Generic SPL framework
=====================

Overview
--------

To unify all existing implementations for a secondary program loader (SPL)
and to allow simply adding of new implementations this generic SPL framework
has been created. With this framework almost all source files for a board
can be reused. No code duplication or symlinking is necessary anymore.


How it works
------------

The object files for SPL are built separately and placed in the "spl" directory.
The final binaries which are generated are u-boot-spl, u-boot-spl.bin and
u-boot-spl.map.

A config option named CONFIG_SPL_BUILD is enabled by Kconfig for SPL.
Source files can therefore be compiled for SPL with different settings.

For example::

   ifeq ($(CONFIG_SPL_BUILD),y)
   obj-y += board_spl.o
   else
   obj-y += board.o
   endif

   obj-$(CONFIG_SPL_BUILD) += foo.o

   #ifdef CONFIG_SPL_BUILD
           foo();
   #endif


The building of SPL images can be enabled by CONFIG_SPL option in Kconfig.

Because SPL images normally have a different text base, one has to be
configured by defining CONFIG_SPL_TEXT_BASE. The linker script has to be
defined with CONFIG_SPL_LDSCRIPT.

To support generic U-Boot libraries and drivers in the SPL binary one can
optionally define CONFIG_SPL_XXX_SUPPORT. Currently following options
are supported:

CONFIG_SPL_LIBCOMMON_SUPPORT (common/libcommon.o)
CONFIG_SPL_LIBDISK_SUPPORT (disk/libdisk.o)
CONFIG_SPL_I2C (drivers/i2c/libi2c.o)
CONFIG_SPL_GPIO (drivers/gpio/libgpio.o)
CONFIG_SPL_MMC (drivers/mmc/libmmc.o)
CONFIG_SPL_SERIAL (drivers/serial/libserial.o)
CONFIG_SPL_SPI_FLASH_SUPPORT (drivers/mtd/spi/libspi_flash.o)
CONFIG_SPL_SPI (drivers/spi/libspi.o)
CONFIG_SPL_FS_FAT (fs/fat/libfat.o)
CONFIG_SPL_FS_EXT4
CONFIG_SPL_LIBGENERIC_SUPPORT (lib/libgeneric.o)
CONFIG_SPL_POWER (drivers/power/libpower.o)
CONFIG_SPL_NAND_SUPPORT (drivers/mtd/nand/raw/libnand.o)
CONFIG_SPL_DRIVERS_MISC (drivers/misc)
CONFIG_SPL_DMA (drivers/dma/libdma.o)
CONFIG_SPL_POST_MEM_SUPPORT (post/drivers/memory.o)
CONFIG_SPL_NAND_LOAD (drivers/mtd/nand/raw/nand_spl_load.o)
CONFIG_SPL_SPI_LOAD (drivers/mtd/spi/spi_spl_load.o)
CONFIG_SPL_RAM_DEVICE (common/spl/spl.c)
CONFIG_SPL_WATCHDOG (drivers/watchdog/libwatchdog.o)

Adding SPL-specific code
------------------------

To check whether a feature is enabled, use CONFIG_IS_ENABLED()::

  if (CONFIG_IS_ENABLED(CLK))
      ...

This checks CONFIG_CLK for the main build, CONFIG_SPL_CLK for the SPL build,
CONFIG_TPL_CLK for the TPL build, etc.

U-Boot Boot Phases
------------------

U-Boot goes through the following boot phases where TPL, VPL, SPL are optional.
While many boards use SPL, less use TPL.

TPL
   Very early init, as tiny as possible. This loads SPL (or VPL if enabled).

VPL
   Optional verification step, which can select one of several SPL binaries,
   if A/B verified boot is enabled. Implementation of the VPL logic is
   work-in-progress. For now it just boots into SPL.

SPL
   Secondary program loader. Sets up SDRAM and loads U-Boot proper. It may also
   load other firmware components.

U-Boot
   U-Boot proper, containing the command line and boot logic.

Further usages of U-Boot SPL comprise:

* Launching BL31 of ARM Trusted Firmware which invokes main U-Boot as BL33
* launching EDK II
* launching Linux kernel
* launching RISC-V OpenSBI which invokes main U-Boot

Checking the boot phase
-----------------------

Use `spl_phase()` to find the current U-Boot phase, e.g. `PHASE_SPL`. You can
also find the previous and next phase and get the phase name.


Device tree
-----------
The U-Boot device tree is filtered by the fdtgrep tools during the build
process to generate a much smaller device tree used in SPL (spl/u-boot-spl.dtb)
with:

- the mandatory nodes (/alias, /chosen, /config)
- the nodes with one pre-relocation property:
  'bootph-all' or 'bootph-pre-ram'

fdtgrep is also used to remove:

- the properties defined in CONFIG_OF_SPL_REMOVE_PROPS
- all the pre-relocation properties
  ('bootph-all', 'bootph-pre-ram' (SPL), 'bootph-pre-sram' (TPL) and
  'bootph-verify' (TPL))

All the nodes remaining in the SPL devicetree are bound
(see doc/driver-model/design.rst).

NOTE: U-Boot migrated to a new schema for the u-boot,dm-* tags in 2023. Please
update to use the new bootph-* tags as described in the
doc/device-tree-bindings/bootph.yaml binding file.

Debugging
---------

When building SPL with DEBUG set you may also need to set CONFIG_PANIC_HANG
as in most cases do_reset is not defined within SPL.


Estimating stack usage
----------------------

With gcc 4.6 (and later) and the use of GNU cflow it is possible to estimate
stack usage at various points in run sequence of SPL.  The -fstack-usage option
to gcc will produce '.su' files (such as arch/arm/cpu/armv7/syslib.su) that
will give stack usage information and cflow can construct program flow.

Must have gcc 4.6 or later, which supports -fstack-usage:

#. Build normally
#. Perform the following shell command to generate a list of C files used in
   SPL:
#. `find spl -name '*.su' | sed -e 's:^spl/::' -e 's:[.]su$:.c:' > used-spl.list`
#. Execute cflow:
   `$ cflow --main=board_init_r $(cat used-spl.list) 2>&1 | $PAGER`

cflow will spit out a number of warnings as it does not parse
the config files and picks functions based on #ifdef.  Parsing the '.i'
files instead introduces another set of headaches.  These warnings are
not usually important to understanding the flow, however.