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