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 83 84Using spl command 85----------------- 86 87spl - SPL configuration 88 89Usage: 90 91spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ] 92 93img : "atags" or "fdt" 94kernel_addr : kernel is loaded as part of the boot process, but it is not started. 95 This is the address where a kernel image is stored. 96initrd_addr : Address of initial ramdisk 97 can be set to "-" if fdt_addr without initrd_addr is used 98fdt_addr : in case of fdt, the address of the device tree. 99 100The spl export command does not write to a storage media. The user is 101responsible to transfer the gathered information (assembled ATAGS list 102or prepared FDT) from temporary storage in RAM into persistant storage 103after each run of 'spl export'. Unfortunately the position of temporary 104storage can not be predicted nor provided at commandline, it depends 105highly on your system setup and your provided data (ATAGS or FDT). 106However at the end of an succesful 'spl export' run it will print the 107RAM address of temporary storage. 108Now the user have to save the generated BLOB from that printed address 109to the pre-defined address in persistent storage 110(CONFIG_CMD_SPL_NAND_OFS in case of NAND). 111The following example shows how to prepare the data for Falcon Mode on 112twister board with ATAGS BLOB. 113 114The "spl export" command is prepared to work with ATAGS and FDT. However, 115using FDT is at the moment untested. The ppc port (see a3m071 example 116later) prepares the fdt blob with the fdt command instead. 117 118 119Usage on the twister board: 120-------------------------------- 121 122Using mtd names with the following (default) configuration 123for mtdparts: 124 125device nand0 <omap2-nand.0>, # parts = 9 126 #: name size offset mask_flags 127 0: MLO 0x00080000 0x00000000 0 128 1: u-boot 0x00100000 0x00080000 0 129 2: env1 0x00040000 0x00180000 0 130 3: env2 0x00040000 0x001c0000 0 131 4: kernel 0x00600000 0x00200000 0 132 5: bootparms 0x00040000 0x00800000 0 133 6: splashimg 0x00200000 0x00840000 0 134 7: mini 0x02800000 0x00a40000 0 135 8: rootfs 0x1cdc0000 0x03240000 0 136 137 138twister => nand read 82000000 kernel 139 140NAND read: device 0 offset 0x200000, size 0x600000 141 6291456 bytes read: OK 142 143Now the kernel is in RAM at address 0x82000000 144 145twister => spl export atags 0x82000000 146## Booting kernel from Legacy Image at 82000000 ... 147 Image Name: Linux-3.5.0-rc4-14089-gda0b7f4 148 Image Type: ARM Linux Kernel Image (uncompressed) 149 Data Size: 3654808 Bytes = 3.5 MiB 150 Load Address: 80008000 151 Entry Point: 80008000 152 Verifying Checksum ... OK 153 Loading Kernel Image ... OK 154OK 155cmdline subcommand not supported 156bdt subcommand not supported 157Argument image is now in RAM at: 0x80000100 158 159The result can be checked at address 0x80000100: 160 161twister => md 0x80000100 16280000100: 00000005 54410001 00000000 00000000 ......AT........ 16380000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot 16480000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs 165 166The parameters generated with this step can be saved into NAND at the offset 1670x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS) 168 169nand erase.part bootparms 170nand write 0x80000100 bootparms 0x4000 171 172Now the parameters are stored into the NAND flash at the address 173CONFIG_CMD_SPL_NAND_OFS (=0x800000). 174 175Next time, the board can be started into Falcon Mode moving the 176setting the gpio (on twister gpio 55 is used) to kernel mode. 177 178The kernel is loaded directly by the SPL without passing through U-Boot. 179 180Example with FDT: a3m071 board 181------------------------------- 182 183To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get 184prepard/patched first. U-Boot usually inserts some dynamic values into 185the DT binary (blob), e.g. autodetected memory size, MAC addresses, 186clocks speeds etc. To generate this patched DT blob, you can use 187the following command: 188 1891. Load fdt blob to SDRAM: 190=> tftp 1800000 a3m071/a3m071.dtb 191 1922. Set bootargs as desired for Linux booting (e.g. flash_mtd): 193=> run mtdargs addip2 addtty 194 1953. Use "fdt" commands to patch the DT blob: 196=> fdt addr 1800000 197=> fdt boardsetup 198=> fdt chosen 199 2004. Display patched DT blob (optional): 201=> fdt print 202 2035. Save fdt to NOR flash: 204=> erase fc060000 fc07ffff 205=> cp.b 1800000 fc060000 10000 206... 207 208 209Falcon Mode was presented at the RMLL 2012. Slides are available at: 210 211http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf 212