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. 44If a valid uImage is not found at the defined location, U-Boot will be 45booted instead. 46 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 83Environment variables 84--------------------- 85 86A board may chose to look at the environment for decisions about falcon 87mode. In this case the following variables may be supported: 88 89boot_os : Set to yes/Yes/true/True/1 to enable booting to OS, 90 any other value to fall back to U-Boot (including 91 unset) 92 93Using spl command 94----------------- 95 96spl - SPL configuration 97 98Usage: 99 100spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ] 101 102img : "atags" or "fdt" 103kernel_addr : kernel is loaded as part of the boot process, but it is not started. 104 This is the address where a kernel image is stored. 105initrd_addr : Address of initial ramdisk 106 can be set to "-" if fdt_addr without initrd_addr is used 107fdt_addr : in case of fdt, the address of the device tree. 108 109The spl export command does not write to a storage media. The user is 110responsible to transfer the gathered information (assembled ATAGS list 111or prepared FDT) from temporary storage in RAM into persistant storage 112after each run of 'spl export'. Unfortunately the position of temporary 113storage can not be predicted nor provided at commandline, it depends 114highly on your system setup and your provided data (ATAGS or FDT). 115However at the end of an succesful 'spl export' run it will print the 116RAM address of temporary storage. 117Now the user have to save the generated BLOB from that printed address 118to the pre-defined address in persistent storage 119(CONFIG_CMD_SPL_NAND_OFS in case of NAND). 120The following example shows how to prepare the data for Falcon Mode on 121twister board with ATAGS BLOB. 122 123The "spl export" command is prepared to work with ATAGS and FDT. However, 124using FDT is at the moment untested. The ppc port (see a3m071 example 125later) prepares the fdt blob with the fdt command instead. 126 127 128Usage on the twister board: 129-------------------------------- 130 131Using mtd names with the following (default) configuration 132for mtdparts: 133 134device nand0 <omap2-nand.0>, # parts = 9 135 #: name size offset mask_flags 136 0: MLO 0x00080000 0x00000000 0 137 1: u-boot 0x00100000 0x00080000 0 138 2: env1 0x00040000 0x00180000 0 139 3: env2 0x00040000 0x001c0000 0 140 4: kernel 0x00600000 0x00200000 0 141 5: bootparms 0x00040000 0x00800000 0 142 6: splashimg 0x00200000 0x00840000 0 143 7: mini 0x02800000 0x00a40000 0 144 8: rootfs 0x1cdc0000 0x03240000 0 145 146 147twister => nand read 82000000 kernel 148 149NAND read: device 0 offset 0x200000, size 0x600000 150 6291456 bytes read: OK 151 152Now the kernel is in RAM at address 0x82000000 153 154twister => spl export atags 0x82000000 155## Booting kernel from Legacy Image at 82000000 ... 156 Image Name: Linux-3.5.0-rc4-14089-gda0b7f4 157 Image Type: ARM Linux Kernel Image (uncompressed) 158 Data Size: 3654808 Bytes = 3.5 MiB 159 Load Address: 80008000 160 Entry Point: 80008000 161 Verifying Checksum ... OK 162 Loading Kernel Image ... OK 163OK 164cmdline subcommand not supported 165bdt subcommand not supported 166Argument image is now in RAM at: 0x80000100 167 168The result can be checked at address 0x80000100: 169 170twister => md 0x80000100 17180000100: 00000005 54410001 00000000 00000000 ......AT........ 17280000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot 17380000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs 174 175The parameters generated with this step can be saved into NAND at the offset 1760x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS) 177 178nand erase.part bootparms 179nand write 0x80000100 bootparms 0x4000 180 181Now the parameters are stored into the NAND flash at the address 182CONFIG_CMD_SPL_NAND_OFS (=0x800000). 183 184Next time, the board can be started into Falcon Mode moving the 185setting the gpio (on twister gpio 55 is used) to kernel mode. 186 187The kernel is loaded directly by the SPL without passing through U-Boot. 188 189Example with FDT: a3m071 board 190------------------------------- 191 192To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get 193prepard/patched first. U-Boot usually inserts some dynamic values into 194the DT binary (blob), e.g. autodetected memory size, MAC addresses, 195clocks speeds etc. To generate this patched DT blob, you can use 196the following command: 197 1981. Load fdt blob to SDRAM: 199=> tftp 1800000 a3m071/a3m071.dtb 200 2012. Set bootargs as desired for Linux booting (e.g. flash_mtd): 202=> run mtdargs addip2 addtty 203 2043. Use "fdt" commands to patch the DT blob: 205=> fdt addr 1800000 206=> fdt boardsetup 207=> fdt chosen 208 2094. Display patched DT blob (optional): 210=> fdt print 211 2125. Save fdt to NOR flash: 213=> erase fc060000 fc07ffff 214=> cp.b 1800000 fc060000 10000 215... 216 217 218Falcon Mode was presented at the RMLL 2012. Slides are available at: 219 220http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf 221