23.2.4 QueryMode(), SetMode(), and Blt() Implementation

There are three functions that make up one method: QueryMode(), SetMode(), and Blt(). The mode pointer is pointing to a structure that has members so that the consumer of the GOP protocol can get information about the current state.

The QueryMode() function is used to return extended information on one of the supported video modes. For example, the protocol consumer could iterate through all of the valid video modes and see what they offer in terms of resolution, color depth, etc. This function has no effect on the hardware or the currently displayed image. It is critical that QueryMode() only return modes that can actually be displayed on the attached display device. This means that the UEFI Driver must evaluate the modes that that graphics controller supports and the modes that the attached display supports and only reports the intersection of those two sets. Otherwise, a consumer of the Graphics Output Protocol may attempt to set a mode that cannot be displayed.

The SetMode() function is how the consumer of the Graphics Output Protocol selected the specific mode to become active. SetMode() is also required to clear the entire display output and reset it all to black.

The Blt() function is for transferring information (Block Transfer) to and from the video frame buffer. This is how graphics content is moved to and from the video frame buffer and also allows graphics content to be moved from one location of the video frame buffer to another location of the video frame buffer. The prototype of the Blt() function is shown below.

Example 232-Graphics Output Protocol Blt() Service

Blt a rectangle of pixels on the graphics screen. Blt stands for BLock Transfer.
@param This Protocol instance pointer.
@param BltBuffer Buffer containing data to blit into video buffer. This buffer
has a size of Width * Height * sizeof(EFI_GRAPHICS_OUTPUT_BLT_PIXEL)
@param BltOperation Operation to perform on BlitBuffer and video memory
@param SourceX X coordinate of source for the BltBuffer.
@param SourceY Y coordinate of source for the BltBuffer.
@param DestinationX X coordinate of destination for the BltBuffer.
@param DestinationY Y coordinate of destination for the BltBuffer.
@param Width Width of rectangle in BltBuffer in pixels.
@param Height Hight of rectangle in BltBuffer in pixels.
@param Delta OPTIONAL
@retval EFI_SUCCESS The Blt operation completed.
@retval EFI_INVALID_PARAMETER BltOperation is not valid.
@retval EFI_DEVICE_ERROR A hardware error occured writting to the video buffer.
IN UINTN DestinationX,
IN UINTN DestinationY,
IN UINTN Height,

In this function the driver must translate the entire Blt operation into the correct commands for the graphics adapter that it is managing. This can by be done by performing PCI memory mapped I/O or port /IO operations or by performing a DMA operation. The exact method is specific to the graphics silicon.

A critical consideration of implementing the Blt() function is to get the highest performance possible for the user. A common problem is that scrolling the screen results in significant lags such that the user experiences a less than optimal perception. This could be caused by the lags that are normally present when reading back from the frame buffer. A possible solution is to have a copy of the current frame buffer in a memory buffer for use in reads.

The screen is defined in terms of pixels and the buffer is formatted as follows. For a given pixel at location X,Y the location in the buffer is Buffer[((Y*<<ScreenWidth>>)+X)]. The screen is described according to the following figure.

Figure 28-Software BLT Buffer

An important optimization to make in graphics drivers is for scrolling. Scrolling is one of the most common operations to occur on a pre-boot graphics adapter due to the common use of text based consoles. A method to scroll the screen can be viewed in EDK II in the GraphicsConsole driver (\MdeModulePkg\Universal\Console\GraphicsConsoleDxe).

The EFI_GRAPHICS_OUTPUT_PROTOCOL_MODE object pointed to by the Mode pointer is populated when the graphics controller is initialized, and must be updated whenever SetMode() is called. The FrameBufferBase member of this object may be used by a UEFI OS Loader or OS Kernel to update the contents of the graphical display after ExitBootServices() is called and the Graphics Output Protocol services are not longer available. A UEFI OS may choose to use this method until an OS driver for the graphics controller can be installed and started.