doc/README.falcon

   1 U-Boot Falcon Mode
   2 ====================
   3
   4 Introduction
   5 ------------
   6
   7 This document provides an overview of how to add support for Falcon Mode
   8 to a board.
   9
  10 Falcon Mode is introduced to speed up the booting process, allowing
  11 to boot a Linux kernel (or whatever image) without a full blown U-Boot.
  12
  13 Falcon Mode relies on the SPL framework. In fact, to make booting faster,
  14 U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
  15 image. In most implementations, SPL is used to start U-Boot when booting from
  16 a mass storage, such as NAND or SD-Card. SPL has now support for other media,
  17 and can generally be seen as a way to start an image performing the minimum
  18 required initialization. SPL mainly initializes the RAM controller, and then
  19 copies U-Boot image into the memory.
  20
  21 The Falcon Mode extends this way allowing to start the Linux kernel directly
  22 from SPL. A new command is added to U-Boot to prepare the parameters that SPL
  23 must pass to the kernel, using ATAGS or Device Tree.
  24
  25 In normal mode, these parameters are generated each time before
  26 loading the kernel, passing to Linux the address in memory where
  27 the parameters can be read.
  28 With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
  29 informed to load it before running the kernel.
  30
  31 To boot the kernel, these steps under a Falcon-aware U-Boot are required:
  32
  33 1. Boot the board into U-Boot.
  34 After loading the desired legacy-format kernel image into memory (and DT as
  35 well, if used), use the "spl export" command to generate the kernel parameters
  36 area or the DT.  U-Boot runs as when it boots the kernel, but stops before
  37 passing the control to the kernel.
  38
  39 2. Save the prepared snapshot into persistent media.
  40 The address where to save it must be configured into board configuration
  41 file (CONFIG_CMD_SPL_NAND_OFS for NAND).
  42
  43 3. Boot the board into Falcon Mode. SPL will load the kernel and copy
  44 the parameters which are saved in the persistent area to the required address.
  45 If a valid uImage is not found at the defined location, U-Boot will be
  46 booted instead.
  47
  48 It is required to implement a custom mechanism to select if SPL loads U-Boot
  49 or another image.
  50
  51 The value of a GPIO is a simple way to operate the selection, as well as
  52 reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
  53
  54 Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
  55 SPL that U-Boot is not the only available image that SPL is able to start.
  56
  57 Configuration
  58 ----------------------------
  59 CONFIG_CMD_SPL          Enable the "spl export" command.
  60                         The command "spl export" is then available in U-Boot
  61                         mode
  62 CONFIG_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
  66 CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored
  67
  68 CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved.
  69
  70 CONFIG_CMD_SPL_WRITE_SIZE       Size of the parameters area to be copied
  71
  72 CONFIG_SPL_OS_BOOT      Activate Falcon Mode.
  73
  74 Function that a board must implement
  75 ------------------------------------
  76
  77 void spl_board_prepare_for_linux(void) : optional
  78         Called from SPL before starting the kernel
  79
  80 spl_start_uboot() : required
  81                 Returns "0" if SPL should start the kernel, "1" if U-Boot
  82                 must be started.
  83
  84 Environment variables
  85 ---------------------
  86
  87 A board may chose to look at the environment for decisions about falcon
  88 mode.  In this case the following variables may be supported:
  89
  90 boot_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)
  93 falcon_args_file :      Filename to load as the 'args' portion of falcon mode
  94                         rather than the hard-coded value.
  95 falcon_image_file :     Filename to load as the OS image portion of falcon
  96                         mode rather than the hard-coded value.
  97
  98 Using spl command
  99 -----------------
 100
 101 spl - SPL configuration
 102
 103 Usage:
 104
 105 spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
 106
 107 img             : "atags" or "fdt"
 108 kernel_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.
 110 initrd_addr     : Address of initial ramdisk
 111                   can be set to "-" if fdt_addr without initrd_addr is used
 112 fdt_addr        : in case of fdt, the address of the device tree.
 113
 114 The spl export command does not write to a storage media. The user is
 115 responsible to transfer the gathered information (assembled ATAGS list
 116 or prepared FDT) from temporary storage in RAM into persistant storage
 117 after each run of 'spl export'. Unfortunately the position of temporary
 118 storage can not be predicted nor provided at commandline, it depends
 119 highly on your system setup and your provided data (ATAGS or FDT).
 120 However at the end of an succesful 'spl export' run it will print the
 121 RAM address of temporary storage. The RAM address of FDT will also be
 122 set in the environment variable 'fdtargsaddr', the new length of the
 123 prepared FDT will be set in the environment variable 'fdtargslen'.
 124 These environment variables can be used in scripts for writing updated
 125 FDT to persistent storage.
 126
 127 Now the user have to save the generated BLOB from that printed address
 128 to the pre-defined address in persistent storage
 129 (CONFIG_CMD_SPL_NAND_OFS in case of NAND).
 130 The following example shows how to prepare the data for Falcon Mode on
 131 twister board with ATAGS BLOB.
 132
 133 The "spl export" command is prepared to work with ATAGS and FDT. However,
 134 using FDT is at the moment untested. The ppc port (see a3m071 example
 135 later) prepares the fdt blob with the fdt command instead.
 136
 137
 138 Usage on the twister board:
 139 --------------------------------
 140
 141 Using mtd names with the following (default) configuration
 142 for mtdparts:
 143
 144 device nand0 <omap2-nand.0>, # parts = 9
 145  #: name                size            offset          mask_flags
 146  0: MLO                 0x00080000      0x00000000      0
 147  1: u-boot              0x00100000      0x00080000      0
 148  2: env1                0x00040000      0x00180000      0
 149  3: env2                0x00040000      0x001c0000      0
 150  4: kernel              0x00600000      0x00200000      0
 151  5: bootparms           0x00040000      0x00800000      0
 152  6: splashimg           0x00200000      0x00840000      0
 153  7: mini                0x02800000      0x00a40000      0
 154  8: rootfs              0x1cdc0000      0x03240000      0
 155
 156
 157 twister => nand read 82000000 kernel
 158
 159 NAND read: device 0 offset 0x200000, size 0x600000
 160  6291456 bytes read: OK
 161
 162 Now the kernel is in RAM at address 0x82000000
 163
 164 twister => spl export atags 0x82000000
 165 ## Booting kernel from Legacy Image at 82000000 ...
 166    Image Name:   Linux-3.5.0-rc4-14089-gda0b7f4
 167    Image Type:   ARM Linux Kernel Image (uncompressed)
 168    Data Size:    3654808 Bytes = 3.5 MiB
 169    Load Address: 80008000
 170    Entry Point:  80008000
 171    Verifying Checksum ... OK
 172    Loading Kernel Image ... OK
 173 OK
 174 cmdline subcommand not supported
 175 bdt subcommand not supported
 176 Argument image is now in RAM at: 0x80000100
 177
 178 The result can be checked at address 0x80000100:
 179
 180 twister => md 0x80000100
 181 80000100: 00000005 54410001 00000000 00000000    ......AT........
 182 80000110: 00000000 00000067 54410009 746f6f72    ....g.....ATroot
 183 80000120: 65642f3d 666e2f76 77722073 73666e20    =/dev/nfs rw nfs
 184
 185 The parameters generated with this step can be saved into NAND at the offset
 186 0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
 187
 188 nand erase.part bootparms
 189 nand write 0x80000100 bootparms 0x4000
 190
 191 Now the parameters are stored into the NAND flash at the address
 192 CONFIG_CMD_SPL_NAND_OFS (=0x800000).
 193
 194 Next time, the board can be started into Falcon Mode moving the
 195 setting the gpio (on twister gpio 55 is used) to kernel mode.
 196
 197 The kernel is loaded directly by the SPL without passing through U-Boot.
 198
 199 Example with FDT: a3m071 board
 200 -------------------------------
 201
 202 To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
 203 prepard/patched first. U-Boot usually inserts some dynamic values into
 204 the DT binary (blob), e.g. autodetected memory size, MAC addresses,
 205 clocks speeds etc. To generate this patched DT blob, you can use
 206 the following command:
 207
 208 1. Load fdt blob to SDRAM:
 209 => tftp 1800000 a3m071/a3m071.dtb
 210
 211 2. Set bootargs as desired for Linux booting (e.g. flash_mtd):
 212 => run mtdargs addip2 addtty
 213
 214 3. Use "fdt" commands to patch the DT blob:
 215 => fdt addr 1800000
 216 => fdt boardsetup
 217 => fdt chosen
 218
 219 4. Display patched DT blob (optional):
 220 => fdt print
 221
 222 5. Save fdt to NOR flash:
 223 => erase fc060000 fc07ffff
 224 => cp.b 1800000 fc060000 10000
 225 ...
 226
 227
 228 Falcon Mode was presented at the RMLL 2012. Slides are available at:
 229
 230 http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf