Some audio devices provide hardware volume-control knobs, mute switches, or other types of manual controls. Applications can respond to changes in these controls by adjusting the volume or otherwise changing the way that the audio stream is played. When the user adjusts a hardware control, the miniport driver uses the IPortEvents interface to inform the port driver that a hardware event has occurred. The port driver, in turn, notifies the application of the event so that it can read the new control setting from the device.
Your miniport driver can query the port driver for the IPortEvents interface at the time that it services the Init call (see IMiniportWavePci::Init, for example) from the port driver. On Microsoft Windows 98 SE, Windows Me, and Windows 2000 and later, that query succeeds. For a code example, see the Sb16 sample audio adapter in the Windows Driver Kit (WDK).
When the port driver calls your driver's IMiniport::GetDescription method, the method outputs a PCFILTER_DESCRIPTOR structure that specifies, among other things, the events that your device supports. Events can be specified in the automation tables for the Pins and Nodes members of PCFILTER_DESCRIPTOR, and in the AutomationTable member, which points to the automation table for the filter itself. Each event is specified by a PCEVENT_ITEM structure. Your driver should set the PCEVENT_ITEM structure's Set and Id members to KSEVENTSETID_AudioControlChange and KSEVENT_CONTROL_CHANGE, and should load a pointer to your driver's EventHandler routine into the Handler member. Your driver should also set the PCEVENT_ITEM_FLAG_BASICSUPPORT bit in the Flags member to indicate basic support for control-change events, and it should set the PCEVENT_ITEM_FLAG_ONESHOT and/or PCEVENT_ITEM_FLAG_ENABLE bits to indicate that it supports one-shot and/or recurring notification.
When an application later calls the mixerOpen function (described in the Microsoft Windows SDK documentation) to request notification of a particular event, the port driver then calls your driver's EventHandler routine with a pointer to a PCEVENT_REQUEST structure. This structure's Verb member is set to PCEVENT_VERB_ADD and its EventItem member specifies the event that is to be enabled. The PCEVENT_REQUEST structure also contains a pointer to a KSEVENT_ENTRY structure that your driver should treat as opaque system data. After enabling the event, your handler should call IPortEvents::AddEventToEventList with the same KSEVENT_ENTRY pointer. With this call, the handler acknowledges that the event is enabled.
When the hardware event occurs and your driver's interrupt-service routine detects a mute or a volume change, your driver signals the event to the port driver by calling IPortEvents::GenerateEventList with a set of parameters that describe the event. For example, the following call describes a control change in a lineout-volume node:
pPE->GenerateEventList(NULL, KSEVENT_CONTROL_CHANGE, FALSE, ULONG(-1), TRUE, LINEOUT_VOL);
During this call, the port driver searches its event list for all events that match the call parameters and sends notification to the clients that are monitoring these events. In this example, pPE is a pointer to the IPortEvents object, and LINEOUT_VOL is the node ID that the miniport driver assigns to the lineout-volume node. Unspecified parameters (such as the event-set GUID and pin ID in the preceding example) are treated as wildcards and always match the corresponding parameters in the list.