[U-Boot] [PATCH] mmc:dcache: Cache line size aligned internal MMC buffers

[Scott Wood <scottwood@freescale.com>]

On 08/29/2011 03:12 PM, Anton Staaf wrote:
> 1) Mikes's macro
> 
> #define DMA_ALIGN_SIZE(size) \
>        (((size) + CONFIG_SYS_CACHELINE_SIZE - 1)
> 
> #define DMA_DECLARE_BUFFER(type, name, size) \
>        void __##name[DMA_ALIGN_SIZE(size * sizeof(type))]; \
>        type * name = __##name & ~(CONFIG_SYS_CACHELINE_SIZE - 1));
> 
> DMA_DECLARE_BUFFER(int, buffer, 100);

This doesn't compile, and it tries to round the buffer down below its
starting point.

After fixing the more obvious issues, I get "error: initializer element
is not constant".

There might be no way to express and-by-constant as a relocation.

You could set the pointer at runtime, though, and remove some of the
macrification:

#define DMA_ALIGN_SIZE(size) \
	((size) + CONFIG_SYS_CACHELINE_SIZE - 1)
#define DMA_ALIGN_ADDR(addr) \
	(DMA_ALIGN_SIZE(addr) & (CONFIG_SYS_CACHELINE_SIZE - 1))

int buffer_unaligned[DMA_ALIGN_SIZE(100)];
int *buffer;

some_init_func()
{
	buffer = (int *)(DMA_ALIGN_ADDR((uintptr_t)buffer_unaligned));
}

> 3) Use GCC specific alignment attribute:
> 
> #define CACHLINE_ALIGNED __attribute__ ((aligned (CONFIG_SYS_CACHELINE_SIZE)))
> 
> int buffer[100] CACHELINE_ALIGNED;
> 
> Pros: The declaration of the buffer is even simpler and more obvious,
> no use of alloca at all.
> 
> Cons: This doesn't work in any version of GCC before October 2010.
> Meaning that it probably doesn't work in whatever compiler you're
> using.
> 
> 
> It's really too bad that this isn't a usable solution.  I suppose that
> we could switch to it at some point when we expect U-Boot to only be
> compiled by versions of GCC that support this.  By the way, the
> failure mode here is pretty bad.  If you compile the above code with
> an older GCC it will silently fail to align the variable.  :(

If the decision is made to depend on newer compilers, U-Boot could check
for it and #error out if the compiler is too old (possibly just in the
files that depend on this feature).

-Scott