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. 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. 44 45It is required to implement a custom mechanism to select if SPL loads U-Boot 46or another image. 47 48The value of a GPIO is a simple way to operate the selection, as well as 49reading a character from the SPL console if CONFIG_SPL_CONSOLE is set. 50 51Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells 52SPL that U-Boot is not the only available image that SPL is able to start. 53 54Configuration 55---------------------------- 56CONFIG_CMD_SPL Enable the "spl export" command. 57 The command "spl export" is then available in U-Boot 58 mode 59CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be 60 copied by SPL. 61 In most cases, it is <start_of_ram> + 0x100 62 63CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored 64 65CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved. 66 67CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied 68 69CONFIG_SPL_OS_BOOT Activate Falcon Mode. 70 71Function that a board must implement 72------------------------------------ 73 74void spl_board_prepare_for_linux(void) : optional 75 Called from SPL before starting the kernel 76 77spl_start_uboot() : required 78 Returns "0" if SPL should start the kernel, "1" if U-Boot 79 must be started. 80 81 82Using spl command 83----------------- 84 85spl - SPL configuration 86 87Usage: 88 89spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ] 90 91img : "atags" or "fdt" 92kernel_addr : kernel is loaded as part of the boot process, but it is not started. 93 This is the address where a kernel image is stored. 94initrd_addr : Address of initial ramdisk 95 can be set to "-" if fdt_addr without initrd_addr is used 96fdt_addr : in case of fdt, the address of the device tree. 97 98The spl export command does not write to a storage media. The user is 99responsible to transfer the gathered information (assembled ATAGS list 100or prepared FDT) from temporary storage in RAM into persistant storage 101after each run of 'spl export'. Unfortunately the position of temporary 102storage can not be predicted nor provided at commandline, it depends 103highly on your system setup and your provided data (ATAGS or FDT). 104However at the end of an succesful 'spl export' run it will print the 105RAM address of temporary storage. 106Now the user have to save the generated BLOB from that printed address 107to the pre-defined address in persistent storage 108(CONFIG_CMD_SPL_NAND_OFS in case of NAND). 109The following example shows how to prepare the data for Falcon Mode on 110twister board with ATAGS BLOB. 111 112The "spl export" command is prepared to work with ATAGS and FDT. However, 113using FDT is at the moment untested. The ppc port (see a3m071 example 114later) prepares the fdt blob with the fdt command instead. 115 116 117Usage on the twister board: 118-------------------------------- 119 120Using mtd names with the following (default) configuration 121for mtdparts: 122 123device nand0 <omap2-nand.0>, # parts = 9 124 #: name size offset mask_flags 125 0: MLO 0x00080000 0x00000000 0 126 1: u-boot 0x00100000 0x00080000 0 127 2: env1 0x00040000 0x00180000 0 128 3: env2 0x00040000 0x001c0000 0 129 4: kernel 0x00600000 0x00200000 0 130 5: bootparms 0x00040000 0x00800000 0 131 6: splashimg 0x00200000 0x00840000 0 132 7: mini 0x02800000 0x00a40000 0 133 8: rootfs 0x1cdc0000 0x03240000 0 134 135 136twister => nand read 82000000 kernel 137 138NAND read: device 0 offset 0x200000, size 0x600000 139 6291456 bytes read: OK 140 141Now the kernel is in RAM at address 0x82000000 142 143twister => spl export atags 0x82000000 144## Booting kernel from Legacy Image at 82000000 ... 145 Image Name: Linux-3.5.0-rc4-14089-gda0b7f4 146 Image Type: ARM Linux Kernel Image (uncompressed) 147 Data Size: 3654808 Bytes = 3.5 MiB 148 Load Address: 80008000 149 Entry Point: 80008000 150 Verifying Checksum ... OK 151 Loading Kernel Image ... OK 152OK 153cmdline subcommand not supported 154bdt subcommand not supported 155Argument image is now in RAM at: 0x80000100 156 157The result can be checked at address 0x80000100: 158 159twister => md 0x80000100 16080000100: 00000005 54410001 00000000 00000000 ......AT........ 16180000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot 16280000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs 163 164The parameters generated with this step can be saved into NAND at the offset 1650x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS) 166 167nand erase.part bootparms 168nand write 0x80000100 bootparms 0x4000 169 170Now the parameters are stored into the NAND flash at the address 171CONFIG_CMD_SPL_NAND_OFS (=0x800000). 172 173Next time, the board can be started into Falcon Mode moving the 174setting the gpio (on twister gpio 55 is used) to kernel mode. 175 176The kernel is loaded directly by the SPL without passing through U-Boot. 177 178Example with FDT: a3m071 board 179------------------------------- 180 181To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get 182prepard/patched first. U-Boot usually inserts some dynamic values into 183the DT binary (blob), e.g. autodetected memory size, MAC addresses, 184clocks speeds etc. To generate this patched DT blob, you can use 185the following command: 186 1871. Load fdt blob to SDRAM: 188=> tftp 1800000 a3m071/a3m071.dtb 189 1902. Set bootargs as desired for Linux booting (e.g. flash_mtd): 191=> run mtdargs addip2 addtty 192 1933. Use "fdt" commands to patch the DT blob: 194=> fdt addr 1800000 195=> fdt boardsetup 196=> fdt chosen 197 1984. Display patched DT blob (optional): 199=> fdt print 200 2015. Save fdt to NOR flash: 202=> erase fc060000 fc07ffff 203=> cp.b 1800000 fc060000 10000 204... 205 206 207Falcon Mode was presented at the RMLL 2012. Slides are available at: 208 209http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf 210