MULTI DTB FIT and SPL_MULTI_DTB_FIT
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The purpose of this feature is to enable U-Boot or the SPL to select its DTB
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from a FIT appended at the end of the binary.
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It comes in two flavors: U-Boot (CONFIG_MULTI_DTB_FIT) and SPL
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(CONFIG_SPL_MULTI_DTB_FIT).
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U-Boot flavor:
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Usually the DTB is selected by the SPL and passed down to U-Boot. But some
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platforms don't use the SPL. In this case MULTI_DTB_FIT can used to provide
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U-Boot with a choice of DTBs.
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The relevant DTBs are packed into a FIT (list provided by CONFIG__OF_LIST). The
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FIT is automatically generated at the end of the compilation and appended to
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u-boot.bin so that U-Boot can locate it and select the correct DTB from inside
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the FIT.
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The selection is done using board_fit_config_name_match() (same as what the SPL
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uses to select the DTB for U-Boot). The selection happens during fdtdec_setup()
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which is called during before relocation by board_init_f().
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SPL flavor:
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the SPL uses only a small subset of the DTB and it usually depends more
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on the SOC than on the board. So it's usually fine to include a DTB in the
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SPL that doesn't exactly match the board. There are howerver some cases
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where it's not possible. In the later case, in order to support multiple
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boards (or board revisions) with the same SPL binary, SPL_MULTI_DTB_FIT
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can be used.
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The relevant DTBs are packed into a FIT. This FIT is automatically generated
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at the end of the compilation, compressed and appended to u-boot-spl.bin, so
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that SPL can locate it and select the correct DTB from inside the FIT.
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CONFIG_SPL__OF_LIST is used to list the relevant DTBs.
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The compression stage is optional but reduces the impact on the size of the
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SPL. LZO and GZIP compressions are supported. By default, the area where the
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FIT is uncompressed is dynamicaly allocated but this behaviour can be changed
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for platforms that don't provide a HEAP big enough to contain the uncompressed
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FIT.
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The SPL uses board_fit_config_name_match() to find the correct DTB within the
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FIT (same as what the SPL uses to select the DTB for U-Boot).
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Uncompression and selection stages happen in fdtdec_setup() which is called
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during the early initialization stage of the SPL (spl_early_init() or
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spl_init())
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Impacts and performances (SPL flavor):
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The impact of this option is relatively small. Here are some numbers measured
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for a TI DRA72 platform:
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+----------+------------+-----------+------------+
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| size | size delta | SPL boot | boot time |
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| (bytes) | (bytes) | time (s) | delta (s) |
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+---------------------------+----------+------------+-----------+------------+
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| 1 DTB | | | | |
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+---------------------------+----------+------------+-----------+------------+
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| reference | 125305 | 0 | 1.389 | 0 |
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| LZO (dynamic allocation) | 125391 | 86 | 1.381 | -0.008 |
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+---------------------------+----------+------------+-----------+------------+
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| 4 DTBs (DRA7, DRA71, | | | | |
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| DRA72, DRA72 revC) | | | | |
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+---------------------------+----------+------------+-----------+------------+
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| LZO (dynamic allocation) | 125991 | 686 | 1.39 | 0.001 |
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| LZO (user defined area) | 125927 | 622 | 1.403 | 0.014 |
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| GZIP (user defined area) | 133880 | 8575 | 1.421 | 0.032 |
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| No compression (in place) | 137472 | 12167 | 1.412 | 0.023 |
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+---------------------------+----------+------------+-----------+------------+
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Note: SPL boot time is the time elapsed between the 'reset' command is entered
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and the time when the first U-Boot (not SPL) version string is displayed.
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