On Tue, Aug 23, 2011 at 2:32 PM, Mike Frysinger vapier@gentoo.org wrote:
On Tuesday, August 23, 2011 17:09:37 Wolfgang Denk wrote:
Mike Frysinger wrote:
On Tuesday, August 23, 2011 16:27:26 Anton Staaf wrote:
So then, to guide our efforts, what is a more suitable solution? Would you prefer we stick with the existing path of calling memalign and passing it the cache size by directly calling get_dcache_line_size? Or would you prefer something more like a dma_buffer_malloc function that allocates on the heap a cache line size aligned buffer and returns it?
memalign() is simply a malloc() with offset fudging, so dma_buffer_malloc() is the way to go imo. anything that involves end code having to figure out how to align things itself is asking for pain.
I would like to avoid using any malloc code here. We have to keep in mind that such code changes will spread, and will be copied into driver code, file systems, etc. which might be used (and even required, for example for NAND or SDCard booting systems) before relocation - but malloc becomes available only after relocation.
Why cannot we define a macro that declares a (sufficiently sized) buffer on the stack and provides and a pointer to a (correctly aligned) address in this buffer?
isnt that what i already posted and you NAK-ed ? :)
DMA_DECLARE_BUFFER(...)
I wasn't going to say it. :) How about something like this, which is very similar to what you had Mike, but doesn't define the array in the macro. It's a bit clearer what is going on, but also requires a bit more work at each use.
#define DCACHE_RESIZE(size) ((size) + get_dcache_line_size() - 1) #define DCACHE_ALIGN(buffer) ((buffer) + get_dcache_line_size() - 1) & ~(get_dcache_line_size() - 1)
char buffer[DCACHE_RESIZE(100)]; char * aligned = DCACHE_ALIGN(buffer);
It would be awesome if the idea below worked, but it can't because the array is popped when the ({...}) scope is exited I believe.
#define allocate_dma_buffer_on_stack(size) \ ({ \ char _dma_buffer[(size) + get_dcache_line_size() - 1]; \ \ _dma_buffer & ~(get_dcache_line_size() - 1); \ })
And you could use it like:
char * buffer = allocate_dma_buffer_on_stack(100);
If anyone can think of a way to make this work that would be excellent...
Thanks, Anton
-mike