On Wednesday, August 24, 2011 13:27:05 Anton Staaf wrote:
On Wed, Aug 24, 2011 at 6:25 AM, Wolfgang Denk wrote:
Lukasz Majewski wrote:
- get_dcache_line_size() can be simply defined as
#define get_dcache_line_size() CONFIG_SYS_CACHE_LINE_SIZE if we _really_ want to save a few bytes.
Actually I fail to understand why we would ever need get_dcache_line_size() in a boot loader.
It is required so that we can correctly allocate buffers that will be used by DMA engines to read or write data. The reason that these buffers need to be cache line size aligned is because unaligned cache invalidates are not possible to do in a safe way. The problem is that invalidating a partial cache line requires invalidating the entire line. And the other part of the line can contain nearby variables (especially if the buffer is stack allocated), so we have to first flush the cache line to be safe. However, that flush will clobber the values that the DMA engine wrote to main memory that are not reflected in the cache.
right, and that's why i want to "hide" it be hind a "dma buffer allocate" API so that people can just say "i want a dma safe buffer" rather than having to delve into these details. they'll inevitable get confused and screw it up. ;) -mike