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Diffstat (limited to 'doc/README.multi-dtb-fit')
| -rw-r--r-- | doc/README.multi-dtb-fit | 56 |
1 files changed, 53 insertions, 3 deletions
diff --git a/doc/README.multi-dtb-fit b/doc/README.multi-dtb-fit index 0d4f068bbf3..6cc4965beac 100644 --- a/doc/README.multi-dtb-fit +++ b/doc/README.multi-dtb-fit @@ -1,8 +1,11 @@ -MULTI DTB FIT +MULTI DTB FIT and SPL_MULTI_DTB_FIT -The purpose of this feature is to enable u-boot to select its DTB from a FIT -appended at the end of the binary. +The purpose of this feature is to enable U-Boot or the SPL to select its DTB +from a FIT appended at the end of the binary. +It comes in two flavors: U-Boot (CONFIG_MULTI_DTB_FIT) and SPL +(CONFIG_SPL_MULTI_DTB_FIT). +U-Boot flavor: Usually the DTB is selected by the SPL and passed down to U-Boot. But some platforms don't use the SPL. In this case MULTI_DTB_FIT can used to provide U-Boot with a choice of DTBs. @@ -13,3 +16,50 @@ the FIT. The selection is done using board_fit_config_name_match() (same as what the SPL uses to select the DTB for U-Boot). The selection happens during fdtdec_setup() which is called during before relocation by board_init_f(). + +SPL flavor: +the SPL uses only a small subset of the DTB and it usually depends more +on the SOC than on the board. So it's usually fine to include a DTB in the +SPL that doesn't exactly match the board. There are howerver some cases +where it's not possible. In the later case, in order to support multiple +boards (or board revisions) with the same SPL binary, SPL_MULTI_DTB_FIT +can be used. +The relevant DTBs are packed into a FIT. This FIT is automatically generated +at the end of the compilation, compressed and appended to u-boot-spl.bin, so +that SPL can locate it and select the correct DTB from inside the FIT. +CONFIG_SPL__OF_LIST is used to list the relevant DTBs. +The compression stage is optional but reduces the impact on the size of the +SPL. LZO and GZIP compressions are supported. By default, the area where the +FIT is uncompressed is dynamicaly allocated but this behaviour can be changed +for platforms that don't provide a HEAP big enough to contain the uncompressed +FIT. +The SPL uses board_fit_config_name_match() to find the correct DTB within the +FIT (same as what the SPL uses to select the DTB for U-Boot). +Uncompression and selection stages happen in fdtdec_setup() which is called +during the early initialization stage of the SPL (spl_early_init() or +spl_init()) + +Impacts and performances (SPL flavor): +The impact of this option is relatively small. Here are some numbers measured +for a TI DRA72 platform: + + +----------+------------+-----------+------------+ + | size | size delta | SPL boot | boot time | + | (bytes) | (bytes) | time (s) | delta (s) | ++---------------------------+----------+------------+-----------+------------+ +| 1 DTB | | | | | ++---------------------------+----------+------------+-----------+------------+ +| reference | 125305 | 0 | 1.389 | 0 | +| LZO (dynamic allocation) | 125391 | 86 | 1.381 | -0.008 | ++---------------------------+----------+------------+-----------+------------+ +| 4 DTBs (DRA7, DRA71, | | | | | +| DRA72, DRA72 revC) | | | | | ++---------------------------+----------+------------+-----------+------------+ +| LZO (dynamic allocation) | 125991 | 686 | 1.39 | 0.001 | +| LZO (user defined area) | 125927 | 622 | 1.403 | 0.014 | +| GZIP (user defined area) | 133880 | 8575 | 1.421 | 0.032 | +| No compression (in place) | 137472 | 12167 | 1.412 | 0.023 | ++---------------------------+----------+------------+-----------+------------+ + +Note: SPL boot time is the time elapsed between the 'reset' command is entered +and the time when the first U-Boot (not SPL) version string is displayed. |