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+<!--
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+ Copyright (c) 2018 Heinrich Schuchardt
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+
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+ SPDX-License-Identifier: GPL-2.0+
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+-->
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+
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+# UEFI on U-Boot
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+
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+The Unified Extensible Firmware Interface Specification (UEFI) [1] has become
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+the default for booting on AArch64 and x86 systems. It provides a stable API for
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+the interaction of drivers and applications with the firmware. The API comprises
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+access to block storage, network, and console to name a few. The Linux kernel
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+and boot loaders like GRUB or the FreeBSD loader can be executed.
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+
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+## Building for UEFI
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+
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+The UEFI standard supports only little endian systems. The UEFI support can be
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+activated for ARM and x86 by specifying
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+
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+ CONFIG_CMD_BOOTEFI=y
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+ CONFIG_EFI_LOADER=y
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+
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+in the .config file.
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+
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+Support for attaching virtual block devices, e.g. iSCSI drives connected by the
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+loaded UEFI application [3], requires
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+
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+ CONFIG_BLK=y
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+ CONFIG_PARTITIONS=y
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+
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+### Executing a UEFI binary
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+
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+The bootefi command is used to start UEFI applications or to install UEFI
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+drivers. It takes two parameters
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+
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+ bootefi <image address> [fdt address]
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+
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+* image address - the memory address of the UEFI binary
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+* fdt address - the memory address of the flattened device tree
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+
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+Below you find the output of an example session starting GRUB.
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+
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+ => load mmc 0:2 ${fdt_addr_r} boot/dtb
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+ 29830 bytes read in 14 ms (2 MiB/s)
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+ => load mmc 0:1 ${kernel_addr_r} efi/debian/grubaa64.efi
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+ reading efi/debian/grubaa64.efi
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+ 120832 bytes read in 7 ms (16.5 MiB/s)
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+ => bootefi ${kernel_addr_r} ${fdt_addr_r}
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+
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+The environment variable 'bootargs' is passed as load options in the UEFI system
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+table. The Linux kernel EFI stub uses the load options as command line
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+arguments.
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+
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+### Executing the boot manager
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+
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+The UEFI specfication foresees to define boot entries and boot sequence via UEFI
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+variables. Booting according to these variables is possible via
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+
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+ bootefi bootmgr [fdt address]
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+
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+As of U-Boot v2018.03 UEFI variables are not persisted and cannot be set at
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+runtime.
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+
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+### Executing the built in hello world application
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+
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+A hello world UEFI application can be built with
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+
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+ CONFIG_CMD_BOOTEFI_HELLO_COMPILE=y
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+
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+It can be embedded into the U-Boot binary with
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+
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+ CONFIG_CMD_BOOTEFI_HELLO=y
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+
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+The bootefi command is used to start the embedded hello world application.
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+
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+ bootefi hello [fdt address]
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+
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+Below you find the output of an example session.
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+
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+ => bootefi hello ${fdtcontroladdr}
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+ ## Starting EFI application at 01000000 ...
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+ WARNING: using memory device/image path, this may confuse some payloads!
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+ Hello, world!
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+ Running on UEFI 2.7
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+ Have SMBIOS table
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+ Have device tree
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+ Load options: root=/dev/sdb3 init=/sbin/init rootwait ro
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+ ## Application terminated, r = 0
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+
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+The environment variable fdtcontroladdr points to U-Boot's internal device tree
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+(if available).
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+
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+### Executing the built-in selftest
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+
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+An UEFI selftest suite can be embedded in U-Boot by building with
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+
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+ CONFIG_CMD_BOOTEFI_SELFTEST=y
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+
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+For testing the UEFI implementation the bootefi command can be used to start the
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+selftest.
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+
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+ bootefi selftest [fdt address]
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+
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+The environment variable 'efi_selftest' can be used to select a single test. If
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+it is not provided all tests are executed except those marked as 'on request'.
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+If the environment variable is set to 'list' a list of all tests is shown.
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+
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+Below you can find the output of an example session.
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+
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+ => setenv efi_selftest simple network protocol
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+ => bootefi selftest
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+ Testing EFI API implementation
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+ Selected test: 'simple network protocol'
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+ Setting up 'simple network protocol'
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+ Setting up 'simple network protocol' succeeded
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+ Executing 'simple network protocol'
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+ DHCP Discover
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+ DHCP reply received from 192.168.76.2 (52:55:c0:a8:4c:02)
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+ as broadcast message.
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+ Executing 'simple network protocol' succeeded
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+ Tearing down 'simple network protocol'
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+ Tearing down 'simple network protocol' succeeded
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+ Boot services terminated
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+ Summary: 0 failures
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+ Preparing for reset. Press any key.
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+
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+## The UEFI life cycle
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+
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+After the U-Boot platform has been initialized the UEFI API provides two kinds
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+of services
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+
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+* boot services and
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+* runtime services.
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+
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+The API can be extended by loading UEFI drivers which come in two variants
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+
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+* boot drivers and
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+* runtime drivers.
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+
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+UEFI drivers are installed with U-Boot's bootefi command. With the same command
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+UEFI applications can be executed.
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+
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+Loaded images of UEFI drivers stay in memory after returning to U-Boot while
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+loaded images of applications are removed from memory.
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+
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+An UEFI application (e.g. an operating system) that wants to take full control
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+of the system calls ExitBootServices. After a UEFI application calls
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+ExitBootServices
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+
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+* boot services are not available anymore
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+* timer events are stopped
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+* the memory used by U-Boot except for runtime services is released
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+* the memory used by boot time drivers is released
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+
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+So this is a point of no return. Afterwards the UEFI application can only return
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+to U-Boot by rebooting.
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+
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+## The UEFI object model
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+
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+UEFI offers a flexible and expandable object model. The objects in the UEFI API
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+are devices, drivers, and loaded images. These objects are referenced by
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+handles.
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+
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+The interfaces implemented by the objects are referred to as protocols. These
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+are identified by GUIDs. They can be installed and uninstalled by calling the
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+appropriate boot services.
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+
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+Handles are created by the InstallProtocolInterface or the
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+InstallMultipleProtocolinterfaces service if NULL is passed as handle.
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+
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+Handles are deleted when the last protocol has been removed with the
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+UninstallProtocolInterface or the UninstallMultipleProtocolInterfaces service.
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+
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+Devices offer the EFI_DEVICE_PATH_PROTOCOL. A device path is the concatenation
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+of device nodes. By their device paths all devices of a system are arranged in a
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+tree.
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+
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+Drivers offer the EFI_DRIVER_BINDING_PROTOCOL. This protocol is used to connect
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+a driver to devices (which are referenced as controllers in this context).
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+
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+Loaded images offer the EFI_LOADED_IMAGE_PROTOCOL. This protocol provides meta
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+information about the image and a pointer to the unload callback function.
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+
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+## The UEFI events
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+
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+In the UEFI terminology an event is a data object referencing a notification
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+function which is queued for calling when the event is signaled. The following
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+types of events exist:
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+
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+* periodic and single shot timer events
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+* exit boot services events, triggered by calling the ExitBootServices() service
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+* virtual address change events
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+* memory map change events
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+* read to boot events
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+* reset system events
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+* system table events
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+* events that are only triggered programmatically
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+
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+Events can be created with the CreateEvent service and deleted with CloseEvent
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+service.
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+
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+Events can be assigned to an event group. If any of the events in a group is
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+signaled, all other events in the group are also set to the signaled state.
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+
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+## The UEFI driver model
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+
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+A driver is specific for a single protocol installed on a device. To install a
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+driver on a device the ConnectController service is called. In this context
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+controller refers to the device for which the driver is installed.
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+
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+The relevant drivers are identified using the EFI_DRIVER_BINDING_PROTOCOL. This
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+protocol has has three functions:
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+
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+* supported - determines if the driver is compatible with the device
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+* start - installs the driver by opening the relevant protocol with
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+ attribute EFI_OPEN_PROTOCOL_BY_DRIVER
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+* stop - uninstalls the driver
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+
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+The driver may create child controllers (child devices). E.g. a driver for block
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+IO devices will create the device handles for the partitions. The child
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+controllers will open the supported protocol with the attribute
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+EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.
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+
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+A driver can be detached from a device using the DisconnectController service.
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+
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+## U-Boot devices mapped as UEFI devices
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+
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+Some of the U-Boot devices are mapped as UEFI devices
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+
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+* block IO devices
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+* console
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+* graphical output
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+* network adapter
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+
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+As of U-Boot 2018.03 the logic for doing this is hard coded.
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+
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+The development target is to integrate the setup of these UEFI devices with the
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+U-Boot driver model. So when a U-Boot device is discovered a handle should be
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+created and the device path protocol and the relevant IO protocol should be
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+installed. The UEFI driver then would be attached by calling ConnectController.
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+When a U-Boot device is removed DisconnectController should be called.
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+
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+## UEFI devices mapped as U-Boot devices
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+
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+UEFI drivers binaries and applications may create new (virtual) devices, install
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+a protocol and call the ConnectController service. Now the matching UEFI driver
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+is determined by iterating over the implementations of the
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+EFI_DRIVER_BINDING_PROTOCOL.
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+
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+It is the task of the UEFI driver to create a corresponding U-Boot device and to
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+proxy calls for this U-Boot device to the controller.
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+
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+In U-Boot 2018.03 this has only been implemented for block IO devices.
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+
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+### UEFI uclass
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+
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+An UEFI uclass driver (lib/efi_driver/efi_uclass.c) has been created that
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+takes care of initializing the UEFI drivers and providing the
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+EFI_DRIVER_BINDING_PROTOCOL implementation for the UEFI drivers.
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+
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+A linker created list is used to keep track of the UEFI drivers. To create an
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+entry in the list the UEFI driver uses the U_BOOT_DRIVER macro specifying
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+UCLASS_EFI as the ID of its uclass, e.g.
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+
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+ /* Identify as UEFI driver */
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+ U_BOOT_DRIVER(efi_block) = {
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+ .name = "EFI block driver",
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+ .id = UCLASS_EFI,
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+ .ops = &driver_ops,
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+ };
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+
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+The available operations are defined via the structure struct efi_driver_ops.
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+
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+ struct efi_driver_ops {
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+ const efi_guid_t *protocol;
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+ const efi_guid_t *child_protocol;
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+ int (*bind)(efi_handle_t handle, void *interface);
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+ };
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+
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+When the supported() function of the EFI_DRIVER_BINDING_PROTOCOL is called the
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+uclass checks if the protocol GUID matches the protocol GUID of the UEFI driver.
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+In the start() function the bind() function of the UEFI driver is called after
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+checking the GUID.
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+The stop() function of the EFI_DRIVER_BINDING_PROTOCOL disconnects the child
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+controllers created by the UEFI driver and the UEFI driver. (In U-Boot v2013.03
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+this is not yet completely implemented.)
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+
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+### UEFI block IO driver
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+
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+The UEFI block IO driver supports devices exposing the EFI_BLOCK_IO_PROTOCOL.
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+
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+When connected it creates a new U-Boot block IO device with interface type
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+IF_TYPE_EFI, adds child controllers mapping the partitions, and installs the
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+EFI_SIMPLE_FILE_SYSTEM_PROTOCOL on these. This can be used together with the
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+software iPXE to boot from iSCSI network drives [3].
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+
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+This driver is only available if U-Boot is configured with
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+
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+ CONFIG_BLK=y
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+ CONFIG_PARTITIONS=y
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+
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+## TODOs as of U-Boot 2018.03
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+
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+* unimplemented or incompletely implemented boot services
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+ * Exit - call unload function, unload applications only
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+ * ReinstallProtocolInterface
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+ * UnloadImage
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+
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+* unimplemented events
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+ * EVT_RUNTIME
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+ * EVT_SIGNAL_VIRTUAL_ADDRESS_CHANGE
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+ * event groups
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+
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+* data model
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+ * manage events in a linked list
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+ * manage configuration tables in a linked list
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+
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+* UEFI drivers
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+ * support DisconnectController for UEFI block devices.
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+
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+* support for CONFIG_EFI_LOADER in the sandbox (CONFIG_SANDBOX=y)
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+
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+* UEFI variables
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+ * persistence
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+ * runtime support
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+
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+## Links
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+
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+* [1](http://uefi.org/specifications)
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+ http://uefi.org/specifications - UEFI specifications
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+* [2](./driver-model/README.txt) doc/driver-model/README.txt - Driver model
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+* [3](./README.iscsi) doc/README.iscsi - iSCSI booting with U-Boot and iPXE
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