xref: /openbmc/u-boot/doc/README.falcon (revision bfc37f3c)
1U-Boot Falcon Mode
2====================
3
4Introduction
5------------
6
7This document provides an overview of how to add support for Falcon Mode
8to a board.
9
10Falcon Mode is introduced to speed up the booting process, allowing
11to boot a Linux kernel (or whatever image) without a full blown U-Boot.
12
13Falcon Mode relies on the SPL framework. In fact, to make booting faster,
14U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
15image. In most implementations, SPL is used to start U-Boot when booting from
16a mass storage, such as NAND or SD-Card. SPL has now support for other media,
17and can generally be seen as a way to start an image performing the minimum
18required initialization. SPL mainly initializes the RAM controller, and then
19copies U-Boot image into the memory.
20
21The Falcon Mode extends this way allowing to start the Linux kernel directly
22from SPL. A new command is added to U-Boot to prepare the parameters that SPL
23must pass to the kernel, using ATAGS or Device Tree.
24
25In normal mode, these parameters are generated each time before
26loading the kernel, passing to Linux the address in memory where
27the parameters can be read.
28With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
29informed to load it before running the kernel.
30
31To boot the kernel, these steps under a Falcon-aware U-Boot are required:
32
331. Boot the board into U-Boot.
34After loading the desired legacy-format kernel image into memory (and DT as
35well, if used), use the "spl export" command to generate the kernel parameters
36area or the DT.  U-Boot runs as when it boots the kernel, but stops before
37passing the control to the kernel.
38
392. Save the prepared snapshot into persistent media.
40The address where to save it must be configured into board configuration
41file (CONFIG_CMD_SPL_NAND_OFS for NAND).
42
433. Boot the board into Falcon Mode. SPL will load the kernel and copy
44the parameters which are saved in the persistent area to the required address.
45If a valid uImage is not found at the defined location, U-Boot will be
46booted instead.
47
48It is required to implement a custom mechanism to select if SPL loads U-Boot
49or another image.
50
51The value of a GPIO is a simple way to operate the selection, as well as
52reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
53
54Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
55SPL that U-Boot is not the only available image that SPL is able to start.
56
57Configuration
58----------------------------
59CONFIG_CMD_SPL		Enable the "spl export" command.
60			The command "spl export" is then available in U-Boot
61			mode
62CONFIG_SYS_SPL_ARGS_ADDR	Address in RAM where the parameters must be
63				copied by SPL.
64				In most cases, it is <start_of_ram> + 0x100
65
66CONFIG_SYS_NAND_SPL_KERNEL_OFFS	Offset in NAND where the kernel is stored
67
68CONFIG_CMD_SPL_NAND_OFS	Offset in NAND where the parameters area was saved.
69
70CONFIG_CMD_SPL_WRITE_SIZE 	Size of the parameters area to be copied
71
72CONFIG_SPL_OS_BOOT	Activate Falcon Mode.
73
74Function that a board must implement
75------------------------------------
76
77void spl_board_prepare_for_linux(void) : optional
78	Called from SPL before starting the kernel
79
80spl_start_uboot() : required
81		Returns "0" if SPL should start the kernel, "1" if U-Boot
82		must be started.
83
84Environment variables
85---------------------
86
87A board may chose to look at the environment for decisions about falcon
88mode.  In this case the following variables may be supported:
89
90boot_os : 		Set to yes/Yes/true/True/1 to enable booting to OS,
91			any other value to fall back to U-Boot (including
92			unset)
93falcon_args_file :	Filename to load as the 'args' portion of falcon mode
94			rather than the hard-coded value.
95falcon_image_file :	Filename to load as the OS image portion of falcon
96			mode rather than the hard-coded value.
97
98Using spl command
99-----------------
100
101spl - SPL configuration
102
103Usage:
104
105spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
106
107img		: "atags" or "fdt"
108kernel_addr	: kernel is loaded as part of the boot process, but it is not started.
109		  This is the address where a kernel image is stored.
110initrd_addr	: Address of initial ramdisk
111		  can be set to "-" if fdt_addr without initrd_addr is used
112fdt_addr	: in case of fdt, the address of the device tree.
113
114The spl export command does not write to a storage media. The user is
115responsible to transfer the gathered information (assembled ATAGS list
116or prepared FDT) from temporary storage in RAM into persistant storage
117after each run of 'spl export'. Unfortunately the position of temporary
118storage can not be predicted nor provided at commandline, it depends
119highly on your system setup and your provided data (ATAGS or FDT).
120However at the end of an succesful 'spl export' run it will print the
121RAM address of temporary storage.
122Now the user have to save the generated BLOB from that printed address
123to the pre-defined address in persistent storage
124(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
125The following example shows how to prepare the data for Falcon Mode on
126twister board with ATAGS BLOB.
127
128The "spl export" command is prepared to work with ATAGS and FDT. However,
129using FDT is at the moment untested. The ppc port (see a3m071 example
130later) prepares the fdt blob with the fdt command instead.
131
132
133Usage on the twister board:
134--------------------------------
135
136Using mtd names with the following (default) configuration
137for mtdparts:
138
139device nand0 <omap2-nand.0>, # parts = 9
140 #: name		size		offset		mask_flags
141 0: MLO                 0x00080000      0x00000000      0
142 1: u-boot              0x00100000      0x00080000      0
143 2: env1                0x00040000      0x00180000      0
144 3: env2                0x00040000      0x001c0000      0
145 4: kernel              0x00600000      0x00200000      0
146 5: bootparms           0x00040000      0x00800000      0
147 6: splashimg           0x00200000      0x00840000      0
148 7: mini                0x02800000      0x00a40000      0
149 8: rootfs              0x1cdc0000      0x03240000      0
150
151
152twister => nand read 82000000 kernel
153
154NAND read: device 0 offset 0x200000, size 0x600000
155 6291456 bytes read: OK
156
157Now the kernel is in RAM at address 0x82000000
158
159twister => spl export atags 0x82000000
160## Booting kernel from Legacy Image at 82000000 ...
161   Image Name:   Linux-3.5.0-rc4-14089-gda0b7f4
162   Image Type:   ARM Linux Kernel Image (uncompressed)
163   Data Size:    3654808 Bytes = 3.5 MiB
164   Load Address: 80008000
165   Entry Point:  80008000
166   Verifying Checksum ... OK
167   Loading Kernel Image ... OK
168OK
169cmdline subcommand not supported
170bdt subcommand not supported
171Argument image is now in RAM at: 0x80000100
172
173The result can be checked at address 0x80000100:
174
175twister => md 0x80000100
17680000100: 00000005 54410001 00000000 00000000    ......AT........
17780000110: 00000000 00000067 54410009 746f6f72    ....g.....ATroot
17880000120: 65642f3d 666e2f76 77722073 73666e20    =/dev/nfs rw nfs
179
180The parameters generated with this step can be saved into NAND at the offset
1810x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
182
183nand erase.part bootparms
184nand write 0x80000100 bootparms 0x4000
185
186Now the parameters are stored into the NAND flash at the address
187CONFIG_CMD_SPL_NAND_OFS (=0x800000).
188
189Next time, the board can be started into Falcon Mode moving the
190setting the gpio (on twister gpio 55 is used) to kernel mode.
191
192The kernel is loaded directly by the SPL without passing through U-Boot.
193
194Example with FDT: a3m071 board
195-------------------------------
196
197To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
198prepard/patched first. U-Boot usually inserts some dynamic values into
199the DT binary (blob), e.g. autodetected memory size, MAC addresses,
200clocks speeds etc. To generate this patched DT blob, you can use
201the following command:
202
2031. Load fdt blob to SDRAM:
204=> tftp 1800000 a3m071/a3m071.dtb
205
2062. Set bootargs as desired for Linux booting (e.g. flash_mtd):
207=> run mtdargs addip2 addtty
208
2093. Use "fdt" commands to patch the DT blob:
210=> fdt addr 1800000
211=> fdt boardsetup
212=> fdt chosen
213
2144. Display patched DT blob (optional):
215=> fdt print
216
2175. Save fdt to NOR flash:
218=> erase fc060000 fc07ffff
219=> cp.b 1800000 fc060000 10000
220...
221
222
223Falcon Mode was presented at the RMLL 2012. Slides are available at:
224
225http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf
226