understanding DMA cache maint

[Neshama Parhoti]

in the following function,
what's the different between the inner_op (dmac_*) and outer_op (outer_*) ?
why do we need two functions to do the cache operation ?
isn't this an action on the cache hardware itself ? are there 2
hardwares involved ?

void dma_cache_maint(const void *start, size_t size, int direction)
{
   ...
	switch (direction) {
	case DMA_FROM_DEVICE:		/* invalidate only */
		inner_op = dmac_inv_range;
		outer_op = outer_inv_range;
		break;
	case DMA_TO_DEVICE:		/* writeback only */
		inner_op = dmac_clean_range;
		outer_op = outer_clean_range;
		break;
	case DMA_BIDIRECTIONAL:		/* writeback and invalidate */
		inner_op = dmac_flush_range;
		outer_op = outer_flush_range;


thank you!
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