ARM: prevent misaligned array inits

Under option -munaligned-access, gcc can perform local char
or 16-bit array initializations using misaligned native
accesses which will throw a data abort exception. Fix files
where these array initializations were unneeded, and for
files known to contain such initializations, enforce gcc
option -mno-unaligned-access.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
[trini: Switch to usign call cc-option for -mno-unaligned-access as
Albert had done previously as that's really correct]
Signed-off-by: Tom Rini <trini@ti.com>

1 files changed, 122 insertions, 0 deletions

diff --git a/doc/README.arm-unaligned-accesses b/doc/README.arm-unaligned-accesses
new file mode 100644
index 0000000..c37d135
--- /dev/null
+++ b/doc/README.arm-unaligned-accesses
@@ -0,0 +1,122 @@
+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.