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