U-Boot Falcon Mode ==================== Introduction ------------ This document provides an overview of how to add support for Falcon Mode to a board. Falcon Mode is introduced to speed up the booting process, allowing to boot a Linux kernel (or whatever image) without a full blown U-Boot. Falcon Mode relies on the SPL framework. In fact, to make booting faster, U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot image. In most implementations, SPL is used to start U-Boot when booting from a mass storage, such as NAND or SD-Card. SPL has now support for other media, and can generally be seen as a way to start an image performing the minimum required initialization. SPL mainly initializes the RAM controller, and then copies U-Boot image into the memory. The Falcon Mode extends this way allowing to start the Linux kernel directly from SPL. A new command is added to U-Boot to prepare the parameters that SPL must pass to the kernel, using ATAGS or Device Tree. In normal mode, these parameters are generated each time before loading the kernel, passing to Linux the address in memory where the parameters can be read. With Falcon Mode, this snapshot can be saved into persistent storage and SPL is informed to load it before running the kernel. To boot the kernel, these steps under a Falcon-aware U-Boot are required: 1. Boot the board into U-Boot. After loading the desired legacy-format kernel image into memory (and DT as well, if used), use the "spl export" command to generate the kernel parameters area or the DT. U-Boot runs as when it boots the kernel, but stops before passing the control to the kernel. 2. Save the prepared snapshot into persistent media. The address where to save it must be configured into board configuration file (CONFIG_CMD_SPL_NAND_OFS for NAND). 3. Boot the board into Falcon Mode. SPL will load the kernel and copy the parameters which are saved in the persistent area to the required address. If a valid uImage is not found at the defined location, U-Boot will be booted instead. It is required to implement a custom mechanism to select if SPL loads U-Boot or another image. The value of a GPIO is a simple way to operate the selection, as well as reading a character from the SPL console if CONFIG_SPL_CONSOLE is set. Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells SPL that U-Boot is not the only available image that SPL is able to start. Configuration ---------------------------- CONFIG_CMD_SPL Enable the "spl export" command. The command "spl export" is then available in U-Boot mode CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be copied by SPL. In most cases, it is <start_of_ram> + 0x100 CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved. CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied CONFIG_SPL_OS_BOOT Activate Falcon Mode. Function that a board must implement ------------------------------------ void spl_board_prepare_for_linux(void) : optional Called from SPL before starting the kernel spl_start_uboot() : required Returns "0" if SPL should start the kernel, "1" if U-Boot must be started. Environment variables --------------------- A board may chose to look at the environment for decisions about falcon mode. In this case the following variables may be supported: boot_os : Set to yes/Yes/true/True/1 to enable booting to OS, any other value to fall back to U-Boot (including unset) falcon_args_file : Filename to load as the 'args' portion of falcon mode rather than the hard-coded value. falcon_image_file : Filename to load as the OS image portion of falcon mode rather than the hard-coded value. Using spl command ----------------- spl - SPL configuration Usage: spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ] img : "atags" or "fdt" kernel_addr : kernel is loaded as part of the boot process, but it is not started. This is the address where a kernel image is stored. initrd_addr : Address of initial ramdisk can be set to "-" if fdt_addr without initrd_addr is used fdt_addr : in case of fdt, the address of the device tree. The spl export command does not write to a storage media. The user is responsible to transfer the gathered information (assembled ATAGS list or prepared FDT) from temporary storage in RAM into persistant storage after each run of 'spl export'. Unfortunately the position of temporary storage can not be predicted nor provided at commandline, it depends highly on your system setup and your provided data (ATAGS or FDT). However at the end of an succesful 'spl export' run it will print the RAM address of temporary storage. Now the user have to save the generated BLOB from that printed address to the pre-defined address in persistent storage (CONFIG_CMD_SPL_NAND_OFS in case of NAND). The following example shows how to prepare the data for Falcon Mode on twister board with ATAGS BLOB. The "spl export" command is prepared to work with ATAGS and FDT. However, using FDT is at the moment untested. The ppc port (see a3m071 example later) prepares the fdt blob with the fdt command instead. Usage on the twister board: -------------------------------- Using mtd names with the following (default) configuration for mtdparts: device nand0 <omap2-nand.0>, # parts = 9 #: name size offset mask_flags 0: MLO 0x00080000 0x00000000 0 1: u-boot 0x00100000 0x00080000 0 2: env1 0x00040000 0x00180000 0 3: env2 0x00040000 0x001c0000 0 4: kernel 0x00600000 0x00200000 0 5: bootparms 0x00040000 0x00800000 0 6: splashimg 0x00200000 0x00840000 0 7: mini 0x02800000 0x00a40000 0 8: rootfs 0x1cdc0000 0x03240000 0 twister => nand read 82000000 kernel NAND read: device 0 offset 0x200000, size 0x600000 6291456 bytes read: OK Now the kernel is in RAM at address 0x82000000 twister => spl export atags 0x82000000 ## Booting kernel from Legacy Image at 82000000 ... Image Name: Linux-3.5.0-rc4-14089-gda0b7f4 Image Type: ARM Linux Kernel Image (uncompressed) Data Size: 3654808 Bytes = 3.5 MiB Load Address: 80008000 Entry Point: 80008000 Verifying Checksum ... OK Loading Kernel Image ... OK OK cmdline subcommand not supported bdt subcommand not supported Argument image is now in RAM at: 0x80000100 The result can be checked at address 0x80000100: twister => md 0x80000100 80000100: 00000005 54410001 00000000 00000000 ......AT........ 80000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot 80000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs The parameters generated with this step can be saved into NAND at the offset 0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS) nand erase.part bootparms nand write 0x80000100 bootparms 0x4000 Now the parameters are stored into the NAND flash at the address CONFIG_CMD_SPL_NAND_OFS (=0x800000). Next time, the board can be started into Falcon Mode moving the setting the gpio (on twister gpio 55 is used) to kernel mode. The kernel is loaded directly by the SPL without passing through U-Boot. Example with FDT: a3m071 board ------------------------------- To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get prepard/patched first. U-Boot usually inserts some dynamic values into the DT binary (blob), e.g. autodetected memory size, MAC addresses, clocks speeds etc. To generate this patched DT blob, you can use the following command: 1. Load fdt blob to SDRAM: => tftp 1800000 a3m071/a3m071.dtb 2. Set bootargs as desired for Linux booting (e.g. flash_mtd): => run mtdargs addip2 addtty 3. Use "fdt" commands to patch the DT blob: => fdt addr 1800000 => fdt boardsetup => fdt chosen 4. Display patched DT blob (optional): => fdt print 5. Save fdt to NOR flash: => erase fc060000 fc07ffff => cp.b 1800000 fc060000 10000 ... Falcon Mode was presented at the RMLL 2012. Slides are available at: http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf