-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.
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.
projects / oweals / u-boot.git / blobdiff