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Learning about PCI-e: Implementing an option ROM

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In this series, we've been implementing a PCI-e GPU and so far we were able to put some pixels on the (emulated) screen via purpose-built userspace programs.

Now it's time to make the GPU available to the system, and we'll start by making it available to UEFI, which is the system firmware that initializes hardware and loads the operating system. UEFI does not have built-in drivers for our custom GPU, but it allows for PCI devices to bring their own driver packaged in something called an Option ROM.

The format for an option rom is defined by two data structures, a 28-byte-long PCI Option ROM and a 24-byte-long PCI Data Structure ("PCIR"), followed by the driver's executable which must be in PE format:

A 'Hello world' driver

For this post, we are going to implement the UEFI driver with EDK2, to set up the dev environment, we follow the official instructions:

git clone
git submodule update --init --recursive
make -C BaseTools

To build the driver, we need to specify in Conf/target.txt our platform's target:


and following the skeleton example we can create

in OptionRom/Rom.inf

  INF_VERSION    = 0x00010005
  BASE_NAME      = OptionRom
  FILE_GUID      = f1f6026b-aa59-4e68-9fbe-6be92e37a225
  ENTRY_POINT    = OptionRomEntry




in OptionRom/rom.c

#include <Uefi.h>
#include <Library/UefiLib.h>
#include <Library/DebugLib.h>

  IN  EFI_HANDLE        ImageHandle,
  IN  EFI_SYSTEM_TABLE  *SystemTable) {
    DEBUG((EFI_D_INFO, "MyOptionRom loaded\n"));
    return EFI_SUCCESS;

EFI_STATUS EFIAPI MyOptionRomUnload (IN  EFI_HANDLE  ImageHandle) {
    return EFI_SUCCESS;

and build the driver with build -m OptionRom/Rom.inf (yes, they named their build command build), which will generate the driver, as a PE file at ./Build/OvmfX64/DEBUG_GCC5/X64/OptionRom.efi

We can now take the driver and build it into an Option Rom with EfiRom by passing the Vendor ID (0x1337) and the Device ID (0x1234):

./BaseTools/Source/C/bin/EfiRom -f 0x1337 -i 0x1234 -o ./Build/OptionRom.rom -e ./Build/OvmfX64/DEBUG_GCC5/X64/OptionRom.efi

Let's dump the Option Rom metadata to confirm:

./BaseTools/Source/C/bin/EfiRom -d ./Build/OptionRom.rom 
Image 1 -- Offset 0x0
  ROM header contents
    Signature              0xAA55
    PCIR offset            0x001C
    Signature               PCIR
    Vendor ID               0x1234
    Device ID               0x1337
    Length                  0x001C
    Revision                0x0003
    DeviceListOffset        0x00
    Class Code              0x000000
    Image size              0x2400
    Code revision:          0x0000
    MaxRuntimeImageLength   0x00
    ConfigUtilityCodeHeaderOffset 0x00
    DMTFCLPEntryPointOffset 0x00
    Indicator               0x80   (last image)
    Code type               0x03   (EFI image)
  EFI ROM header contents
    EFI Signature          0x0EF1
    Compression Type       0x0000 (not compressed)
    Machine type           0x8664 (X64)
    Subsystem              0x000B (EFI boot service driver)
    EFI image offset       0x0100 (@0x100)

Testing the 'Hello World' driver

Let's put the driver in a FAT filesystem:

truncate -s 32M disk.raw
mkfs.fat disk.raw
mcopy -o -i disk.raw Build/OptionRom.rom ::OptionRom.rom

Now we can load it manually with QEMU:

$ qemu-system-x86_64 -hda disk.raw -bios /usr/share/ovmf/OVMF.fd
UEFI Interactive Shell v2.2
Shell> fs0:
FS0:\> loadpcirom OptionRom.rom
MyOptionRom loaded
Image 'FS0:\OptionRom.rom' load result: Success

This confirms that the toolchain is working, and we can now start the real implementation of the driver, let's jump into how UEFI drivers interact with the system.

Driver model

UEFI drivers are expected1 to follow the driver model which provides a standardized way to initialize hardware during the boot process. These drivers are only really required to implement the Driver Binding protocol, which handles the attachment and detachment of drivers to devices by requiring three functions:

  • Supported(): Check whether the driver supports a particular device, usually by checking Vendor/Device IDs and/or device class. Must undo any changes it applies to the device before returning.
  • Start(): Initialize the device, allocate resources, install protocols, etc. Must store the original state of the device.
  • Stop(): Free all allocated resources & restore the device to its original state.

First, UEFI scans the PCI bus for all present devices, registering each Option ROM as a driver.

Once all Option ROMs are registered, UEFI calls the Supported function on each driver, for every PCI device. When Supported returns true, UEFI proceeds with initializing the device by calling the driver's Start function.

In a diagram, something like this:

The goal of drivers is to install protocols on their respective device's handle. These protocols abstract away the hardware, into higher level operations that the device supports.

A real driver

The goal of a graphics driver is to abstract away the specific PCI commands needed to interact with the device, and provide a more convenient interface, such as a linear framebuffer.

In our case, we want to get a handle to our device via the Pci I/O protocol on top of which we want to install Graphics Output Protocol.

To build a driver that follows the UEFI model, first we want to define the driver's state:

typedef struct {
  // Driver's own handle
  EFI_HANDLE Handle;
  // PCI device

  // Our installed GOP

  // Graphics mode (resolution)

along with the type to hold our Driver Binding definition:

  0x10, // version

As UEFI code is quite verbose, all code samples are abbreviated, find sources here

Then imlpement the Supported() function to determine which PCI devices work with this driver:

EFI_STATUS EFIAPI GpuVideoControllerDriverSupported(...) {
  PCI_TYPE00           Pci;
  // Get a handle for the PCI I/O Protocol
  gBS->OpenProtocol(&gEfiPciIoProtocolGuid, (VOID **)&PciIo, ...);
  // Read the PCI Configuration Header from the PCI Device
  PciIo->Pci.Read(PciIo, EfiPciIoWidthUint32, 0, sizeof (Pci) / sizeof (UINT32), &Pci);

  // Release the PCI I/O Protocol
  gBS->CloseProtocol(&gEfiPciIoProtocolGuid, ...);

  // Validate we are talking with the custom GPU
  if (Pci.Hdr.VendorId == 0x1234 && Pci.Hdr.DeviceId == 0x1337) {
    return EFI_SUCCESS;

and we get to the meat of the driver — configuring the device:

EFI_STATUS EFIAPI GpuVideoControllerDriverStart (...) {
  // Raise task priority to prevent `Start` from being interrupted
  MY_GPU_PRIVATE_DATA *Private = AllocateZeroPool(sizeof(MY_GPU_PRIVATE_DATA));;
  // Get a handle for the PCI I/O Protocol
  gBS->OpenProtocol(&gEfiPciIoProtocolGuid, (VOID **)&Private->PciIo, ...);

  // Read supported attributes
  UINT64 SupportedAttrs;
  Private->PciIo->Attributes(Private->PciIo, EfiPciIoAttributeOperationSupported, 0, &SupportedAttrs);

  // Update attributes
  SupportedAttrs |= EFI_PCI_DEVICE_ENABLE;
  Private->PciIo->Attributes (Private->PciIo, EfiPciIoAttributeOperationEnable, SupportedAttrs, NULL);

  // Install Graphics Output Protocol on this driver
  gBS->InstallMultipleProtocolInterfaces(&Private->Handle, &gEfiGraphicsOutputProtocolGuid, Private->Gop, NULL);

  // Restore the priority of this task
  gBS->RestoreTPL (OldTpl);

Now we only need to connect our Driver to the EFI application's entrypoint:

EFI_STATUS EFIAPI OptionRomEntry(...) {
  EFI_STATUS Status = EfiLibInstallDriverBindingComponentName2 (
  return Status;

at this point we have a driver that registers itself as a Graphics Output (GOP) and does nothing.

Configuring the output adapter

To do something, we need to set up the information for the output adapter (the QEMU device we implemented in part 1) in the GOP structure

  EFI_STATUS Status;
  // Initialize the GOP protocol with our 3 callbacks
  Private->Gop.QueryMode = MyGpuQueryMode;
  Private->Gop.SetMode = MyGpuSetMode;
  Private->Gop.Blt = MyGpuBlt;

  // Fill in the available modes, for now, this is single, static entry
  Private->Info.Version = 0;
  Private->Info.HorizontalResolution = 640; // hardcoded on the adapter
  Private->Info.VerticalResolution = 480;
  Private->Info.PixelFormat = PixelBlueGreenRedReserved8BitPerColor;
  Private->Info.PixelsPerScanLine = Private->Info.HorizontalResolution;

  Private->Gop.Mode = AllocateZeroPool(sizeof(EFI_GRAPHICS_OUTPUT_PROTOCOL_MODE));
  Private->Gop.Mode->MaxMode = 1;
  Private->Gop.Mode->Mode = 0;
  Private->Gop.Mode->Info = &Private->Info;
  Private->Gop.Mode->SizeOfInfo = sizeof(EFI_GRAPHICS_OUTPUT_MODE_INFORMATION);

  UINT32 FbSize = Private->Info.HorizontalResolution * Private->Info.VerticalResolution * sizeof(EFI_GRAPHICS_OUTPUT_BLT_PIXEL);
  Private->Gop.Mode->FrameBufferBase = AllocateZeroPool(FbSize);
  Private->Gop.Mode->FrameBufferSize = FbSize;

and making sure the GOP is set up before we install the protocol

@@ GpuVideoControllerDriverStart (
  Private->PciIo->Attributes (Private->PciIo, EfiPciIoAttributeOperationEnable, SupportedAttrs, NULL);

+ GopSetup(Private)

  // Install Graphics Output Protocol on this driver
  gBS->InstallMultipleProtocolInterfaces(&Private->Handle, &gEfiGraphicsOutputProtocolGuid, Private->Gop, NULL);

With the GOP structure populated, we need to implement the 3 callbacks for the protocol:


Returns a newly-allocated EFI_GRAPHICS_OUTPUT_MODE_INFORMATION** which specifies the supported graphics resolutions, as we only support 1 resolution, we copy it from the driver state

  // Info must be a newly allocated pool
  *Info = AllocateCopyPool (*SizeOfInfo, &Private->Info);
  return EFI_SUCCESS;

the emphasis in newly-allocated is because I spent hours trying to figure out why the return from the QueryMode call was tripping this assertion

ASSERT edk2/MdeModulePkg/Core/Dxe/Mem/Pool.c(721): Head->Signature == ((('p') | ('h' << 8)) | ((('d') | ('0' << 8)) << 16)) ||
                                                   Head->Signature == ((('p') | ('h' << 8)) | ((('d') | ('1' << 8)) << 16))

when not using the AllocatePool family for the output buffer (eg: *Info = &Private->Info); the docs do state:

Info\* is a pointer to a callee allocated buffer that returns information about ModeNumber.

which seems to imply FreePool() will be called on the pointer after the return of the function.


Re-configure the output to one of the supported modes -- but as we don't really support other resolutions, it's a no-op:

    return EFI_SUCCESS;


Blt a rectangle of pixels, through multiple seemingly-unnecessary modes.

Let's start by only supporting full blits, with the most naive implementation: setting the value of each pixel, one at a time, via raw PCI writes

    ...) {
    for(int y=0; y<Private->Info.VerticalResolution;y++){
        for(int x=0; x<Private->Info.HorizontalResolution;x++){
            UINT32 i = (Private->Info.HorizontalResolution * y + x) * 4;
            UINT32 r = ((char*)Private->Gop.Mode->FrameBufferBase)[i+2];
            UINT32 g = ((char*)Private->Gop.Mode->FrameBufferBase)[i+1];
            UINT32 b = ((char*)Private->Gop.Mode->FrameBufferBase)[i+0];
            UINT32 pixval = b | (g << 8) | (r << 16);
            Private->PciIo->Mem.Write (
                    EfiPciIoWidthUint32,  // Width
                    0,                    // BarIndex
                    i,                    // Offset
                    1,                    // Count
                    &pixval               // Value

And now, for the grand reveal (be patient):

which is amazing as it shows the option ROM works with a completely unmodified UEFI.

It also may be a tad slow, so I spent about 45 min researching and implementing DMA transfers, as we did in the last entry, which ended up looking something like this:

@@ MyGpuBlt (
-    for(int y=0; y<Private->Info.VerticalResolution;y++){
-       for(int x=0; x<Private->Info.HorizontalResolution;x++){
-           ...
-       }
-    }
+    CopyBufferDMA()

and it was so much faster


at this point I realized that:

  • GOP is a Boot Service
  • ExitBootServices is a thing

A sidetrack on UEFI services & framebuffers

UEFI provides a bunch of "services", which are just functions. These services are split into two categories: Boot services and Runtime services.

Boot Services are the functions which are only available before the operating system takes control, and these are the functions for interacting with hardware, registering drivers, protocols, allocating memory, etc.

Runtime Services are the functions that remain available after the operating system has taken control, but these are simple management functions to deal with UEFI variables, system time, etc.

The operating system "taking control" is defined as the point in time on which ExitBootServices is called.

So, whenever Linux starts booting, it'll call ExitBootServices(), and our GOP's Blt function will no longer be callable.

There's light at the end of the tunnel though, as the resources which were allocated during Boot Services do not get automatically cleaned up, so, the operating system is free to find the framebuffer's hardware address and keep writing to it (which assumes memory-mapped IO)

If we look at how Gop->Mode is defined:

typedef struct {
    UINT32 MaxMode;
    UINT32 Mode;
    UINTN SizeOfInfo;
    UINTN FrameBufferSize;

we can see that there's a physical address (FrameBufferBase) and len (FrameBufferSize), if we can map our PCI device's framebuffer memory here, we no longer need to call Blt (and we lose DMA, so it'll be less efficient)

Memory mapped framebuffer

On the PCI device which we implemented in QEMU, we can now map the framebuffer (GpuState->framebuffer2) as a new memory region, and assign it BAR#1

@@ pci_gpu_realize(
+    memory_region_init_ram_ptr(&gpu->fbmem, OBJECT(gpu), "gpu-fb-mem", 16 * MiB, (void*)ptr);
+    pci_register_bar(pdev, 1, PCI_BASE_ADDRESS_SPACE_MEMORY, &gpu->fbmem);

Now the UEFI driver can map GOP's framebuffer to the address referenced by BAR#1

@@ GpuVideoControllerDriverStart (
   Private->PciIo->Attributes (Private->PciIo, EfiPciIoAttributeOperationEnable, SupportedAttrs, NULL);

+  UINTN FbBarIdx = 1;
+  Private->PciIo->GetBarAttributes(Private->PciIo, FbBarIdx, NULL, (VOID **)&Resources);
+  Private->PciFbMemBase = Resources->AddrRangeMin;


and the only thing that's left is to render into the buffer directly, without using DMA or single pixel writes — conveniently EDK2 provides the function FrameBufferBlt, which is succintly documented to "Perform a UEFI Graphics Output Protocol Blt operation".

@@ MyGpuBlt (
-  CopyBufferDMA()
+  // This Configure points to PciFbMemBase
+  // so FrameBufferBlt is writing to the PCI FB directly,
+  // which is what efifb will do later
+  FrameBufferBlt(Private->FrameBufferBltConfigure, ...)

with that, we should be done!


This is an UEFI system, booting an unmodified kernel (no custom drivers), with the PCI option rom:

and even Windows

A detour into a framebuffer driver

The above video shows that any system booting with efifb or able to use the efi framebuffer will be able to use the adapter, but what about other systems? Those may either be systems which were booted via BIOS, but also systems that are not using EFISTUB to boot.

In these cases, it'd still be nice to be able to use the adapter after the system has booted — luckily, the kernel has a framebuffer API, which allows implementing custom framebuffer drivers very easily

We don't need to do almost anything, as there are existing functions for the actual blitting (cfb_fillrect, cfb_copyarea, cfb_imageblit) -- the main thing that we need to do is populating a struct fb_info with the physical and virtual addresses for the PCI-e BAR address, but we already had those:

    // virtual
    gpu->info->screen_base = gpu->fbmem;
    // physical
    gpu->info->fix.smem_start = gpu->fb_phys;

then, calling register_framebuffer with this struct fb_info.

With that adjustment to the driver, now we can also boot a BIOS system, with the driver module

That's it for now, next time, we go physical.


  1. UEFI Specification
  2. Uefi Driver Writer's Guide
  3. Using an Option ROM to overwrite SMM/SMI handlers in QEMU
  4. UEFI Option ROM Bootkit
  5. UEFI Driver Development Guide for Graphics Controller Device Classe

  1. but you can also completely ignore the driver model and just install a protocol during your entrypoint ¯\_(ツ)_/¯ 

  2. the framebuffer should be 4k aligned, as a QEMU requirement