Cancel-efi-console-redirect.c

#define EFI_GLOBAL_VARIABLE_GUID \
  { 0x8B843E20, 0x8110, 0x4318, { 0xCC, 0x69, 0x87, 0xCE, 0x47, 0x46, 0x61, 0x03 } }

// Standard UEFI Terminal Type GUIDs used by BIOS redirection
static EFI_GUID gEfiPcAnsiGuid    = { 0xE0C1ECE1, 0x5544, 0x11D4, { 0x9A, 0x39, 0x00, 0x90, 0x27, 0x3F, 0xC1, 0x4D } };
static EFI_GUID gEfiVt100Guid     = { 0xDFA666B1, 0x1E59, 0x11D4, { 0x9A, 0x38, 0x00, 0x90, 0x27, 0x3F, 0xC1, 0x4D } };
static EFI_GUID gEfiVt100PlusGuid = { 0x7BA0BCC0, 0x1B58, 0x4114, { 0xA7, 0xA9, 0x54, 0x90, 0xA3, 0x84, 0x2B, 0x8B } };
static EFI_GUID gEfiVtUtf8Guid    = { 0xAD15A0D6, 0x8BEC, 0x4B4A, { 0xBA, 0x20, 0x00, 0x80, 0xC7, 0x7E, 0x59, 0x37 } };

EFI_STATUS FilterConsoleVariable(CHAR16 *VarName) {
    EFI_GUID GlobalGuid = EFI_GLOBAL_VARIABLE_GUID;
    UINTN Size = 0;
    EFI_DEVICE_PATH_PROTOCOL *DevicePathList = NULL;
    EFI_STATUS Status;

    // Determine required buffer size
    Status = gRT->GetVariable(VarName, &GlobalGuid, NULL, &Size, NULL);
    if (Status == EFI_BUFFER_TOO_SMALL) {
        DevicePathList = AllocatePool(Size);
        Status = gRT->GetVariable(VarName, &GlobalGuid, NULL, &Size, DevicePathList);
    }
    
    if (EFI_ERROR(Status) || !DevicePathList) return Status;
    
    // Proceed to parse and strip nodes...
}
    EFI_DEVICE_PATH_PROTOCOL *CurrentPath = DevicePathList;
    EFI_DEVICE_PATH_PROTOCOL *NewPathList = NULL;

    while (!IsDevicePathEnd(CurrentPath)) {
        BOOLEAN IsSerialRedirection = FALSE;
        EFI_DEVICE_PATH_PROTOCOL *Node = CurrentPath;

        // Inspect nodes within the current instance channel
        while (!IsDevicePathEndType(Node)) {
            if (Node->Type == MESSAGING_DEVICE_PATH) {
                if (Node->SubType == MSG_UART_DP) {
                    IsSerialRedirection = TRUE;
                } 
                else if (Node->SubType == MSG_VENDOR_DP) {
                    VENDOR_DEVICE_PATH *VendorNode = (VENDOR_DEVICE_PATH *)Node;
                    if (CompareGuid(&VendorNode->Guid, &gEfiPcAnsiGuid) ||
                        CompareGuid(&VendorNode->Guid, &gEfiVt100Guid)  ||
                        CompareGuid(&VendorNode->Guid, &gEfiVt100PlusGuid) ||
                        CompareGuid(&VendorNode->Guid, &gEfiVtUtf8Guid)) {
                        IsSerialRedirection = TRUE;
                    }
                }
            }
            Node = NextDevicePathNode(Node);
        }

        // If it's a clean graphic/keyboard node, append it to our new safe path list
        if (!IsSerialRedirection) {
            NewPathList = AppendDevicePathInstance(NewPathList, CurrentPath);
        }

        CurrentPath = NextDevicePathInstance(CurrentPath);
    }

    // Update NVRAM
    UINTN NewSize = GetDevicePathSize(NewPathList);
    Status = gRT->SetVariable(VarName, &GlobalGuid, 
                              EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS | EFI_VARIABLE_NON_VOLATILE, 
                              NewSize, NewPathList);

    // Re-evaluate the active interfaces 
    if (!EFI_ERROR(Status)) {
        // Disconnect and reconnect system console handles to apply modifications
        gBS->DisconnectController(gST->ConsoleOutHandle, NULL, NULL);
        gBS->ConnectController(gST->ConsoleOutHandle, NULL, NULL, TRUE);
    }

    FreePool(DevicePathList);
    if (NewPathList) FreePool(NewPathList);
    return Status;

To detect if the BIOS is performing serial console redirection and to control it from the FreeBSD bootloader, you must interact with the UEFI ⁠EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL⁠ and the ⁠EFI_CONSOLE_CONTROL_PROTOCOL⁠ (where supported).

ConOut

MSG_UART_DP

Goal: if in comconsole both aerial and efi are set at the same time, check if bios is redirecting efi to serial itself, if it is interrupt it at this point and let the bootloader write to efi and serial for itself.

Question: why does the bios know when the boot loader has started the kernel, maybe tje bios just needs to reinitialize the serial port to take ownership of it and this abort console redirection by the bios?