23.2.3 Driver Binding Protocol Implementation

Like all drivers that follow the UEFI driver model, the image entry point of a graphics driver installs the Driver Binding Protocol instance on the image handle. The driver must implement all three of the services in the Driver Binding Protocol-Supported(), Start(), and Stop().
The Supported() function tests to see whether the given handle is a manageable adapter. The driver should check that EFI_DEVICE_PATH_PROTOCOL and EFI_PCI_IO_PROTOCOL are present to ensure the handle that is passed in represents a PCI device. Then the driver should verify that the device is conformant and manageable by reading the ClassCode, VendorId, and DeviceId from the device's PCI configuration header.
The Start() function tells the Graphics driver to start managing the controller. In this function, a Graphics driver should use chip-specific knowledge to do the following:
  1. 1.
    Initialize the adapter.
  2. 2.
    Enable the PCI device.
  3. 3.
    Allocate resources.
  4. 4.
    Construct data structures for the driver to use (if required by the device).
  5. 5.
    Enumerate the outputs that are enabled on the device.
  6. 6.
    Create child handles for each detected (and enabled) physical output (physical child handles) and install EFI_DEVICE_PATH_PROTOCOL.
  7. 7.
    Get EDID information from each physical output device connected and install EFI_EDID_DISCOVERED_PROTOCOL on the child handle.
  8. 8.
    Create child handles for each valid combination of 2 or more video outputs (logical child handles) and install EFI_DEVICE_PATH_PROTOCOL.
  9. 9.
    Check RemainingDevicePath to see if the correct child or children were created or if NULL select a default set. If incorrect children (no defaults) clean up memory and return EFI_UNSUPPORTED. If default or correct children set them active.
  10. 10.
    Call GetEdid() function to check for overrides on each active physical child handle and produce EFI_EDID_ACTIVE_PROTOCOL on each child protocol based on the result.
  11. 11.
    Install EFI_GRAPHICS_OUTPUT_PROTOCOL on each active child handle (physical or logical).
  12. 12.
  13. 13.
    In order to support faster boot times, a default mode set and clear screen operation must not be performed in the Start() function. This allows the UEFI Boot Manager to select the best mode for the current boot scenario and set the mode one time.
The Start() function should not scan for devices every time the driver is started. It should depend on RemainingDevicePath parameter to determine what to start. Only if NULL was passed in should the driver should create a device handle for each device that was found in the scan behind the controller. Otherwise the driver should only start what was specified in RemainingDevicePath.
The Stop() function performs the opposite operations as Start(). Generally speaking, a Graphics driver is required to do the following:
  1. 1.
    Uninstall all protocols on all child handles and close all the child handles.
  2. 2.
    Uninstall all protocols that were attached on the host controller handle.
  3. 3.
    Close all protocol instances that were opened in the Start() function.
  4. 4.
    Release all resources that were allocated for this driver.
  5. 5.
    Disable the adapter.
In general, if it is possible to support RemainingDevicePath, the driver should do so to support the rapid boot capability in the UEFI driver model.