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Update user guide further to linker scripts changes

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This patch updates the user guide section about the memory layout.
  - Explain the verifications that the linker scripts does on the
    global memory layout.
  - Refer to the new linker symbols.
  - Describe the linker symbols exported to the trusted firmware code.

Change-Id: I033ab2b867e8b9776deb4185b9986bcb8218f286
This commit is contained in:
Sandrine Bailleux 2013-11-27 10:32:17 +00:00 committed by Dan Handley
parent 65f546a14f
commit cd29b0a60c
3 changed files with 201 additions and 31 deletions
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14
docs/change-log.md
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@ -26,6 +26,20 @@ Detailed changes since last release
* Build products are now created in a separate build directory tree.
* Analyze at link-time whether bootloader images will fit in memory and won't
overlap each other at run time. If it is not the case then image linking
will now fail.
* Reduce the size of the bootloader images by cutting some sections out of
their disk images and allocating them at load time, whenever possible.
* Properly initialise the C runtime environment. C code can now safely assume
that global variables are initialised to 0 and that initialised data holds
the correct value.
* General changes on the memory layout: some sections have been moved, some of
them have been merged together, and some alignment constraints on sections
have changed.
ARM Trusted Firmware - version 0.2
==================================

3
docs/porting-guide.md
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@ -164,11 +164,12 @@ constants defined. In the ARM FVP port, this file is found in
* **#define : BL2_BASE**
Defines the base address in secure RAM where BL1 loads the BL2 binary image.
Must be aligned on a page-size boundary.
* **#define : BL31_BASE**
Defines the base address in secure RAM where BL2 loads the BL3-1 binary
image.
image. Must be aligned on a page-size boundary.
### Other mandatory modifications

215
docs/user-guide.md
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@ -883,15 +883,133 @@ world software image at the highest available Exception Level (EL2 if
available, otherwise EL1).
### Memory layout on Base FVP
### Memory layout on FVP platforms
On FVP platforms, we use the Trusted ROM and Trusted SRAM to store the trusted
firmware binaries. BL1 is originally sitting in the Trusted ROM. Its read-write
data are relocated at the base of the Trusted SRAM at runtime. BL1 loads BL2
image near the top of the the trusted SRAM. BL2 loads BL3-1 image between BL1
and BL2. This memory layout is illustrated by the following diagram.
Trusted SRAM
+----------+ 0x04040000
| |
|----------|
| BL2 |
|----------|
| |
|----------|
| BL31 |
|----------|
| |
|----------|
| BL1 (rw) |
+----------+ 0x04000000
Trusted ROM
+----------+ 0x04000000
| BL1 (ro) |
+----------+ 0x00000000
Each bootloader stage image layout is described by its own linker script. The
linker scripts export some symbols into the program symbol table. Their values
correspond to particular addresses. The trusted firwmare code can refer to these
symbols to figure out the image memory layout.
Linker symbols follow the following naming convention in the trusted firmware.
* `__<SECTION>_START__`
Start address of a given section named `<SECTION>`.
* `__<SECTION>_END__`
End address of a given section named `<SECTION>`. If there is an alignment
constraint on the section's end address then `__<SECTION>_END__` corresponds
to the end address of the section's actual contents, rounded up to the right
boundary. Refer to the value of `__<SECTION>_UNALIGNED_END__` to know the
actual end address of the section's contents.
* `__<SECTION>_UNALIGNED_END__`
End address of a given section named `<SECTION>` without any padding or
rounding up due to some alignment constraint.
* `__<SECTION>_SIZE__`
Size (in bytes) of a given section named `<SECTION>`. If there is an
alignment constraint on the section's end address then `__<SECTION>_SIZE__`
corresponds to the size of the section's actual contents, rounded up to the
right boundary. In other words, `__<SECTION>_SIZE__ = __<SECTION>_END__ -
_<SECTION>_START__`. Refer to the value of `__<SECTION>_UNALIGNED_SIZE__`
to know the actual size of the section's contents.
* `__<SECTION>_UNALIGNED_SIZE__`
Size (in bytes) of a given section named `<SECTION>` without any padding or
rounding up due to some alignment constraint. In other words,
`__<SECTION>_UNALIGNED_SIZE__ = __<SECTION>_UNALIGNED_END__ -
__<SECTION>_START__`.
Some of the linker symbols are mandatory as the trusted firmware code relies on
them to be defined. They are listed in the following subsections. Some of them
must be provided for each bootloader stage and some are specific to a given
bootloader stage.
The linker scripts define some extra, optional symbols. They are not actually
used by any code but they help in undertanding the bootloader images' memory
layout as they are easy to spot in the link map files.
#### Common linker symbols
Early setup code needs to know the extents of the BSS section to zero-initialise
it before executing any C code. The following linker symbols are defined for
this purpose:
* `__BSS_START__` This address must be aligned on a 16-byte boundary.
* `__BSS_SIZE__`
Similarly, the coherent memory section must be zero-initialised. Also, the MMU
setup code needs to know the extents of this section to set the right memory
attributes for it. The following linker symbols are defined for this purpose:
* `__COHERENT_RAM_START__` This address must be aligned on a page-size boundary.
* `__COHERENT_RAM_END__` This address must be aligned on a page-size boundary.
* `__COHERENT_RAM_UNALIGNED_SIZE__`
#### BL1's linker symbols
BL1's early setup code needs to know the extents of the .data section to
relocate it from ROM to RAM before executing any C code. The following linker
symbols are defined for this purpose:
* `__DATA_ROM_START__` This address must be aligned on a 16-byte boundary.
* `__DATA_RAM_START__` This address must be aligned on a 16-byte boundary.
* `__DATA_SIZE__`
BL1's platform setup code needs to know the extents of its read-write data
region to figure out its memory layout. The following linker symbols are defined
for this purpose:
* `__BL1_RAM_START__` This is the start address of BL1 RW data.
* `__BL1_RAM_END__` This is the end address of BL1 RW data.
#### BL2's and BL3-1's linker symbols
Both BL2 and BL3-1 need to know the extents of their read-only section to set
the right memory attributes for this memory region in their MMU setup code. The
following linker symbols are defined for this purpose:
* `__RO_START__`
* `__RO_END__`
#### How to choose the right base address for each bootloader stage image
The current implementation of the image loader has some limitations. It is
designed to load images dynamically, at a load address chosen to minimize memory
fragmentation. The chosen image location can be either at the top or the bottom
of free memory. However, until this feature is fully functional, the code also
contains support for loading images at a link-time fixed address. The code that
dynamically calculates the load address is bypassed and the load address is
specified statically by the platform.
contains support for loading images at a link-time fixed address.
BL1 is always loaded at address `0x0`. BL2 and BL3-1 are loaded at specified
locations in Trusted SRAM. The lack of dynamic image loader support means these
@ -899,42 +1017,79 @@ load addresses must currently be adjusted as the code grows. The individual
images must be linked against their ultimate runtime locations.
BL2 is loaded near the top of the Trusted SRAM. BL3-1 is loaded between BL1
and BL2. As a general rule, the following constraints must always be enforced:
and BL2. All three images are resident concurrently in Trusted RAM during boot
so overlaps are not permitted.
1. `BL2_MAX_ADDR <= (<Top of Trusted SRAM>)`
2. `BL31_BASE >= BL1_MAX_ADDR`
3. `BL2_BASE >= BL31_MAX_ADDR`
The image end addresses can be determined from the link map files of the
different images. These are the `build/<platform>/<build-type>/bl<x>/bl<x>.map`
files, with `<x>` the stage bootloader.
Constraint 1 is enforced by BL2's linker script. If it is violated then the
* `bl1.map` link map file provides `__BL1_RAM_END__` address.
* `bl2.map` link map file provides `__BL2_END__` address.
* `bl31.map` link map file provides `__BL31_END__` address.
To prevent images from overlapping each other, the following constraints must be
enforced:
1. `__BL1_RAM_END__ <= BL31_BASE`
2. `__BL31_END__ <= BL2_BASE`
3. `__BL2_END__ <= (<Top of Trusted SRAM>)`
This is illustrated by the following memory layout diagram:
+----------+ 0x04040000
| |
|----------| __BL2_END__
| BL2 |
|----------| BL2_BASE
| |
|----------| __BL31_END__
| BL31 |
|----------| BL31_BASE
| |
|----------| __BL1_RAM_END__
| BL1 (rw) |
+----------+ 0x04000000
Overlaps are detected during image linking as follows.
Constraint 1 is enforced by BL1's linker script. If it is violated then the
linker will report an error while building BL1 to indicate that it doesn't
fit:
aarch64-none-elf-ld: BL31 image overlaps BL1 image.
This error means that the BL3-1 base address needs to be incremented. Ensure
that the new memory layout still obeys all constraints.
Constraint 2 is enforced by BL3-1's linker script. If it is violated then the
linker will report an error while building BL3-1 to indicate that it doesn't
fit:
aarch64-none-elf-ld: BL31 image overlaps BL2 image.
This error can either mean that the BL3-1 base address needs to be decremented
or that BL2 base address needs to be incremented. Ensure that the new memory
layout still obeys all constraints.
Constraint 3 is enforced by BL2's linker script. If it is violated then the
linker will report an error while building BL2 to indicate that it doesn't
fit. For example:
aarch64-none-elf-ld: address 0x40400c8 of bl2.elf section `.bss' is not
within region `RAM'
This error means that the BL2 base address needs to be moved down. Be sure that
the new BL2 load address still obeys constraint 3.
This error means that the BL2 base address needs to be decremented. Ensure that
the new memory layout still obeys all constraints.
Constraints 2 & 3 must currently be checked by hand. To ensure they are
enforced, first determine the maximum addresses used by BL1 and BL3-1. This can
be deduced from the link map files of the different images.
Since constraint checks are scattered across linker scripts, it is required to
`make clean` prior to building to ensure that all possible overlapping scenarios
are checked.
The BL1 link map file (`bl1.map`) gives these 2 values:
* `FIRMWARE_RAM_COHERENT_START`
* `FIRMWARE_RAM_COHERENT_SIZE`
The maximum address used by BL1 can then be easily determined:
BL1_MAX_ADDR = FIRMWARE_RAM_COHERENT_START + FIRMWARE_RAM_COHERENT_SIZE
The BL3-1 link map file (`bl31.map`) gives the following value:
* `BL31_DATA_STOP`. This is the the maximum address used by BL3-1.
The current implementation can result in wasted space because a simplified
`meminfo` structure represents the extents of free memory. For example, to load
BL2 at address `0x04020000`, the resulting memory layout should be as follows:
The current implementation of the image loader can result in wasted space
because of the simplified data structure used to represent the extents of free
memory. For example, to load BL2 at address `0x0402D000`, the resulting memory
layout should be as follows:
------------ 0x04040000
| | <- Free space (1)