Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

From: Christoph Hellwig
Date: Fri Jun 18 2021 - 00:25:34 EST

On Fri, Jun 18, 2021 at 12:21:33PM +0900, Tomasz Figa wrote:
> > On Thu, Jun 17, 2021 at 06:40:58PM +0900, Tomasz Figa wrote:
> > > Sorry, I meant dma_alloc_attrs() and yes, it's indeed a misnomer. Our
> > > use case basically has no need for the additional coherent mapping, so
> > > creation of it can be skipped to save some vmalloc space. (Yes, it
> > > probably only matters for 32-bit architectures.)
> >
> > Yes, that is the normal use case, and it is solved by using
> > dma_alloc_noncoherent or dma_alloc_noncontigous without the vmap
> > step.
> True, silly me. Probably not enough coffee at the time I was looking at it.
> With that, wouldn't it be possible to completely get rid of
> dma_alloc_{coherent,attrs}() and use dma_alloc_noncontiguous() +
> optional kernel and/or userspace mapping helper everywhere? (Possibly
> renaming it to something as simple as dma_alloc().

Well, dma_alloc_coherent users want a non-cached mapping. And while
some architectures provide that using a vmap with "uncached" bits in the
PTE to provide that, this:

a) is not possibly everywhere
b) even where possible is not always the best idea as it creates mappings
with differnet cachability bets

And even without that dma_alloc_noncoherent causes less overhead than
dma_alloc_noncontigious if you only need a single contiguous range.

So while I'm happy we have something useful for more complex drivers like
v4l I think the simple dma_alloc_coherent API, including some of the less
crazy flags for dma_alloc_attrs is the right thing to use for more than
90% of the use cases.