dma-mapping: document dma_{alloc,free}_pages

Document the new dma_alloc_pages and dma_free_pages APIs, and fix
up the documentation for dma_alloc_noncoherent and dma_free_noncoherent.
Reported-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

Documentation/core-api/dma-api.rst

@@ -519,10 +519,9 @@ routines, e.g.:::
 Part II - Non-coherent DMA allocations
 --------------------------------------
 
-These APIs allow to allocate pages in the kernel direct mapping that are
-guaranteed to be DMA addressable.  This means that unlike dma_alloc_coherent,
-virt_to_page can be called on the resulting address, and the resulting
-struct page can be used for everything a struct page is suitable for.
+These APIs allow to allocate pages that are guaranteed to be DMA addressable
+by the passed in device, but which need explicit management of memory ownership
+for the kernel vs the device.
 
 If you don't understand how cache line coherency works between a processor and
 an I/O device, you should not be using this part of the API.
@@ -537,7 +536,7 @@ an I/O device, you should not be using this part of the API.
 This routine allocates a region of <size> bytes of consistent memory.  It
 returns a pointer to the allocated region (in the processor's virtual address
 space) or NULL if the allocation failed.  The returned memory may or may not
-be in the kernels direct mapping.  Drivers must not call virt_to_page on
+be in the kernel direct mapping.  Drivers must not call virt_to_page on
 the returned memory region.
 
 It also returns a <dma_handle> which may be cast to an unsigned integer the
@@ -565,7 +564,45 @@ reused.
 Free a region of memory previously allocated using dma_alloc_noncoherent().
 dev, size and dma_handle and dir must all be the same as those passed into
 dma_alloc_noncoherent().  cpu_addr must be the virtual address returned by
-the dma_alloc_noncoherent().
+dma_alloc_noncoherent().
+
+::
+
+	struct page *
+	dma_alloc_pages(struct device *dev, size_t size, dma_addr_t *dma_handle,
+			enum dma_data_direction dir, gfp_t gfp)
+
+This routine allocates a region of <size> bytes of non-coherent memory.  It
+returns a pointer to first struct page for the region, or NULL if the
+allocation failed. The resulting struct page can be used for everything a
+struct page is suitable for.
+
+It also returns a <dma_handle> which may be cast to an unsigned integer the
+same width as the bus and given to the device as the DMA address base of
+the region.
+
+The dir parameter specified if data is read and/or written by the device,
+see dma_map_single() for details.
+
+The gfp parameter allows the caller to specify the ``GFP_`` flags (see
+kmalloc()) for the allocation, but rejects flags used to specify a memory
+zone such as GFP_DMA or GFP_HIGHMEM.
+
+Before giving the memory to the device, dma_sync_single_for_device() needs
+to be called, and before reading memory written by the device,
+dma_sync_single_for_cpu(), just like for streaming DMA mappings that are
+reused.
+
+::
+
+	void
+	dma_free_pages(struct device *dev, size_t size, struct page *page,
+			dma_addr_t dma_handle, enum dma_data_direction dir)
+
+Free a region of memory previously allocated using dma_alloc_pages().
+dev, size and dma_handle and dir must all be the same as those passed into
+dma_alloc_noncoherent().  page must be the pointer returned by
+dma_alloc_pages().
 
 ::