+++ /dev/null
-If you are reading this because of a data abort: the following MIGHT
-be relevant to your abort, if it was caused by an alignment violation.
-In order to determine this, use the PC from the abort dump along with
-an objdump -s -S of the u-boot ELF binary to locate the function where
-the abort happened; then compare this function with the examples below.
-If they match, then you've been hit with a compiler generated unaligned
-access, and you should rewrite your code or add -mno-unaligned-access
-to the command line of the offending file.
-
-Note that the PC shown in the abort message is relocated. In order to
-be able to match it to an address in the ELF binary dump, you will need
-to know the relocation offset. If your target defines CONFIG_CMD_BDI
-and if you can get to the prompt and enter commands before the abort
-happens, then command "bdinfo" will give you the offset. Otherwise you
-will need to try a build with DEBUG set, which will display the offset,
-or use a debugger and set a breakpoint at relocate_code() to see the
-offset (passed as an argument).
-
-*
-
-Since U-Boot runs on a variety of hardware, some only able to perform
-unaligned accesses with a strong penalty, some unable to perform them
-at all, the policy regarding unaligned accesses is to not perform any,
-unless absolutely necessary because of hardware or standards.
-
-Also, on hardware which permits it, the core is configured to throw
-data abort exceptions on unaligned accesses in order to catch these
-unallowed accesses as early as possible.
-
-Until version 4.7, the gcc default for performing unaligned accesses
-(-mno-unaligned-access) is to emulate unaligned accesses using aligned
-loads and stores plus shifts and masks. Emulated unaligned accesses
-will not be caught by hardware. These accesses may be costly and may
-be actually unnecessary. In order to catch these accesses and remove
-or optimize them, option -munaligned-access is explicitly set for all
-versions of gcc which support it.
-
-From gcc 4.7 onward starting at armv7 architectures, the default for
-performing unaligned accesses is to use unaligned native loads and
-stores (-munaligned-access), because the cost of unaligned accesses
-has dropped on armv7 and beyond. This should not affect U-Boot's
-policy of controlling unaligned accesses, however the compiler may
-generate uncontrolled unaligned accesses on its own in at least one
-known case: when declaring a local initialized char array, e.g.
-
-function foo()
-{
- char buffer[] = "initial value";
-/* or */
- char buffer[] = { 'i', 'n', 'i', 't', 0 };
- ...
-}
-
-Under -munaligned-accesses with optimizations on, this declaration
-causes the compiler to generate native loads from the literal string
-and native stores to the buffer, and the literal string alignment
-cannot be controlled. If it is misaligned, then the core will throw
-a data abort exception.
-
-Quite probably the same might happen for 16-bit array initializations
-where the constant is aligned on a boundary which is a multiple of 2
-but not of 4:
-
-function foo()
-{
- u16 buffer[] = { 1, 2, 3 };
- ...
-}
-
-The long term solution to this issue is to add an option to gcc to
-allow controlling the general alignment of data, including constant
-initialization values.
-
-However this will only apply to the version of gcc which will have such
-an option. For other versions, there are four workarounds:
-
-a) Enforce as a rule that array initializations as described above
- are forbidden. This is generally not acceptable as they are valid,
- and usual, C constructs. The only case where they could be rejected
- is when they actually equate to a const char* declaration, i.e. the
- array is initialized and never modified in the function's scope.
-
-b) Drop the requirement on unaligned accesses at least for ARMv7,
- i.e. do not throw a data abort exception upon unaligned accesses.
- But that will allow adding badly aligned code to U-Boot, only for
- it to fail when re-used with a stricter target, possibly once the
- bad code is already in mainline.
-
-c) Relax the -munaligned-access rule globally. This will prevent native
- unaligned accesses of course, but that will also hide any bug caused
- by a bad unaligned access, making it much harder to diagnose it. It
- is actually what already happens when building ARM targets with a
- pre-4.7 gcc, and it may actually already hide some bugs yet unseen
- until the target gets compiled with -munaligned-access.
-
-d) Relax the -munaligned-access rule only for for files susceptible to
- the local initialized array issue and for armv7 architectures and
- beyond. This minimizes the quantity of code which can hide unwanted
- misaligned accesses.
-
-The option retained is d).
-
-Considering that actual occurrences of the issue are rare (as of this
-writing, 5 files out of 7840 in U-Boot, or .3%, contain an initialized
-local char array which cannot actually be replaced with a const char*),
-contributors should not be required to systematically try and detect
-the issue in their patches.
-
-Detecting files susceptible to the issue can be automated through a
-filter installed as a hook in .git which recognizes local char array
-initializations. Automation should err on the false positive side, for
-instance flagging non-local arrays as if they were local if they cannot
-be told apart.
-
-In any case, detection shall not prevent committing the patch, but
-shall pre-populate the commit message with a note to the effect that
-this patch contains an initialized local char or 16-bit array and thus
-should be protected from the gcc 4.7 issue.
-
-Upon a positive detection, either $(PLATFORM_NO_UNALIGNED) should be
-added to CFLAGS for the affected file(s), or if the array is a pseudo
-const char*, it should be replaced by an actual one.
projects / profile / common / platform / kernel / u-boot-artik.git / commitdiff