Querying for Information from the GPU
The Direct3D runtime might require information from the graphics processing unit (GPU) other than an output render target or output vertex buffer. Because the GPU executes in parallel with the CPU, the user-mode display driver should supply functions that expose the asynchronous nature of communication with the GPU efficiently.
The query object is the resource that the runtime and driver use for asynchronous notification. To create a query object, the runtime first calls the driver's CalcPrivateQuerySize function so that the driver can supply the size of the memory region that the driver requires for the query object. The runtime then calls the driver's CreateQuery(D3D10) function to create the query object. In the CalcPrivateQuerySize and CreateQuery(D3D10) calls, the runtime supplies a query-type value from the D3D10DDI_QUERY enumeration in the Query member of the D3D10DDIARG_CREATEQUERY structure that the pCreateQuery parameters point to.
Each query object instance exists in one of three states: building, issued, and signaled. The runtime calls the driver's QueryBegin function to transition the query object to the building state.
The runtime calls the driver's QueryEnd function to transition the query object to the issued state. Transitions to the signaled state occur asynchronously some time later. The runtime calls the driver's QueryGetData function to detect whether the query has transitioned to the signaled state. If the query is in the signaled state, QueryGetData can pass back data that applies to the query in the memory region that the pData parameter points to.
All query objects of the same type are FIFO (that is, first-in, first-out). For example, all query objects of type D3D10DDI_QUERY_EVENT complete in FIFO order based on their issued order. However, query objects of different types can complete or signal in an overlapping order. For example, a query of type D3D10DDI_QUERY_EVENT can complete before a query of type D3D10DDI_QUERY_OCCLUSION, even if the runtime issued the D3D10DDI_QUERY_EVENT query after the runtime issued the D3D10DDI_QUERY_OCCLUSION query.
When the runtime no longer requires the query object, the runtime frees the memory region that the runtime previously allocated for the object and calls the driver's DestroyQuery(D3D10) function to notify the driver that the driver can no longer access this memory region.