On Mon, Aug 29, 2011 at 1:47 PM, Scott Wood <scottw...@freescale.com> wrote: > 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.
You are correct. I wrote that one as a modification of mikes initial proposal. I should have caught the incorrect rounding when I did. The patch that Lukasz sent titled "dcache: Dcache line size aligned stack buffer allocation" has a correct implementation. > After fixing the more obvious issues, I get "error: initializer element > is not constant". I think this requires the use of -std=c99 or GCC extensions. More specifics can be found here: http://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html > 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)); > } :) This was one of my suggestions earlier on a different thread. It was rejected there, I believe because it makes things less clear. >> 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). Yup, I think whatever macro we come up with we can simplify it eventually using the GCC attribute solution and we can make the macro conditional on compiler version. Thanks, Anton > -Scott > > _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
