a saner API for allocating DMA addressable pages v2

[Matthew Wilcox]

On Mon, Sep 14, 2020 at 04:44:16PM +0200, Christoph Hellwig wrote:
> I'm still a little unsure about the API naming, as alloc_pages sort of
> implies a struct page return value, but we return a kernel virtual
> address.

Erm ... dma_alloc_pages() returns a struct page, so is this sentence
stale?

>From patch 14:

+struct page *dma_alloc_pages(struct device *dev, size_t size,
+               dma_addr_t *dma_handle, enum dma_data_direction dir, gfp_t gfp);

> The other alternative would be to name the API
> dma_alloc_noncoherent, but the whole non-coherent naming seems to put
> people off.

You say that like it's a bad thing.  I think the problem is more that
people don't understand what non-coherent means and think they're
supporting it when they're not.

dma_alloc_manual_flushing()?

> As a follow up I plan to move the implementation of the
> DMA_ATTR_NO_KERNEL_MAPPING flag over to this framework as well, given
> that is also is a fundamentally non coherent allocation.  The replacement
> for that flag would then return a struct page, as it is allowed to
> actually return pages without a kernel mapping as the name suggested
> (although most of the time they will actually have a kernel mapping..)

If the page doesn't have a kernel mapping, shouldn't it return a PFN
or a phys_addr?