[U-Boot] [PATCH v4 1/3] Add README for the "Falcon" mode

[Vikram Narayanan]

Hi Stefano,

Sorry for bumping in at v4. Below are some of my comments.

On 11/23/2012 9:01 PM, Stefano Babic wrote:
> Simple howto to add support to a board
> for booting the kernel from SPL ("Falcon" mode).
>
> Signed-off-by: Stefano Babic<sbabic at denx.de>
> ---
> Changes in v4:
> - fix capitalization, styling, in spl help (Andreas Biessmann)
> - move CONFIG_SPL_OS_BOOT before function in doc (Andreas Biessmann)
>
<snip>
> diff --git a/doc/README.falcon b/doc/README.falcon
> new file mode 100644
> index 0000000..1c041ea
> --- /dev/null
> +++ b/doc/README.falcon
> @@ -0,0 +1,173 @@
> +U-Boot Falcon Mode
> +====================
> +
> +Introduction
> +------------
> +
> +This document provides an overview how to add support for Falcon Mode

s/an overview/an overview of/

> +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 be generalized seen as a way to start an image performing the minimum

Rephrase the above line as,
and can generally be seen ....

> +required initialization. SPL initializes mainly the RAM controller, and after
> +that copies U-Boot image into the memory. The Falcon Mode extends this way

Rephrase ..

SPL mainly initializes the RAM controller, and then copies U-boot image 
to the main memory.

> +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.
> +
> +Falcon Mode adds a command under U-Boot to reuse all code responsible to prepare

Already it is mentioned that 'a new command is added'. The above line is 
redundant then. Please rephrase it.

> +the interface with the kernel. In usual U-Boot systems, 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.
> +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 area to the required address.

copy the parameters which is saved in the persistent media to the ....

> +
> +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

A space in between is and '<'

> +
> +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.

Can the above be renamed to include the word 'PARAMS'? But becomes 
lengthy. Any suggestions?

> +
> +CONFIG_CMD_SPL_WRITE_SIZE 	Size of the parameters area to be copied
> +
> +CONFIG_SPL_OS_BOOT	Activate Falcon Mode.
> +			A board should implement the following functions:

Is there a big difference between the line above and the one below?

> +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 starts the kernel, "1" if U-Boot
> +		must be started.
> +
> +
> +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 img is used
> +fdt_addr	: in case of fdt, the address of the device tree.
> +
> +The spl puts its result at a self gained position. The position is defined at compile
> +time or when generating the uImage but not at command line for 'spl export'
> +(see spl_export(): gd->bd->bi_boot_params vs. images.ft_addr).
> +
> +spl export' does not write directly to a storage media. This command is intended to save
> +the prepared information in RAM.  This information must then be transferred to a final destination

^ [1]
Final destination? How is it getting transferred?
Shall lines/content [1] and [2] merged?

> +where the SPL will load it, that is defined at compile time
> +(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
> +
> +The user is responsible to save the data into the required media, as described
> +on the following example.

^ [2]

> +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

If the offset is known at compile time, why should the end user use the 
above commands to write it? Can't it be automated? Just an idea.

> +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.
> +
> +Falcon Mode was presented at the RMLL 2012. Slides are available at:
> +
> +http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf

Regards,
Vikram