doc: Convert SPL documentation to ReST

Move this documentation over to .rst format.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/spl.rst

@@ -0,0 +1,115 @@
+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)
+
+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:
+  'u-boot,dm-pre-reloc' or 'u-boot,dm-spl'
+
+fdtgrep is also used to remove:
+
+- the properties defined in CONFIG_OF_SPL_REMOVE_PROPS
+- all the pre-relocation properties
+  ('u-boot,dm-pre-reloc', 'u-boot,dm-spl' and 'u-boot,dm-tpl')
+
+All the nodes remaining in the SPL devicetree are bound
+(see doc/driver-model/design.rst).
+
+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.