6.1.6 Device driver protocol management

Device drivers consume one or more I/O-related protocols and use the services of those protocols to produce one or more I/O-related protocols. The Supported() and Start() functions of the Driver Binding Protocol are responsible for opening the I/Orelated protocols being consumed using the EFI Boot Service OpenProtocol(). The Stop() function is responsible for closing the consumed I/O-related protocols using CloseProtocol().

A protocol can be opened in several different modes, but the most common is EFI_OPEN_PROTOCOL_BY_DRIVER. When a protocol is opened by EFI_OPEN_PROTOCOL_BY_DRIVER, a test is made to see if that protocol is already being consumed by any other drivers. The open operation succeeds only if the protocol is not being consumed by any other drivers.

Caution: Using the OpenProtocol() service with EFIOPEN_PROTOCOL_BY_DRIVER is how resource conflicts are avoided in the UEFI driver model. However, it requires that every driver present in the system follow the driver interoperability rules for all resource conflicts to be avoided._

The following figure shows the image handle for a device driver as LoadImage() and StartImage() are called. In addition, it shows the states of three different controller handles as the Driver Binding Protocol services Supported(), Start(), and Stop() are called. Controller Handle 1 and Controller Handle 3 pass the Supported() test, so the Start() function can be called. In this case, the Supported() service tests to see if the controller handle supports Protocol A. Start() is then called for Controller Handle 1 and Controller Handle 3. In the Start() function, Protocol A is opened EFI_OPEN_PROTOCOL_BY_DRIVER, and Protocol B is installed onto the same controller handle. The implementation of Protocol B uses the services of Protocol A to produce the services of Protocol B.

All drivers that follow the UEFI driver model must support the Stop() service. The Stop() service must put the handles back into their previous state, before Start() was called, so the Stop() service uninstalls Protocol B and closes Protocol A.

Figure 12-Device driver protocol management

The figure below shows a more complex device driver that requires Protocol A and Protocol B to produce Protocol C. Notice that the controller handles that do not support either Protocol A or Protocol B do not pass the Supported() test. In addition, controller handles that only support Protocol A or only Protocol B also do not pass the Supported() test. Finally, note that Controller Handle 6 already has Protocol A opened by EFI_OPEN_PROTOCOL_BY_DRIVER, so this device driver requiring both Protocol A and Protocol B also does not pass the Supported() test.

This example highlights some of the flexibility of the UEFI driver model. Because the Supported() and Start() services are functions, a driver writer can implement simple or complex algorithms to test driver support for a specific controller handle.

Figure 13-Complex device driver protocol management

TIP: The best way to design the algorithm for the opening protocols is to write a Boolean expression for the protocols that a device driver consumes. Then, expand this Boolean expression into the sum of products form. Each product in the expanded expression requires its own Driver Binding Protocol.

This scenario is another way that a device driver may be required to produce multiple instances of the Driver Binding Protocol. The Supported() service for each Driver Binding Protocol attempts to open each protocol in a product term. If any of those open operations fail, then Supported() fails. If all the opens succeed, then the Supported() test passes. The Start() function should open each protocol in the product term, and the Stop() function should close each protocol in the product term.

For example, the two examples above would have the following Boolean expressions:

  • (Protocol A)

  • (Protocol A AND Protocol B)

These two expressions have only one product term, so only one EFI_DRIVER_BINDING_PROTOCOL is required. A more complex expression would be as follows:

  • (Protocol A AND (Protocol B OR Protocol C))

If this Boolean expression is expanded into a sum of product form, it would yield the following:

  • ((Protocol A AND Protocol C) OR (Protocol B AND Protocol C))

This expression would require a driver with two instances of the EFI_DRIVER_BINDING_PROTOCOL. One would test for Protocol A and Protocol C, and the other would test for Protocol B and Protocol C