Hi Rob and all,
Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
On 06/05/2012 11:47 AM, Lucas Stach wrote:
Recent toolchains default to using the hardware feature for unaligned access on ARM v7, rather than doing the software fallback. According to ARM this is safe as all v7 implementations have to support this feature. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFA...)
To avoid CPU hangs when doing unaligned memory access, we have to turn off alignment checking in our CPU initialisation code. (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCF...)
Does this behavior change trickle down to Linux/... too, or would an OS completely re-initialize this state, and hence not be affected?
Linux in particular does reinitialize this state and I expect any reasonable OS to do so.
Then what is the point of enabling it on U-Boot? Does it fix some issue whereby some mis-aligned piece of data cannot be properly aligned?
This is a new optimization feature in gcc 4.7 (and backported to some 4.6 versions like the ubuntu 12.04 arm cross compiler (4.6.3)):
http://lists.linaro.org/pipermail/linaro-dev/2012-June/012360.html
http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/
If you don't want to enable unaligned accesses, then "-mno-unaligned-access" needs to be added.
I verified it. Option "-mno-unaligned-access" works good.
include/mtd/cfi_flash.h
/* CFI standard query structure */ struct cfi_qry { u8 qry[3]; u16 p_id; <-- unaligned! ... } __attribute__((packed));
$ ${CROSS_COMPILE}gcc --version arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.6.2 20110921 (release) [ARM/embedded-4_6-branch revision 182083] Copyright (C) 2011 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
===================================================================== Compiled without --mno-unaligned-access $ ${CROSS_COMPILE}objdump -d -S u-boot
info->vendor = le16_to_cpu(qry.p_id); cc88: e3003a1c movw r3, #2588 ; 0xa1c cc8c: e1dd11bb ldrh r1, [sp, #27] <-- this is unaligned access cc90: ... cc94: e18410b3 strh r1, [r4, r3]
===================================================================== Compiled with --mno-unaligned-access $ ${CROSS_COMPILE}objdump -d -S u-boot
info->vendor = le16_to_cpu(qry.p_id); cce8: e5dd101c ldrb r1, [sp, #28] <-- ccec: e5dd301b ldrb r3, [sp, #27] <-- separated 2 byte accesses ccf0: ... ccf4: e1831401 orr r1, r3, r1, lsl #8 ccf8: e3003a1c movw r3, #2588 ; 0xa1c ccfc: e18410b3 strh r1, [r4, r3]