties:

In addition to implementing mediums and medium GUIDs correctly, it is necessary to follow other guidelines to ensure processes can work with multiple filter graphs. The minidriver must not lock any hardware resources until the filter graph transitions to the KSSTATE_ACQUIRE value of KSSTATE. This helps to ensure that two built, but not running filter graphs can coexist without interfering with one another.

For more information about mediums, including how to implement them, see the AVStream Simulated Hardware Sample Driver (AVSHwS) in the MSDN Code Gallery.

Note : When deriving a new minidriver from sample code in the Windows Driver Kit, you must generate new GUID values for the mediums to reflect the unique hardware topology of the device. Failure to do so can result in the mediums for one device colliding with the mediums that are defined for another device.

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