2015-01-07 19:41 GMT+09:00 Masahiro Yamada yamada.m@jp.panasonic.com:
The DDR PHY training function, ddrphy_prepare_training() would not work if compiled with GCC 4.9.
The struct ddrphy (arch/arm/include/asm/arch-uniphier/ddrphy-regs.h) is specified with __packed because it represents a hardware register mapping, but it turned out to cause a problem on GCC 4.9.
If -mno-unaligned-access is specified (actually it is in arch/arm/cpu/armv7/config.mk), GCC 4.9 is aware of the __attribute__((packed)) and generates extra instructions to perform the memory access in a way that does not cause unaligned access. (Actually it is bogus here because the register base, the first argument of the ddrphy_prepare_training(), is always given with a 4-byte aligned address.)
Anyway, as a result, readl() / writel() is divided into byte-wise accesses. The problem is that this hardware only accepts 4-byte register access. Byte-wise accesses lead to unexpected behavior.
There are some options to avoid this problem.
[1] Remove -mno-unaligned-access [2] Add __aligned(4) along with __packed to struct ddrphy [3] Remove __packed from struct ddrphy
[1] solves the problem for ARMv7, but it does not for pre-ARMv6 and ARMv6-M architectures where -mno-unaligned-access is default. So, [1] does not seem reasonable in terms of code portability.
Both [2] and [3] work well, but [2] seems too much. All the members of struct ddrphy have the u32 type. No padding would be inserted even if __packed is dropped.
Signed-off-by: Masahiro Yamada yamada.m@jp.panasonic.com
Applied to u-boot-uniphier/master