doc/develop/falcon.rst - u-boot - Git at Google


            
            .. SPDX-License-Identifier: GPL-2.0-or-later


            Falcon Mode

            
            ===========


            Introduction

            
            ------------


            This document provides an overview of how to add support for Falcon Mode

            
            to a board.


            Falcon Mode is introduced to speed up the booting process, allowing

            
            to boot a Linux kernel (or whatever image) without a full blown U-Boot.


            Falcon Mode relies on the SPL framework. In fact, to make booting faster,

            
            U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot

            
            image. In most implementations, SPL is used to start U-Boot when booting from

            
            a mass storage, such as NAND or SD-Card. SPL has now support for other media,

            
            and can generally be seen as a way to start an image performing the minimum

            
            required initialization. SPL mainly initializes the RAM controller, and then

            
            copies U-Boot image into the memory.


            The Falcon Mode extends this way allowing to start the Linux kernel directly

            
            from SPL. A new command is added to U-Boot to prepare the parameters that SPL

            
            must pass to the kernel, using ATAGS or Device Tree.


            In normal mode, these parameters are generated each time before

            
            loading the kernel, passing to Linux the address in memory where

            
            the parameters can be read.

            
            With Falcon Mode, this snapshot can be saved into persistent storage and SPL is

            
            informed to load it before running the kernel.


            To boot the kernel, these steps under a Falcon-aware U-Boot are required:


            1. Boot the board into U-Boot.

            
                After loading the desired legacy-format kernel image into memory (and DT as

            
                well, if used), use the "spl export" command to generate the kernel

            
                parameters area or the DT.  U-Boot runs as when it boots the kernel, but

            
                stops before passing the control to the kernel.


            2. Save the prepared snapshot into persistent media.

            
                The address where to save it must be configured into board configuration

            
                file (CONFIG_CMD_SPL_NAND_OFS for NAND).


            3. Boot the board into Falcon Mode. SPL will load the kernel and copy

            
                the parameters which are saved in the persistent area to the required

            
                address. If a valid uImage is not found at the defined location, U-Boot

            
                will be booted instead.


            It is required to implement a custom mechanism to select if SPL loads U-Boot

            
            or another image.


            The value of a GPIO is a simple way to operate the selection, as well as

            
            reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.


            Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells

            
            SPL that U-Boot is not the only available image that SPL is able to start.


            Configuration

            
            -------------


            CONFIG_CMD_SPL

            
                Enable the "spl export" command.

            
                The command "spl export" is then available in U-Boot mode.


            CONFIG_SPL_PAYLOAD_ARGS_ADDR

            
                Address in RAM where the parameters must be copied by SPL.

            
                In most cases, it is <start_of_ram> + 0x100.


            CONFIG_SYS_NAND_SPL_KERNEL_OFFS

            
                Offset in NAND where the kernel is stored


            CONFIG_CMD_SPL_NAND_OFS

            
                Offset in NAND where the parameters area was saved.


            CONFIG_CMD_SPL_NOR_OFS

            
                Offset in NOR where the parameters area was saved.


            CONFIG_CMD_SPL_WRITE_SIZE

            
                Size of the parameters area to be copied


            CONFIG_SPL_OS_BOOT

            
                Activate Falcon Mode.


            Function that a board must implement

            
            ------------------------------------


            void spl_board_prepare_for_linux(void)

            
                optional, called from SPL before starting the kernel


            spl_start_uboot()

            
                required, returns "0" if SPL should start the kernel, "1" if U-Boot

            
                must be started.


            Environment variables

            
            ---------------------


            A board may chose to look at the environment for decisions about falcon

            
            mode.  In this case the following variables may be supported:


            boot_os

            
                Set to yes/Yes/true/True/1 to enable booting to OS,

            
                any other value to fall back to U-Boot (including unset)


            falcon_args_file

            
                Filename to load as the 'args' portion of falcon mode rather than the

            
                hard-coded value.


            falcon_image_file

            
                Filename to load as the OS image portion of falcon mode rather than the

            
                hard-coded value.


            Using spl command

            
            -----------------


            spl - SPL configuration


            Usage::


                spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]


            img

            
                "atags" or "fdt"


            kernel_addr

            
                kernel is loaded as part of the boot process, but it is not started.

            
                This is the address where a kernel image is stored.


            initrd_addr

            
                Address of initial ramdisk

            
                can be set to "-" if fdt_addr without initrd_addr is used


            fdt_addr

            
                in case of fdt, the address of the device tree.


            The *spl export* command does not write to a storage media. The user is

            
            responsible to transfer the gathered information (assembled ATAGS list

            
            or prepared FDT) from temporary storage in RAM into persistent storage

            
            after each run of *spl export*. Unfortunately the position of temporary

            
            storage can not be predicted nor provided at command line, it depends

            
            highly on your system setup and your provided data (ATAGS or FDT).

            
            However at the end of an successful *spl export* run it will print the

            
            RAM address of temporary storage. The RAM address of FDT will also be

            
            set in the environment variable *fdtargsaddr*, the new length of the

            
            prepared FDT will be set in the environment variable *fdtargslen*.

            
            These environment variables can be used in scripts for writing updated

            
            FDT to persistent storage.


            Now the user have to save the generated BLOB from that printed address

            
            to the pre-defined address in persistent storage

            
            (CONFIG_CMD_SPL_NAND_OFS in case of NAND).

            
            The following example shows how to prepare the data for Falcon Mode on

            
            twister board with ATAGS BLOB.


            The *spl export* command is prepared to work with ATAGS and FDT. However,

            
            using FDT is at the moment untested. The ppc port (see a3m071 example

            
            later) prepares the fdt blob with the fdt command instead.


            Usage on the twister board

            
            --------------------------


            Using mtd names with the following (default) configuration

            
            for mtdparts::


                device nand0 <omap2-nand.0>, # parts = 9

            
                 #: name        size        offset      mask_flags

            
                 0: MLO                 0x00080000      0x00000000      0

            
                 1: u-boot              0x00100000      0x00080000      0

            
                 2: env1                0x00040000      0x00180000      0

            
                 3: env2                0x00040000      0x001c0000      0

            
                 4: kernel              0x00600000      0x00200000      0

            
                 5: bootparms           0x00040000      0x00800000      0

            
                 6: splashimg           0x00200000      0x00840000      0

            
                 7: mini                0x02800000      0x00a40000      0

            
                 8: rootfs              0x1cdc0000      0x03240000      0


            ::


                twister => nand read 82000000 kernel


                NAND read: device 0 offset 0x200000, size 0x600000

            
                6291456 bytes read: OK


            Now the kernel is in RAM at address 0x82000000::


                twister => spl export atags 0x82000000

            
                ## Booting kernel from Legacy Image at 82000000 ...

            
                   Image Name:   Linux-3.5.0-rc4-14089-gda0b7f4

            
                   Image Type:   ARM Linux Kernel Image (uncompressed)

            
                   Data Size:    3654808 Bytes = 3.5 MiB

            
                   Load Address: 80008000

            
                   Entry Point:  80008000

            
                   Verifying Checksum ... OK

            
                   Loading Kernel Image ... OK

            
                OK

            
                cmdline subcommand not supported

            
                bdt subcommand not supported

            
                Argument image is now in RAM at: 0x80000100


            The result can be checked at address 0x80000100::


                twister => md 0x80000100

            
                80000100: 00000005 54410001 00000000 00000000    ......AT........

            
                80000110: 00000000 00000067 54410009 746f6f72    ....g.....ATroot

            
                80000120: 65642f3d 666e2f76 77722073 73666e20    =/dev/nfs rw nfs


            The parameters generated with this step can be saved into NAND at the offset

            
            0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)::


                nand erase.part bootparms

            
                nand write 0x80000100 bootparms 0x4000


            Now the parameters are stored into the NAND flash at the address

            
            CONFIG_CMD_SPL_NAND_OFS (=0x800000).


            Next time, the board can be started into Falcon Mode moving the

            
            setting the GPIO (on twister GPIO 55 is used) to kernel mode.


            The kernel is loaded directly by the SPL without passing through U-Boot.


            Example with FDT: a3m071 board

            
            ------------------------------


            To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get

            
            prepared/patched first. U-Boot usually inserts some dynamic values into

            
            the DT binary (blob), e.g. autodetected memory size, MAC addresses,

            
            clocks speeds etc. To generate this patched DT blob, you can use

            
            the following command:


            1. Load fdt blob to SDRAM::


                    => tftp 1800000 a3m071/a3m071.dtb


            2. Set bootargs as desired for Linux booting (e.g. flash_mtd)::


                    => run mtdargs addip2 addtty


            3. Use "fdt" commands to patch the DT blob::


                    => fdt addr 1800000

            
                    => fdt boardsetup

            
                    => fdt chosen


            4. Display patched DT blob (optional)::


                    => fdt print


            5. Save fdt to NOR flash::


                    => erase fc060000 fc07ffff

            
                    => cp.b 1800000 fc060000 10000

            
                    ...


            Falcon Mode was presented at the RMLL 2012. Slides are available at:


            http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf


            Falcon Mode Boot on RISC-V

            
            --------------------------


            Introduction

            
            ~~~~~~~~~~~~


            In the RISC-V environment, OpenSBI is required to enable a supervisor mode

            
            binary to execute certain privileged operations. The typical boot sequence on

            
            RISC-V is SPL -> OpenSBI -> U-Boot -> Linux kernel. SPL will load and start

            
            the OpenSBI initializations, then OpenSBI will bring up the next image, U-Boot

            
            proper. The OpenSBI binary must be prepared in advance of the U-Boot build

            
            process and it will be packed together with U-Boot into a file called

            
            u-boot.itb.


            The Falcon Mode on RISC-V platforms is a distinct boot sequence. Borrowing

            
            ideas from the U-Boot Falcon Mode on ARM, it skips the U-Boot proper phase

            
            in the normal boot process and allows OpenSBI to load and start the Linux

            
            kernel. Its boot sequence is SPL -> OpenSBI -> Linux kernel. The OpenSBI

            
            binary and Linux kernel binary must be prepared prior to the U-Boot build

            
            process and they will be packed together as a FIT image named linux.itb in

            
            this process.


            CONFIG_SPL_LOAD_FIT_OPENSBI_OS_BOOT enables the Falcon Mode boot on RISC-V.

            
            This configuration setting tells OpenSBI that Linux kernel is its next OS

            
            image and makes it load and start the kernel afterwards.


            Note that the Falcon Mode boot bypasses a lot of initializations by U-Boot.

            
            If the Linux kernel expects hardware initializations by U-Boot, make sure to

            
            port the relevant code to the SPL build process.


            Configuration

            
            ~~~~~~~~~~~~~


            CONFIG_SPL_LOAD_FIT_ADDRESS

            
                Specifies the address to load u-boot.itb in a normal boot. When the Falcon

            
                Mode boot is enabled, it specifies the load address of linux.itb.


            CONFIG_SYS_TEXT_BASE

            
                Specifies the address of the text section for a u-boot proper in a normal

            
                boot. When the Falcon Mode boot is enabled, it specifies the text section

            
                address for the Linux kernel image.


            CONFIG_SPL_PAYLOAD_ARGS_ADDR

            
                The address in the RAM to which the FDT blob is to be moved by the SPL.

            
                SPL places the FDT blob right after the kernel. As the kernel does not

            
                include the BSS section in its size calculation, SPL ends up placing

            
                the FDT blob within the BSS section of the kernel. This may cause the

            
                FDT blob to be cleared during kernel BSS initialization. To avoid the

            
                issue, be sure to move the FDT blob out of the kernel first.


            CONFIG_SPL_LOAD_FIT_OPENSBI_OS_BOOT

            
                Activates the Falcon Mode boot on RISC-V.


            Example for Andes AE350 Board

            
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


            A FDT blob is required to boot the Linux kernel from the SPL. Andes AE350

            
            platforms generally come with a builtin dtb. To load a custom DTB, follow

            
            these steps:


            1. Load the custom DTB to SDRAM::


                    => fatload mmc 0:1 0x20000000 user_custom.dtb


            2. Set the SPI speed::


                    => sf probe 0:0 50000000 0


            3. Erase sectors from the SPI Flash::


                    => sf erase 0xf0000 0x10000


            4. Write the FDT blob to the erased sectors of the Flash::


                    => sf write 0x20000000 0xf0000 0x10000


            Console Log of AE350 Falcon Mode Boot

            
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


            ::


                    U-Boot SPL 2023.01-00031-g777ecdea66 (Oct 31 2023 - 18:41:36 +0800)

            
                    Trying to boot from RAM


                    OpenSBI v1.2-51-g7304e42

            
                       ____                    _____ ____ _____

            
                      / __ \                  / ____|  _ \_   _|

            
                     | |  | |_ __   ___ _ __ | (___ | |_) || |

            
                     | |  | | '_ \ / _ \ '_ \ \___ \|  _ < | |

            
                     | |__| | |_) |  __/ | | |____) | |_) || |_

            
                      \____/| .__/ \___|_| |_|_____/|____/_____|

            
                            | |

            
                            |_|


                    Platform Name             : andestech,ax25

            
                    Platform Features         : medeleg

            
                    Platform HART Count       : 1

            
                    Platform IPI Device       : andes_plicsw

            
                    Platform Timer Device     : andes_plmt @ 60000000Hz

            
                    Platform Console Device   : uart8250

            
                    Platform HSM Device       : andes_smu

            
                    Platform PMU Device       : andes_pmu

            
                    Platform Reboot Device    : atcwdt200

            
                    Platform Shutdown Device  : ---

            
                    Firmware Base             : 0x0

            
                    Firmware Size             : 196 KB

            
                    Runtime SBI Version       : 1.0


                    Domain0 Name              : root

            
                    Domain0 Boot HART         : 0

            
                    Domain0 HARTs             : 0*

            
                    Domain0 Region00          : 0x0000000000000000-0x000000000003ffff ()

            
                    Domain0 Region01          : 0x00000000e6000000-0x00000000e60fffff (I,R)

            
                    Domain0 Region02          : 0x00000000e6400000-0x00000000e67fffff (I)

            
                    Domain0 Region03          : 0x0000000000000000-0xffffffffffffffff (R,W,X)

            
                    Domain0 Next Address      : 0x0000000001800000

            
                    Domain0 Next Arg1         : 0x0000000001700000

            
                    Domain0 Next Mode         : S-mode

            
                    Domain0 SysReset          : yes


                    Boot HART ID              : 0

            
                    Boot HART Domain          : root

            
                    Boot HART Priv Version    : v1.11

            
                    Boot HART Base ISA        : rv64imafdcx

            
                    Boot HART ISA Extensions  : none

            
                    Boot HART PMP Count       : 8

            
                    Boot HART PMP Granularity : 4

            
                    Boot HART PMP Address Bits: 31

            
                    Boot HART MHPM Count      : 4

            
                    Boot HART MHPM Bits       : 64

            
                    Boot HART MIDELEG         : 0x0000000000000222

            
                    Boot HART MEDELEG         : 0x000000000000b109

            
                    [    0.000000] Linux version 6.1.47-09019-g0584b09ad862-dirty

            
                    [    0.000000] OF: fdt: Ignoring memory range 0x0 - 0x1800000

            
                    [    0.000000] Machine model: andestech,ax25

            
                    [    0.000000] earlycon: sbi0 at I/O port 0x0 (options '')

            
                    [    0.000000] printk: bootconsole [sbi0] enabled

            
                    [    0.000000] Disabled 4-level and 5-level paging

            
                    [    0.000000] efi: UEFI not found.

            
                    [    0.000000] Zone ranges:

            
                    [    0.000000]   DMA32    [mem 0x0000000001800000-0x000000003fffffff]

            
                    [    0.000000]   Normal   empty

            
                    [    0.000000] Movable zone start for each node

            
                    [    0.000000] Early memory node ranges

            
                    [    0.000000]   node   0: [mem 0x0000000001800000-0x000000003fffffff]

            
                    [    0.000000] Initmem setup node 0 [mem 0x0000000001800000-0x000000003fffffff]

            
                    [    0.000000] SBI specification v1.0 detected

            
                    [    0.000000] SBI implementation ID=0x1 Version=0x10002

            
                    [    0.000000] SBI TIME extension detected

            
                    [    0.000000] SBI IPI extension detected

            
                    [    0.000000] SBI RFENCE extension detected

            
                    [    0.000000] SBI SRST extension detected

            
                    [    0.000000] SBI HSM extension detected

            
                    [    0.000000] riscv: base ISA extensions acim

            
                    [    0.000000] riscv: ELF capabilities acim

            
                    [    0.000000] percpu: Embedded 18 pages/cpu s35000 r8192 d30536 u73728

            
                    [    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 252500
	.. SPDX-License-Identifier: GPL-2.0-or-later

	Falcon Mode
	===========

	Introduction
	------------

	This document provides an overview of how to add support for Falcon Mode
	to a board.

	Falcon Mode is introduced to speed up the booting process, allowing
	to boot a Linux kernel (or whatever image) without a full blown U-Boot.

	Falcon Mode relies on the SPL framework. In fact, to make booting faster,
	U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
	image. In most implementations, SPL is used to start U-Boot when booting from
	a mass storage, such as NAND or SD-Card. SPL has now support for other media,
	and can generally be seen as a way to start an image performing the minimum
	required initialization. SPL mainly initializes the RAM controller, and then
	copies U-Boot image into the memory.

	The Falcon Mode extends this way allowing to start the Linux kernel directly
	from SPL. A new command is added to U-Boot to prepare the parameters that SPL
	must pass to the kernel, using ATAGS or Device Tree.

	In normal mode, these parameters are generated each time before
	loading the kernel, passing to Linux the address in memory where
	the parameters can be read.
	With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
	informed to load it before running the kernel.

	To boot the kernel, these steps under a Falcon-aware U-Boot are required:

	1. Boot the board into U-Boot.
	After loading the desired legacy-format kernel image into memory (and DT as
	well, if used), use the "spl export" command to generate the kernel
	parameters area or the DT. U-Boot runs as when it boots the kernel, but
	stops before passing the control to the kernel.

	2. Save the prepared snapshot into persistent media.
	The address where to save it must be configured into board configuration
	file (CONFIG_CMD_SPL_NAND_OFS for NAND).

	3. Boot the board into Falcon Mode. SPL will load the kernel and copy
	the parameters which are saved in the persistent area to the required
	address. If a valid uImage is not found at the defined location, U-Boot
	will be booted instead.

	It is required to implement a custom mechanism to select if SPL loads U-Boot
	or another image.

	The value of a GPIO is a simple way to operate the selection, as well as
	reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.

	Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
	SPL that U-Boot is not the only available image that SPL is able to start.

	Configuration
	-------------

	CONFIG_CMD_SPL
	Enable the "spl export" command.
	The command "spl export" is then available in U-Boot mode.

	CONFIG_SPL_PAYLOAD_ARGS_ADDR
	Address in RAM where the parameters must be copied by SPL.
	In most cases, it is <start_of_ram> + 0x100.

	CONFIG_SYS_NAND_SPL_KERNEL_OFFS
	Offset in NAND where the kernel is stored

	CONFIG_CMD_SPL_NAND_OFS
	Offset in NAND where the parameters area was saved.

	CONFIG_CMD_SPL_NOR_OFS
	Offset in NOR where the parameters area was saved.

	CONFIG_CMD_SPL_WRITE_SIZE
	Size of the parameters area to be copied

	CONFIG_SPL_OS_BOOT
	Activate Falcon Mode.

	Function that a board must implement
	------------------------------------

	void spl_board_prepare_for_linux(void)
	optional, called from SPL before starting the kernel

	spl_start_uboot()
	required, returns "0" if SPL should start the kernel, "1" if U-Boot
	must be started.

	Environment variables
	---------------------

	A board may chose to look at the environment for decisions about falcon
	mode. In this case the following variables may be supported:

	boot_os
	Set to yes/Yes/true/True/1 to enable booting to OS,
	any other value to fall back to U-Boot (including unset)

	falcon_args_file
	Filename to load as the 'args' portion of falcon mode rather than the
	hard-coded value.

	falcon_image_file
	Filename to load as the OS image portion of falcon mode rather than the
	hard-coded value.

	Using spl command
	-----------------

	spl - SPL configuration

	Usage::

	spl export <img=atags\|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]

	img
	"atags" or "fdt"

	kernel_addr
	kernel is loaded as part of the boot process, but it is not started.
	This is the address where a kernel image is stored.

	initrd_addr
	Address of initial ramdisk
	can be set to "-" if fdt_addr without initrd_addr is used

	fdt_addr
	in case of fdt, the address of the device tree.

	The spl export command does not write to a storage media. The user is
	responsible to transfer the gathered information (assembled ATAGS list
	or prepared FDT) from temporary storage in RAM into persistent storage
	after each run of spl export. Unfortunately the position of temporary
	storage can not be predicted nor provided at command line, it depends
	highly on your system setup and your provided data (ATAGS or FDT).
	However at the end of an successful spl export run it will print the
	RAM address of temporary storage. The RAM address of FDT will also be
	set in the environment variable fdtargsaddr, the new length of the
	prepared FDT will be set in the environment variable fdtargslen.
	These environment variables can be used in scripts for writing updated
	FDT to persistent storage.

	Now the user have to save the generated BLOB from that printed address
	to the pre-defined address in persistent storage
	(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
	The following example shows how to prepare the data for Falcon Mode on
	twister board with ATAGS BLOB.

	The spl export command is prepared to work with ATAGS and FDT. However,
	using FDT is at the moment untested. The ppc port (see a3m071 example
	later) prepares the fdt blob with the fdt command instead.


	Usage on the twister board
	--------------------------

	Using mtd names with the following (default) configuration
	for mtdparts::

	device nand0 <omap2-nand.0>, # parts = 9
	#: name size offset mask_flags
	0: MLO 0x00080000 0x00000000 0
	1: u-boot 0x00100000 0x00080000 0
	2: env1 0x00040000 0x00180000 0
	3: env2 0x00040000 0x001c0000 0
	4: kernel 0x00600000 0x00200000 0
	5: bootparms 0x00040000 0x00800000 0
	6: splashimg 0x00200000 0x00840000 0
	7: mini 0x02800000 0x00a40000 0
	8: rootfs 0x1cdc0000 0x03240000 0

	::

	twister => nand read 82000000 kernel

	NAND read: device 0 offset 0x200000, size 0x600000
	6291456 bytes read: OK

	Now the kernel is in RAM at address 0x82000000::

	twister => spl export atags 0x82000000
	## Booting kernel from Legacy Image at 82000000 ...
	Image Name: Linux-3.5.0-rc4-14089-gda0b7f4
	Image Type: ARM Linux Kernel Image (uncompressed)
	Data Size: 3654808 Bytes = 3.5 MiB
	Load Address: 80008000
	Entry Point: 80008000
	Verifying Checksum ... OK
	Loading Kernel Image ... OK
	OK
	cmdline subcommand not supported
	bdt subcommand not supported
	Argument image is now in RAM at: 0x80000100

	The result can be checked at address 0x80000100::

	twister => md 0x80000100
	80000100: 00000005 54410001 00000000 00000000 ......AT........
	80000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot
	80000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs

	The parameters generated with this step can be saved into NAND at the offset
	0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)::

	nand erase.part bootparms
	nand write 0x80000100 bootparms 0x4000

	Now the parameters are stored into the NAND flash at the address
	CONFIG_CMD_SPL_NAND_OFS (=0x800000).

	Next time, the board can be started into Falcon Mode moving the
	setting the GPIO (on twister GPIO 55 is used) to kernel mode.

	The kernel is loaded directly by the SPL without passing through U-Boot.

	Example with FDT: a3m071 board
	------------------------------

	To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
	prepared/patched first. U-Boot usually inserts some dynamic values into
	the DT binary (blob), e.g. autodetected memory size, MAC addresses,
	clocks speeds etc. To generate this patched DT blob, you can use
	the following command:

	1. Load fdt blob to SDRAM::

	=> tftp 1800000 a3m071/a3m071.dtb

	2. Set bootargs as desired for Linux booting (e.g. flash_mtd)::

	=> run mtdargs addip2 addtty

	3. Use "fdt" commands to patch the DT blob::

	=> fdt addr 1800000
	=> fdt boardsetup
	=> fdt chosen

	4. Display patched DT blob (optional)::

	=> fdt print

	5. Save fdt to NOR flash::

	=> erase fc060000 fc07ffff
	=> cp.b 1800000 fc060000 10000
	...


	Falcon Mode was presented at the RMLL 2012. Slides are available at:

	http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf

	Falcon Mode Boot on RISC-V
	--------------------------

	Introduction
	~~~~~~~~~~~~

	In the RISC-V environment, OpenSBI is required to enable a supervisor mode
	binary to execute certain privileged operations. The typical boot sequence on
	RISC-V is SPL -> OpenSBI -> U-Boot -> Linux kernel. SPL will load and start
	the OpenSBI initializations, then OpenSBI will bring up the next image, U-Boot
	proper. The OpenSBI binary must be prepared in advance of the U-Boot build
	process and it will be packed together with U-Boot into a file called
	u-boot.itb.

	The Falcon Mode on RISC-V platforms is a distinct boot sequence. Borrowing
	ideas from the U-Boot Falcon Mode on ARM, it skips the U-Boot proper phase
	in the normal boot process and allows OpenSBI to load and start the Linux
	kernel. Its boot sequence is SPL -> OpenSBI -> Linux kernel. The OpenSBI
	binary and Linux kernel binary must be prepared prior to the U-Boot build
	process and they will be packed together as a FIT image named linux.itb in
	this process.

	CONFIG_SPL_LOAD_FIT_OPENSBI_OS_BOOT enables the Falcon Mode boot on RISC-V.
	This configuration setting tells OpenSBI that Linux kernel is its next OS
	image and makes it load and start the kernel afterwards.

	Note that the Falcon Mode boot bypasses a lot of initializations by U-Boot.
	If the Linux kernel expects hardware initializations by U-Boot, make sure to
	port the relevant code to the SPL build process.

	Configuration
	~~~~~~~~~~~~~

	CONFIG_SPL_LOAD_FIT_ADDRESS
	Specifies the address to load u-boot.itb in a normal boot. When the Falcon
	Mode boot is enabled, it specifies the load address of linux.itb.

	CONFIG_SYS_TEXT_BASE
	Specifies the address of the text section for a u-boot proper in a normal
	boot. When the Falcon Mode boot is enabled, it specifies the text section
	address for the Linux kernel image.

	CONFIG_SPL_PAYLOAD_ARGS_ADDR
	The address in the RAM to which the FDT blob is to be moved by the SPL.
	SPL places the FDT blob right after the kernel. As the kernel does not
	include the BSS section in its size calculation, SPL ends up placing
	the FDT blob within the BSS section of the kernel. This may cause the
	FDT blob to be cleared during kernel BSS initialization. To avoid the
	issue, be sure to move the FDT blob out of the kernel first.

	CONFIG_SPL_LOAD_FIT_OPENSBI_OS_BOOT
	Activates the Falcon Mode boot on RISC-V.

	Example for Andes AE350 Board
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

	A FDT blob is required to boot the Linux kernel from the SPL. Andes AE350
	platforms generally come with a builtin dtb. To load a custom DTB, follow
	these steps:

	1. Load the custom DTB to SDRAM::

	=> fatload mmc 0:1 0x20000000 user_custom.dtb

	2. Set the SPI speed::

	=> sf probe 0:0 50000000 0

	3. Erase sectors from the SPI Flash::

	=> sf erase 0xf0000 0x10000

	4. Write the FDT blob to the erased sectors of the Flash::

	=> sf write 0x20000000 0xf0000 0x10000

	Console Log of AE350 Falcon Mode Boot
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

	::

	U-Boot SPL 2023.01-00031-g777ecdea66 (Oct 31 2023 - 18:41:36 +0800)
	Trying to boot from RAM

	OpenSBI v1.2-51-g7304e42
	____ _____ ____ _____
	/ __ \ / ____\| _ \_ _\|
	\| \| \| \|_ __ ___ _ __ \| (___ \| \|_) \|\| \|
	\| \| \| \| '_ \ / _ \ '_ \ \___ \\| _ < \| \|
	\| \|__\| \| \|_) \| __/ \| \| \|____) \| \|_) \|\| \|_
	\____/\| .__/ \___\|_\| \|_\|_____/\|____/_____\|
	\| \|
	\|_\|

	Platform Name : andestech,ax25
	Platform Features : medeleg
	Platform HART Count : 1
	Platform IPI Device : andes_plicsw
	Platform Timer Device : andes_plmt @ 60000000Hz
	Platform Console Device : uart8250
	Platform HSM Device : andes_smu
	Platform PMU Device : andes_pmu
	Platform Reboot Device : atcwdt200
	Platform Shutdown Device : ---
	Firmware Base : 0x0
	Firmware Size : 196 KB
	Runtime SBI Version : 1.0

	Domain0 Name : root
	Domain0 Boot HART : 0
	Domain0 HARTs : 0*
	Domain0 Region00 : 0x0000000000000000-0x000000000003ffff ()
	Domain0 Region01 : 0x00000000e6000000-0x00000000e60fffff (I,R)
	Domain0 Region02 : 0x00000000e6400000-0x00000000e67fffff (I)
	Domain0 Region03 : 0x0000000000000000-0xffffffffffffffff (R,W,X)
	Domain0 Next Address : 0x0000000001800000
	Domain0 Next Arg1 : 0x0000000001700000
	Domain0 Next Mode : S-mode
	Domain0 SysReset : yes

	Boot HART ID : 0
	Boot HART Domain : root
	Boot HART Priv Version : v1.11
	Boot HART Base ISA : rv64imafdcx
	Boot HART ISA Extensions : none
	Boot HART PMP Count : 8
	Boot HART PMP Granularity : 4
	Boot HART PMP Address Bits: 31
	Boot HART MHPM Count : 4
	Boot HART MHPM Bits : 64
	Boot HART MIDELEG : 0x0000000000000222
	Boot HART MEDELEG : 0x000000000000b109
	[ 0.000000] Linux version 6.1.47-09019-g0584b09ad862-dirty
	[ 0.000000] OF: fdt: Ignoring memory range 0x0 - 0x1800000
	[ 0.000000] Machine model: andestech,ax25
	[ 0.000000] earlycon: sbi0 at I/O port 0x0 (options '')
	[ 0.000000] printk: bootconsole [sbi0] enabled
	[ 0.000000] Disabled 4-level and 5-level paging
	[ 0.000000] efi: UEFI not found.
	[ 0.000000] Zone ranges:
	[ 0.000000] DMA32 [mem 0x0000000001800000-0x000000003fffffff]
	[ 0.000000] Normal empty
	[ 0.000000] Movable zone start for each node
	[ 0.000000] Early memory node ranges
	[ 0.000000] node 0: [mem 0x0000000001800000-0x000000003fffffff]
	[ 0.000000] Initmem setup node 0 [mem 0x0000000001800000-0x000000003fffffff]
	[ 0.000000] SBI specification v1.0 detected
	[ 0.000000] SBI implementation ID=0x1 Version=0x10002
	[ 0.000000] SBI TIME extension detected
	[ 0.000000] SBI IPI extension detected
	[ 0.000000] SBI RFENCE extension detected
	[ 0.000000] SBI SRST extension detected
	[ 0.000000] SBI HSM extension detected
	[ 0.000000] riscv: base ISA extensions acim
	[ 0.000000] riscv: ELF capabilities acim
	[ 0.000000] percpu: Embedded 18 pages/cpu s35000 r8192 d30536 u73728
	[ 0.000000] Built 1 zonelists, mobility grouping on. Total pages: 252500