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DOC: Update ROMLIB page with memory impact info 

Complete the Library at ROM documentation with information regarding
the memory impact of the feature.

Change-Id: I5a10620a8e94f123021bb19523a36d558b330deb
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
This commit is contained in:
Louis Mayencourt 2019-10-11 15:27:01 +01:00
parent e7b390892d
commit 4685b64fc3
1 changed files with 15 additions and 0 deletions
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docs/components/romlib-design.rst
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@ -111,6 +111,21 @@ The calling sequence for a patched function is as follows:
BL image --> function
Memory impact
~~~~~~~~~~~~~
Using library at ROM will modify the memory layout of the BL images:
- The ROM library needs a page aligned RAM section to hold the RW data. This
section is defined by the ROMLIB_RW_BASE and ROMLIB_RW_END macros.
On Arm platforms a section of 1 page (0x1000) is allocated at the top of SRAM.
This will have for effect to shift down all the BL images by 1 page.
- Depending on the functions moved to the ROM library, the size of the BL images
will be reduced.
For example: moving MbedTLS function into the ROM library reduces BL1 and
BL2, but not BL31.
- This change in BL images size can be taken into consideration to optimize the
memory layout when defining the BLx_BASE macros.
Build library at ROM
~~~~~~~~~~~~~~~~~~~~~