[U-Boot] [PATCH v3] ARM: Avoid compiler optimization for usages of readb, writeb and friends.

Dirk Behme
2011-01-01T12:04:17Z

On 22.12.2010 12:04, 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.
>
> I haven't add the definitions which are using a memory barrier because I haven't found
> a place in the kernel where they were actually enabled (CONFIG_ARM_DMA_MEM_BUFFERABLE).
>
> 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..068ed17 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) do { __iowmb(); __arch_putb(v,c); } while (0)
> +#define writew(v,c) do { __iowmb(); __arch_putw(v,c); } while (0)
> +#define writel(v,c) do { __iowmb(); __arch_putl(v,c); } while (0)
>
> -#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; })

Do you like to test the patch in the attachment? I named it 'v4'.

After some thinking and testing, it seems to me that the volatile
optimization issue this patch shall fix is only with the readx()
macros. So the idea is to drop all writex() changes done in the v3
version of this patch. With dropping the writex() changes, we would
drop all issues we discussed with e.g. the GCC statement-expression
and the do while workaround, too.

Thanks

Dirk
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