Function bus_dmamem_alloc

freebsd/arm/arm/busdma_machdep.c:777–871

* Allocate a piece of memory that can be efficiently mapped into bus device * space based on the constraints listed in the dma tag. Returns a pointer to * the allocated memory, and a pointer to an associated bus_dmamap. */

 * the allocated memory, and a pointer to an associated bus_dmamap.
 */
int
bus_dmamem_alloc(bus_dma_tag_t dmat, void **vaddr, int flags,
    bus_dmamap_t *mapp)
{
	busdma_bufalloc_t ba;
	struct busdma_bufzone *bufzone;
	bus_dmamap_t map;
	vm_memattr_t memattr;
	int mflags;

	if (flags & BUS_DMA_NOWAIT)
		mflags = M_NOWAIT;
	else
		mflags = M_WAITOK;
	if (flags & BUS_DMA_ZERO)
		mflags |= M_ZERO;

	*mapp = map = allocate_map(dmat, mflags);
	if (map == NULL) {
		CTR4(KTR_BUSDMA, "%s: tag %p tag flags 0x%x error %d",
		    __func__, dmat, dmat->flags, ENOMEM);
		return (ENOMEM);
	}
	map->flags = DMAMAP_DMAMEM_ALLOC;

	/* For coherent memory, set the map flag that disables sync ops. */
	if (flags & BUS_DMA_COHERENT)
		map->flags |= DMAMAP_COHERENT;

	/*
	 * Choose a busdma buffer allocator based on memory type flags.
	 * If the tag's COHERENT flag is set, that means normal memory
	 * is already coherent, use the normal allocator.
	 */
	if ((flags & BUS_DMA_COHERENT) &&
	    ((dmat->flags & BUS_DMA_COHERENT) == 0)) {
		memattr = VM_MEMATTR_UNCACHEABLE;
		ba = coherent_allocator;
	} else {
		memattr = VM_MEMATTR_DEFAULT;
		ba = standard_allocator;
	}

	/*
	 * Try to find a bufzone in the allocator that holds a cache of buffers
	 * of the right size for this request.  If the buffer is too big to be
	 * held in the allocator cache, this returns NULL.
	 */
	bufzone = busdma_bufalloc_findzone(ba, dmat->maxsize);

	/*
	 * Allocate the buffer from the uma(9) allocator if...
	 *  - It's small enough to be in the allocator (bufzone not NULL).
	 *  - The alignment constraint isn't larger than the allocation size
	 *    (the allocator aligns buffers to their size boundaries).
	 *  - There's no need to handle lowaddr/highaddr exclusion zones.
	 * else allocate non-contiguous pages if...
	 *  - The page count that could get allocated doesn't exceed