EmbeddedPkg : Add FdtPlatformDxe driver
[mirror_edk2.git] / EmbeddedPkg / Drivers / FdtPlatformDxe / README.txt
1 /** @file
2
3 Copyright (c) 2015, ARM Ltd. All rights reserved.<BR>
4
5 This program and the accompanying materials
6 are licensed and made available under the terms and conditions of the BSD License
7 which accompanies this distribution. The full text of the license may be found at
8 http://opensource.org/licenses/bsd-license.php
9
10 THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
11 WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
12
13 **/
14
15 The purpose of the FdtPlatformDxe UEFI driver is to install the Flat Device
16 Tree (FDT) of the platform the UEFI frimware is running on into the UEFI
17 Configuration Table. The FDT is identified within the UEFI Configuration
18 Table by the "gFdtTableGuid" GUID defined in EmbeddedPkg.dec.
19
20 Once installed, an UEFI application or OS boot loader can get from the UEFI
21 Configuration Table the FDT of the platform from the "gFdtTableGuid" GUID.
22
23 The installation is done after each boot at the end of the DXE phase,
24 just before the BDS phase. It is done at the end of the DXE phase to be sure
25 that all drivers have been dispatched. That way, all UEFI protocols that may
26 be needed to retrieve the FDT can be made available. It is done before the BDS
27 phase to be able to provide the FDT during that phase.
28
29 The present driver tries to retrieve the FDT from the device paths defined in the
30 "gEmbeddedTokenSpaceGuid.PcdFdtDevicePaths" PCD. The "PcdFdtDevicePaths" PCD
31 contains a list a device paths. The device paths are in the text form and
32 separated by semi-colons. The present driver tries the device paths in the order
33 it finds them in the "PcdFdtDevicePaths" PCD as long as he did not install
34 succesfully a FDT.
35
36 The "PcdFdtDevicePaths" PCD is a dynamic PCD that can be modified during the
37 DXE phase. This allows for exemple to select the right FDT when a binary is
38 intended to run on several platforms and/or variants of a platform.
39
40 If the driver manages to download a FDT from one of the device paths mentioned
41 above then it installs it in the UEFI Configuration table and the run over the
42 device paths is stopped.
43
44 For development purposes only, if the feature PCD "gEmbeddedTokenSpaceGuid.
45 PcdOverridePlatformFdt" is equal to TRUE, then before to try to install the
46 FDT from the device paths listed in the "PcdFdtDevicePaths" PCD, the present
47 driver tries to install it using the device path defined by the UEFI variable
48 "Fdt". If the variable does not exist or the installation using the device path
49 defined by the UEFI variable fails then the installation proceeds as described
50 above.