README.falcon 8.5 KB

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  1. U-Boot Falcon Mode
  2. ====================
  3. Introduction
  4. ------------
  5. This document provides an overview of how to add support for Falcon Mode
  6. to a board.
  7. Falcon Mode is introduced to speed up the booting process, allowing
  8. to boot a Linux kernel (or whatever image) without a full blown U-Boot.
  9. Falcon Mode relies on the SPL framework. In fact, to make booting faster,
  10. U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
  11. image. In most implementations, SPL is used to start U-Boot when booting from
  12. a mass storage, such as NAND or SD-Card. SPL has now support for other media,
  13. and can generally be seen as a way to start an image performing the minimum
  14. required initialization. SPL mainly initializes the RAM controller, and then
  15. copies U-Boot image into the memory.
  16. The Falcon Mode extends this way allowing to start the Linux kernel directly
  17. from SPL. A new command is added to U-Boot to prepare the parameters that SPL
  18. must pass to the kernel, using ATAGS or Device Tree.
  19. In normal mode, these parameters are generated each time before
  20. loading the kernel, passing to Linux the address in memory where
  21. the parameters can be read.
  22. With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
  23. informed to load it before running the kernel.
  24. To boot the kernel, these steps under a Falcon-aware U-Boot are required:
  25. 1. Boot the board into U-Boot.
  26. After loading the desired legacy-format kernel image into memory (and DT as
  27. well, if used), use the "spl export" command to generate the kernel parameters
  28. area or the DT. U-Boot runs as when it boots the kernel, but stops before
  29. passing the control to the kernel.
  30. 2. Save the prepared snapshot into persistent media.
  31. The address where to save it must be configured into board configuration
  32. file (CONFIG_CMD_SPL_NAND_OFS for NAND).
  33. 3. Boot the board into Falcon Mode. SPL will load the kernel and copy
  34. the parameters which are saved in the persistent area to the required address.
  35. If a valid uImage is not found at the defined location, U-Boot will be
  36. booted instead.
  37. It is required to implement a custom mechanism to select if SPL loads U-Boot
  38. or another image.
  39. The value of a GPIO is a simple way to operate the selection, as well as
  40. reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
  41. Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
  42. SPL that U-Boot is not the only available image that SPL is able to start.
  43. Configuration
  44. ----------------------------
  45. CONFIG_CMD_SPL Enable the "spl export" command.
  46. The command "spl export" is then available in U-Boot
  47. mode
  48. CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be
  49. copied by SPL.
  50. In most cases, it is <start_of_ram> + 0x100
  51. CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored
  52. CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved.
  53. CONFIG_CMD_SPL_NOR_OFS Offset in NOR where the parameters area was saved.
  54. CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied
  55. CONFIG_SPL_OS_BOOT Activate Falcon Mode.
  56. Function that a board must implement
  57. ------------------------------------
  58. void spl_board_prepare_for_linux(void) : optional
  59. Called from SPL before starting the kernel
  60. spl_start_uboot() : required
  61. Returns "0" if SPL should start the kernel, "1" if U-Boot
  62. must be started.
  63. Environment variables
  64. ---------------------
  65. A board may chose to look at the environment for decisions about falcon
  66. mode. In this case the following variables may be supported:
  67. boot_os : Set to yes/Yes/true/True/1 to enable booting to OS,
  68. any other value to fall back to U-Boot (including
  69. unset)
  70. falcon_args_file : Filename to load as the 'args' portion of falcon mode
  71. rather than the hard-coded value.
  72. falcon_image_file : Filename to load as the OS image portion of falcon
  73. mode rather than the hard-coded value.
  74. Using spl command
  75. -----------------
  76. spl - SPL configuration
  77. Usage:
  78. spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
  79. img : "atags" or "fdt"
  80. kernel_addr : kernel is loaded as part of the boot process, but it is not started.
  81. This is the address where a kernel image is stored.
  82. initrd_addr : Address of initial ramdisk
  83. can be set to "-" if fdt_addr without initrd_addr is used
  84. fdt_addr : in case of fdt, the address of the device tree.
  85. The spl export command does not write to a storage media. The user is
  86. responsible to transfer the gathered information (assembled ATAGS list
  87. or prepared FDT) from temporary storage in RAM into persistant storage
  88. after each run of 'spl export'. Unfortunately the position of temporary
  89. storage can not be predicted nor provided at commandline, it depends
  90. highly on your system setup and your provided data (ATAGS or FDT).
  91. However at the end of an succesful 'spl export' run it will print the
  92. RAM address of temporary storage. The RAM address of FDT will also be
  93. set in the environment variable 'fdtargsaddr', the new length of the
  94. prepared FDT will be set in the environment variable 'fdtargslen'.
  95. These environment variables can be used in scripts for writing updated
  96. FDT to persistent storage.
  97. Now the user have to save the generated BLOB from that printed address
  98. to the pre-defined address in persistent storage
  99. (CONFIG_CMD_SPL_NAND_OFS in case of NAND).
  100. The following example shows how to prepare the data for Falcon Mode on
  101. twister board with ATAGS BLOB.
  102. The "spl export" command is prepared to work with ATAGS and FDT. However,
  103. using FDT is at the moment untested. The ppc port (see a3m071 example
  104. later) prepares the fdt blob with the fdt command instead.
  105. Usage on the twister board:
  106. --------------------------------
  107. Using mtd names with the following (default) configuration
  108. for mtdparts:
  109. device nand0 <omap2-nand.0>, # parts = 9
  110. #: name size offset mask_flags
  111. 0: MLO 0x00080000 0x00000000 0
  112. 1: u-boot 0x00100000 0x00080000 0
  113. 2: env1 0x00040000 0x00180000 0
  114. 3: env2 0x00040000 0x001c0000 0
  115. 4: kernel 0x00600000 0x00200000 0
  116. 5: bootparms 0x00040000 0x00800000 0
  117. 6: splashimg 0x00200000 0x00840000 0
  118. 7: mini 0x02800000 0x00a40000 0
  119. 8: rootfs 0x1cdc0000 0x03240000 0
  120. twister => nand read 82000000 kernel
  121. NAND read: device 0 offset 0x200000, size 0x600000
  122. 6291456 bytes read: OK
  123. Now the kernel is in RAM at address 0x82000000
  124. twister => spl export atags 0x82000000
  125. ## Booting kernel from Legacy Image at 82000000 ...
  126. Image Name: Linux-3.5.0-rc4-14089-gda0b7f4
  127. Image Type: ARM Linux Kernel Image (uncompressed)
  128. Data Size: 3654808 Bytes = 3.5 MiB
  129. Load Address: 80008000
  130. Entry Point: 80008000
  131. Verifying Checksum ... OK
  132. Loading Kernel Image ... OK
  133. OK
  134. cmdline subcommand not supported
  135. bdt subcommand not supported
  136. Argument image is now in RAM at: 0x80000100
  137. The result can be checked at address 0x80000100:
  138. twister => md 0x80000100
  139. 80000100: 00000005 54410001 00000000 00000000 ......AT........
  140. 80000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot
  141. 80000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs
  142. The parameters generated with this step can be saved into NAND at the offset
  143. 0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
  144. nand erase.part bootparms
  145. nand write 0x80000100 bootparms 0x4000
  146. Now the parameters are stored into the NAND flash at the address
  147. CONFIG_CMD_SPL_NAND_OFS (=0x800000).
  148. Next time, the board can be started into Falcon Mode moving the
  149. setting the gpio (on twister gpio 55 is used) to kernel mode.
  150. The kernel is loaded directly by the SPL without passing through U-Boot.
  151. Example with FDT: a3m071 board
  152. -------------------------------
  153. To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
  154. prepard/patched first. U-Boot usually inserts some dynamic values into
  155. the DT binary (blob), e.g. autodetected memory size, MAC addresses,
  156. clocks speeds etc. To generate this patched DT blob, you can use
  157. the following command:
  158. 1. Load fdt blob to SDRAM:
  159. => tftp 1800000 a3m071/a3m071.dtb
  160. 2. Set bootargs as desired for Linux booting (e.g. flash_mtd):
  161. => run mtdargs addip2 addtty
  162. 3. Use "fdt" commands to patch the DT blob:
  163. => fdt addr 1800000
  164. => fdt boardsetup
  165. => fdt chosen
  166. 4. Display patched DT blob (optional):
  167. => fdt print
  168. 5. Save fdt to NOR flash:
  169. => erase fc060000 fc07ffff
  170. => cp.b 1800000 fc060000 10000
  171. ...
  172. Falcon Mode was presented at the RMLL 2012. Slides are available at:
  173. http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf