Filter drivers can process OID requests that are originated by overlying drivers. NDIS calls the FilterOidRequest function to process each OID request. Filter drivers can forward OID requests to underlying drivers by calling the NdisFOidRequest function.
NDIS can call a filter driver's FilterCancelOidRequest function to cancel an OID request. When NDIS calls FilterCancelOidRequest, the filter driver should try to call the NdisFOidRequest function as soon as possible.
The following figure illustrates a filtered OID request.
The filter driver can complete the OID request synchronously or asynchronously by returning NDIS_STATUS_SUCCESS or NDIS_STATUS_PENDING, respectively, from FilterOidRequest. FilterOidRequest can also complete synchronously with an error status.
A filter driver that successfully handles an OID set request must set the SupportedRevision member in the NDIS_OID_REQUEST structure upon return from the OID set request. The SupportedRevision member notifies the initiator of the OID request about which revision the driver supported. For more information about version information in NDIS structures, see Specifying NDIS Version Information.
If FilterOidRequest returns NDIS_STATUS_PENDING, it must call the NdisFOidRequestComplete function after it completes the OID request. In this case, the driver passes the results of the request at the OidRequest parameter of NdisFOidRequestComplete. The driver passes the final status of the request at the Status parameter of NdisFOidRequestComplete.
If FilterOidRequest returns NDIS_STATUS_SUCCESS, it returns the results of a query request in the NDIS_OID_REQUEST structure at the OidRequest parameter. In this case, the driver does not call the NdisFOidRequestComplete function.
To forward an OID request to underlying drivers, a filter driver calls the NdisFOidRequest function. If a request should not be forwarded to the underlying drivers, a filter driver can complete the request immediately. To complete the request without forwarding, the driver can return NDIS_STATUS_SUCCESS (or an error status) from FilterOidRequest, or it can call NdisFOidRequestComplete after returning NDIS_STATUS_PENDING.
The forwarded request proceeds the same as a request initiated by a filter driver. For more information, see Generating OID Requests from an NDIS Filter Driver.
After the underlying drivers complete a forwarded request, the filter driver can modify the response, if necessary, and pass it to overlying drivers.
A filter driver can receive OID requests from overlying drivers when it is in the Restarting, Running, Pausing, or Paused state.
Note Like miniport drivers, filter drivers can receive only one OID request at a time. Because NDIS serializes requests that are sent to a filter module, a filter driver cannot be called at FilterOidRequest before it completes the previous request.
The following is an example of a filter driver modifying an OID request:
- A filter driver adds a header. In this case, after the driver receives a response to a query for OID_GEN_MAXIMUM_FRAME_SIZE from the underlying drivers, the filter subtracts the size of its header from the response. The driver subtracts its header size because the driver inserts a header in front of each sent packet and removes the header in each received packet.