/** @file
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Copyright (c) 2015, ARM Ltd. All rights reserved.<BR>
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SPDX-License-Identifier: BSD-2-Clause-Patent
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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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"gFdtPlatformDxeTokenSpaceGuid.PcdFdtDevicePaths" PCD. The "PcdFdtDevicePaths"
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PCD 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 "gFdtPlatformDxeTokenSpaceGuid.
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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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Furthermore and again for development purposes only, if the feature PCD
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"PcdOverridePlatformFdt" is equal to TRUE, the current driver provides the EFI
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Shell command "setfdt" to define the location of the FDT by the mean of an EFI
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Shell file path (like "fs2:\boot\fdt.dtb") or a device path.
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If the path passed in to the command is a valid EFI Shell file path, the
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command translates it into the corresponding device path and stores that
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device path in the "Fdt" UEFI variable asking for the variable to be non
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volatile.
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If the path passed in to the command is not recognised as a valid EFI
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Shell device path, the command handles it as device path and stored
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in the "Fdt" UEFI variable as it is.
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Finally, the "-i" option of the "setfdt" command allows to trigger the FDT
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installation process. The installation process is completed when the command
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returns. The command can be invoked with the "-i" option only and in that
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case the "Fdt" UEFI variable is not updated and the command just runs the
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FDT installation process. If the command is invoked with the "-i" option and
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an EFI Shell file path then first the "Fdt" UEFI variable is updated accordingly
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and then the FDT installation process is run.
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