On 14 June 2012 04:22, Rob Herring <robherri...@gmail.com> wrote: > On 06/10/2012 05:31 PM, Michael Hope wrote: >> There's an interaction between Linaro GCC or FSF GCC 4.7 and Linux >> kernels before 3.2 which causes the kernel to halt straight after >> showing 'Uncompressing Linux'. The question comes up every couple of >> months so I've blogged about it: >> http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/ >> >> """ >> Is your ARM Linux kernel not booting when building with Linaro GCC or >> FSF GCC 4.7? Does it halt shortly after showing ‘Uncompressing Linux’? >> You may have run into an interaction between older kernels and the new >> unaligned access support in GCC. This affects Linaro GCC from >> 4.6-2011.11 onwards, GCC from 4.7.0 on, and kernels earlier than 3.2 >> including the Galaxy Nexus Icecream Sandwich release. >> >> The work-around is to add -mno-unaligned-access to KBUILD_CFLAGS in >> the top level kernel Makefile or to backport >> 8428e84d42179c2a00f5f6450866e70d802d1d05 from the current kernel tree. >> >> ARMv6K and later processors have hardware support for doing unaligned >> loads and stores which is faster than the old byte-by-byte/recombine >> that was done in software. Later versions of GCC use this to do >> quicker loads when working on known unaligned data, such as when >> working on a protocol buffer or a packed structure. >> >> The CPU can be configured to trap on unaligned access. This trap is >> off at reset, but pre 3.2 kernels turn this on during the initial >> boot. An interaction between -fconserve-stack and -munaligned-access >> on a char buffer lead to an unaligned access, which causes a trap, >> which causes the kernel to halt. >> >> This does not affect userspace programs as they run with the trap turned off. >> """ > > I've also hit this with u-boot if I enable armv7-a builds. Mainline > u-boot generally builds using -march=armv5 and unaligned accesses > disabled in h/w. Generally u-boot starts but dies on certain commands. I > think there may be other u-boot issues with v7 compiles on newer gcc > versions, but haven't debugged things further.
Note that this is done through a unaligned access trap that is off by default. A quick grep through u-boot git shows that it defines CR_A but doesn't use it. -- Michael _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
