[gnu-efi]: How to call InitializeGlobalIoDevice funtion when I wanna call CMOS Read/Write? - uefi

Question

Below is the code to initializing the IO device, and I wanna to call a function to do CMOS read/write, but I don't know the DevicePath and Protocol? Does anyone know this? thanks a lot;
/*
Routine Description:
Check to see if DevicePath exists for a given Protocol. Return Error if it
exists. Return GlobalIoFuncs set match the DevicePath
Arguments:
DevicePath - to operate on
Protocol - to check the DevicePath against
ErrorStr - ASCII string to display on error
GlobalIoFncs - Returned with DeviceIoProtocol for the DevicePath
Returns:
Pass or Fail based on wether GlobalIoFncs where found
*/
EFI_STATUS
InitializeGlobalIoDevice (
IN EFI_DEVICE_PATH *DevicePath,
IN EFI_GUID *Protocol,
IN CHAR8 *ErrorStr EFI_UNUSED,
OUT EFI_DEVICE_IO_INTERFACE **GlobalIoFncs
)
{
EFI_STATUS Status;
EFI_HANDLE Handle;
//
// Check to see if this device path already has Protocol on it.
// if so we are loading recursivly and should exit with an error
//
Status = uefi_call_wrapper(BS->LocateDevicePath, 3, Protocol, &DevicePath, &Handle);
if (!EFI_ERROR(Status)) {
DEBUG ((D_INIT, "Device Already Loaded for %a device\n", ErrorStr));
return EFI_LOAD_ERROR;
}
Status = uefi_call_wrapper(BS->LocateDevicePath, 3, &DeviceIoProtocol, &DevicePath, &Handle);
if (!EFI_ERROR(Status)) {
Status = uefi_call_wrapper(BS->HandleProtocol, 3, Handle, &DeviceIoProtocol, (VOID*)GlobalIoFncs);
}
ASSERT (!EFI_ERROR(Status));
return Status;
}
Need your help....

Answer 1

In UEFI spec, there is no Protocol or Device Path defined for CMOS.
If you'd like to read/write the CMOS RTC, there are GetTime() and SetTime() APIs defined in Runtime Services. They can be directly called from RT in gnu-efi.
If you want to read/write other CMOS registers, simply access I/O port 0x70/0x71 with inb/outb instruction with inline assembly in C. That's also how EDK2 accesses CMOS.
https://github.com/tianocore/edk2/blob/master/OvmfPkg/Library/PlatformInitLib/Cmos.c
https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseIoLibIntrinsic/IoLib.c