Generic TPL framework
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=====================
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Overview
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--------
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TPL---Third Program Loader.
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Due to the SPL on some boards(powerpc mpc85xx) has a size limit and cannot
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be compatible with all the external device(e.g. DDR). So add a tertiary
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program loader (TPL) to enable a loader stub loaded by the code from the
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SPL. It loads the final uboot image into DDR, then jump to it to begin
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execution. Now, only the powerpc mpc85xx has this requirement and will
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implemente it.
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Keep consistent with SPL, with this framework almost all source files for a
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board can be reused. No code duplication or symlinking is necessary anymore.
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How it works
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------------
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There has been a directory $(srctree)/spl which contains only a Makefile. The
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Makefile is shared by SPL and TPL.
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The object files are built separately for SPL/TPL and placed in the
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directory spl/tpl. The final binaries which are generated are
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u-boot-{spl|tpl}, u-boot-{spl|tpl}.bin and u-boot-{spl|tpl}.map.
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During the TPL build a variable named CONFIG_TPL_BUILD is exported in the
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make environment and also appended to CPPFLAGS with -DCONFIG_TPL_BUILD.
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The SPL options are shared by SPL and TPL, the board config file should
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determine which SPL options to choose based on whether CONFIG_TPL_BUILD
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is set. Source files can be compiled for TPL with options choosed in the
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board config file.
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For example:
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spl/Makefile:
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LIBS-$(CONFIG_SPL_LIBCOMMON_SUPPORT) += common/libcommon.o
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CONFIG_SPL_LIBCOMMON_SUPPORT is defined in board config file:
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#ifdef CONFIG_TPL_BUILD
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#define CONFIG_SPL_LIBCOMMON_SUPPORT
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#endif
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