Handling Asynchronous Queries
The driver creates resources for a query after it receives a D3DDP2OP_CREATEQUERY operation code along with a D3DHAL_DP2CREATEQUERY structure in the command stream.
The driver starts to process a query after it receives a D3DDP2OP_ISSUEQUERY operation code along with a D3DHAL_DP2ISSUEQUERY structure in the command stream.
If previously submitted queries using the D3DDP2OP_ISSUEQUERY operation completed, the driver sets the size of the response buffer in the dwErrorOffset member of the D3DHAL_DRAWPRIMITIVES2DATA structure and sets the ddrval member of D3DHAL_DRAWPRIMITIVES2DATA to D3D_OK for successful completion. The driver overwrites the command buffer in the incoming command stream with the response buffer in the outgoing stream. The driver sets the D3DHAL_DP2RESPONSE structure's bCommand member to D3DDP2OP_RESPONSEQUERY to indicate that responses to previously issued queries are available in the response buffer. Each D3DHAL_DP2RESPONSEQUERY in the response buffer is followed by the following data related to the query:
- BOOL for D3DQUERYTYPE_EVENT. Before responding with D3DDP2OP_RESPONSEQUERY for an event, the driver must ensure that the graphics processing unit (GPU) is finished processing all D3DHAL_DP2OPERATION operations that are related to the event. That is, the driver only responds after the event's ISSUE_END state occurs. Before the driver sets the event to the signaled state (set to TRUE), the GPU might be required to perform a flush to ensure that the pixels are finished rasterizing, blts are completed, resources are no longer being used, and so on. The driver must always set the event's BOOL value to TRUE when responding.
- DWORD for D3DQUERYTYPE_OCCLUSION. The driver sets this DWORD to the number of pixels for which the z-test passed for all primitives between the begin and end of the query. If the depth buffer is multisampled, the driver determines the number of pixels from the number of samples. However, if the display device is capable of per-multisample z-test accuracy, the conversion to number of pixels should generally be rounded up. An application can then check the occlusion result against 0, to effectively mean "fully occluded." Drivers that convert multisampled quantities to pixel quantities should detect render target multisampling changes and continue to compute the query results appropriately.
- D3DDEVINFO_VCACHE structure for D3DQUERYTYPE_VCACHE.
If the supplied command buffer is too small for the driver to write all the responses, the driver also sends D3DDP2OP_RESPONSECONTINUE in the outgoing stream.
If the runtime determines that the driver's D3dDrawPrimitives2 function succeeded (ddrval member of D3DHAL_DRAWPRIMITIVES2DATA set to D3D_OK), the runtime examines the dwErrorOffset member of D3DHAL_DRAWPRIMITIVES2DATA to determine if responses are available from the driver. This dwErrorOffset member is zero if no responses are available; otherwise, dwErrorOffset is the size of the response buffer in bytes. Therefore, on success of D3dDrawPrimitives2 (ddrval set to D3D_OK), the driver must ensure that it only sets dwErrorOffset to nonzero when responses are available.
The runtime parses the returned response buffer and updates its internal data structures.
If the driver sent D3DDP2OP_RESPONSECONTINUE, the runtime submits an empty command buffer in the incoming command stream so that the driver can continue to write more responses. The driver must ensure that it can process empty command buffers.