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 231
