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Stefano Babic3e1b3932013-02-23 00:53:25 +00001U-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.
34Use the "spl export" command to generate the kernel parameters area or the DT.
35U-Boot runs as when it boots the kernel, but stops before passing the control
36to the kernel.
37
382. Save the prepared snapshot into persistent media.
39The address where to save it must be configured into board configuration
40file (CONFIG_CMD_SPL_NAND_OFS for NAND).
41
423. Boot the board into Falcon Mode. SPL will load the kernel and copy
43the parameters which are saved in the persistent area to the required address.
Tom Rinid118d762013-07-08 12:15:14 -040044If a valid uImage is not found at the defined location, U-Boot will be
45booted instead.
Stefano Babic3e1b3932013-02-23 00:53:25 +000046
47It is required to implement a custom mechanism to select if SPL loads U-Boot
48or another image.
49
50The value of a GPIO is a simple way to operate the selection, as well as
51reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
52
53Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
54SPL that U-Boot is not the only available image that SPL is able to start.
55
56Configuration
57----------------------------
58CONFIG_CMD_SPL Enable the "spl export" command.
59 The command "spl export" is then available in U-Boot
60 mode
61CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be
62 copied by SPL.
63 In most cases, it is <start_of_ram> + 0x100
64
65CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored
66
67CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved.
68
69CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied
70
71CONFIG_SPL_OS_BOOT Activate Falcon Mode.
72
73Function that a board must implement
74------------------------------------
75
76void spl_board_prepare_for_linux(void) : optional
77 Called from SPL before starting the kernel
78
79spl_start_uboot() : required
80 Returns "0" if SPL should start the kernel, "1" if U-Boot
81 must be started.
82
83
84Using spl command
85-----------------
86
87spl - SPL configuration
88
89Usage:
90
91spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
92
93img : "atags" or "fdt"
94kernel_addr : kernel is loaded as part of the boot process, but it is not started.
95 This is the address where a kernel image is stored.
96initrd_addr : Address of initial ramdisk
97 can be set to "-" if fdt_addr without initrd_addr is used
98fdt_addr : in case of fdt, the address of the device tree.
99
100The spl export command does not write to a storage media. The user is
101responsible to transfer the gathered information (assembled ATAGS list
102or prepared FDT) from temporary storage in RAM into persistant storage
103after each run of 'spl export'. Unfortunately the position of temporary
104storage can not be predicted nor provided at commandline, it depends
105highly on your system setup and your provided data (ATAGS or FDT).
106However at the end of an succesful 'spl export' run it will print the
107RAM address of temporary storage.
108Now the user have to save the generated BLOB from that printed address
109to the pre-defined address in persistent storage
110(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
111The following example shows how to prepare the data for Falcon Mode on
112twister board with ATAGS BLOB.
113
114The "spl export" command is prepared to work with ATAGS and FDT. However,
115using FDT is at the moment untested. The ppc port (see a3m071 example
116later) prepares the fdt blob with the fdt command instead.
117
118
119Usage on the twister board:
120--------------------------------
121
122Using mtd names with the following (default) configuration
123for mtdparts:
124
125device nand0 <omap2-nand.0>, # parts = 9
126 #: name size offset mask_flags
127 0: MLO 0x00080000 0x00000000 0
128 1: u-boot 0x00100000 0x00080000 0
129 2: env1 0x00040000 0x00180000 0
130 3: env2 0x00040000 0x001c0000 0
131 4: kernel 0x00600000 0x00200000 0
132 5: bootparms 0x00040000 0x00800000 0
133 6: splashimg 0x00200000 0x00840000 0
134 7: mini 0x02800000 0x00a40000 0
135 8: rootfs 0x1cdc0000 0x03240000 0
136
137
138twister => nand read 82000000 kernel
139
140NAND read: device 0 offset 0x200000, size 0x600000
141 6291456 bytes read: OK
142
143Now the kernel is in RAM at address 0x82000000
144
145twister => spl export atags 0x82000000
146## Booting kernel from Legacy Image at 82000000 ...
147 Image Name: Linux-3.5.0-rc4-14089-gda0b7f4
148 Image Type: ARM Linux Kernel Image (uncompressed)
149 Data Size: 3654808 Bytes = 3.5 MiB
150 Load Address: 80008000
151 Entry Point: 80008000
152 Verifying Checksum ... OK
153 Loading Kernel Image ... OK
154OK
155cmdline subcommand not supported
156bdt subcommand not supported
157Argument image is now in RAM at: 0x80000100
158
159The result can be checked at address 0x80000100:
160
161twister => md 0x80000100
16280000100: 00000005 54410001 00000000 00000000 ......AT........
16380000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot
16480000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs
165
166The parameters generated with this step can be saved into NAND at the offset
1670x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
168
169nand erase.part bootparms
170nand write 0x80000100 bootparms 0x4000
171
172Now the parameters are stored into the NAND flash at the address
173CONFIG_CMD_SPL_NAND_OFS (=0x800000).
174
175Next time, the board can be started into Falcon Mode moving the
176setting the gpio (on twister gpio 55 is used) to kernel mode.
177
178The kernel is loaded directly by the SPL without passing through U-Boot.
179
180Example with FDT: a3m071 board
181-------------------------------
182
183To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
184prepard/patched first. U-Boot usually inserts some dynamic values into
185the DT binary (blob), e.g. autodetected memory size, MAC addresses,
186clocks speeds etc. To generate this patched DT blob, you can use
187the following command:
188
1891. Load fdt blob to SDRAM:
190=> tftp 1800000 a3m071/a3m071.dtb
191
1922. Set bootargs as desired for Linux booting (e.g. flash_mtd):
193=> run mtdargs addip2 addtty
194
1953. Use "fdt" commands to patch the DT blob:
196=> fdt addr 1800000
197=> fdt boardsetup
198=> fdt chosen
199
2004. Display patched DT blob (optional):
201=> fdt print
202
2035. Save fdt to NOR flash:
204=> erase fc060000 fc07ffff
205=> cp.b 1800000 fc060000 10000
206...
207
208
209Falcon Mode was presented at the RMLL 2012. Slides are available at:
210
211http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf