EmbeddedPkg : Add FdtPlatformDxe driver

[mirror_edk2.git] / EmbeddedPkg / Drivers / FdtPlatformDxe / README.txt

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+/** @file
+
+  Copyright (c) 2015, ARM Ltd. All rights reserved.<BR>
+
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD License
+  which accompanies this distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+
+The purpose of the FdtPlatformDxe UEFI driver is to install the Flat Device
+Tree (FDT) of the platform the UEFI frimware is running on into the UEFI
+Configuration Table. The FDT is identified within the UEFI Configuration
+Table by the "gFdtTableGuid" GUID defined in EmbeddedPkg.dec.
+
+Once installed, an UEFI application or OS boot loader can get from the UEFI
+Configuration Table the FDT of the platform from the "gFdtTableGuid" GUID.
+
+The installation is done after each boot at the end of the DXE phase,
+just before the BDS phase. It is done at the end of the DXE phase to be sure
+that all drivers have been dispatched. That way, all UEFI protocols that may
+be needed to retrieve the FDT can be made available. It is done before the BDS
+phase to be able to provide the FDT during that phase.
+
+The present driver tries to retrieve the FDT from the device paths defined in the
+"gEmbeddedTokenSpaceGuid.PcdFdtDevicePaths" PCD. The "PcdFdtDevicePaths" PCD
+contains a list a device paths. The device paths are in the text form and
+separated by semi-colons. The present driver tries the device paths in the order
+it finds them in the "PcdFdtDevicePaths" PCD as long as he did not install
+succesfully a FDT.
+
+The "PcdFdtDevicePaths" PCD is a dynamic PCD that can be modified during the
+DXE phase. This allows for exemple to select the right FDT when a binary is
+intended to run on several platforms and/or variants of a platform.
+
+If the driver manages to download a FDT from one of the device paths mentioned
+above then it installs it in the UEFI Configuration table and the run over the
+device paths is stopped.
+
+For development purposes only, if the feature PCD "gEmbeddedTokenSpaceGuid.
+PcdOverridePlatformFdt" is equal to TRUE, then before to try to install the
+FDT from the device paths listed in the "PcdFdtDevicePaths" PCD, the present
+driver tries to install it using the device path defined by the UEFI variable
+"Fdt". If the variable does not exist or the installation using the device path
+defined by the UEFI variable fails then the installation proceeds as described
+above.