[U-Boot] [RFC PATCH v2] ARM: Avoid compiler optimization for usages of readb, writeb and friends.

Dirk Behme
2010-12-19T07:51:48Z

On 18.12.2010 23:27, Alexander Holler wrote:
> gcc 4.5.1 seems to ignore (at least some) volatile definitions,
> avoid that as done in the kernel.
>
> Reading C99 6.7.3 8 and the comment 114) there, I think it is a bug of that
> gcc version to ignore the volatile type qualifier used e.g. in __arch_getl().
> Anyway, using a definition as in the kernel headers avoids such optimizations when
> gcc 4.5.1 is used.
>
> Maybe the headers as used in the current linux-kernel should be used,
> but to avoid large changes, I've just added a small change to the current headers.
>
> Signed-off-by: Alexander Holler<holler@ahsoftware.de>
> ---
> arch/arm/include/asm/io.h | 20 ++++++++++++++------
> 1 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
> index ff1518e..5364b78 100644
> --- a/arch/arm/include/asm/io.h
> +++ b/arch/arm/include/asm/io.h
> @@ -125,13 +125,21 @@ extern inline void __raw_readsl(unsigned int addr, void *data, int longlen)
> #define __raw_readw(a) __arch_getw(a)
> #define __raw_readl(a) __arch_getl(a)
>
> -#define writeb(v,a) __arch_putb(v,a)
> -#define writew(v,a) __arch_putw(v,a)
> -#define writel(v,a) __arch_putl(v,a)
> +/*
> + * TODO: The kernel offers some more advanced versions of barriers, it might
> + * have some advantages to use them instead of the simple one here.
> + */
> +#define dmb() __asm__ __volatile__ ("" : : : "memory")
> +#define __iormb() dmb()
> +#define __iowmb() dmb()
> +
> +#define writeb(v,c) ({ __iowmb(); __arch_putb(v,c); })
> +#define writew(v,c) ({ __iowmb(); __arch_putw(v,c); })
> +#define writel(v,c) ({ __iowmb(); __arch_putl(v,c); })
>
> -#define readb(a) __arch_getb(a)
> -#define readw(a) __arch_getw(a)
> -#define readl(a) __arch_getl(a)
> +#define readb(c) ({ u8 __v = __arch_getb(c); __iormb(); __v; })
> +#define readw(c) ({ u16 __v = __arch_getw(c); __iormb(); __v; })
> +#define readl(c) ({ u32 __v = __arch_getl(c); __iormb(); __v; })

Using the test code below [1] and then looking at the disassembly from
the two tool chains gcc version 4.3.3 (Sourcery G++ Lite 2009q1-203)
versus gcc version 4.5.1 (Sourcery G++ Lite 2010.09-50): Yes, without
the additional dmb() the gcc 4.5.1 just creates

00000000 :
0: e3a00000 mov r0, #0
4: e12fff1e bx lr

while with the additional dmb() it creates

00000000 :
0: e59f300c ldr r3, [pc, #12] ; 14 <main+0x14>
4: e5932028 ldr r2, [r3, #40] ; 0x28
8: e5930028 ldr r0, [r3, #40] ; 0x28
c: e0620000 rsb r0, r2, r0
10: e12fff1e bx lr
14: 48318000

what looks correct. And 4.3.3 does the same code for both readl()
versions. So:

Acked-by: Dirk Behme <dirk.behme@googlemail.com>

Thanks

Dirk

[1]

arm-none-linux-gnueabi-gcc -Wall -O2 -c foo.c -o foo.o
arm-none-linux-gnueabi-objdump -D foo.o > foo.dis

-- foo.c --
struct gptimer {
unsigned int tidr; /* 0x00 r */
unsigned char res[0xc];
unsigned int tiocp_cfg; /* 0x10 rw */
unsigned int tistat; /* 0x14 r */
unsigned int tisr; /* 0x18 rw */
unsigned int tier; /* 0x1c rw */
unsigned int twer; /* 0x20 rw */
unsigned int tclr; /* 0x24 rw */
unsigned int tcrr; /* 0x28 rw */
unsigned int tldr; /* 0x2c rw */
unsigned int ttgr; /* 0x30 rw */
unsigned int twpc; /* 0x34 r*/
unsigned int tmar; /* 0x38 rw*/
unsigned int tcar1; /* 0x3c r */
unsigned int tcicr; /* 0x40 rw */
unsigned int tcar2; /* 0x44 r */
};

#define dmb() __asm__ __volatile__ ("" : : : "memory")
#define __iormb() dmb()

#define __arch_getl(a) (*(volatile unsigned int *)(a))
#define readl(a) __arch_getl(a)
//#define readl(c) ({ unsigned int __v = __arch_getl(c); __iormb();
__v; })

int main(void) {

struct gptimer *gpt1_base = (struct gptimer *)0x48318000;
unsigned int cdiff, cstart, cend;

cstart = readl(&gpt1_base->tcrr);

cend = readl(&gpt1_base->tcrr);

cdiff = cend - cstart;

return cdiff;

}
-- foo.c --