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

[Tomasz Figa <tfiga@chromium.org>]

On Tue, Jun 22, 2021 at 4:33 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Fri, Jun 18, 2021 at 01:44:08PM +0900, Tomasz Figa wrote:
> > > 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
> >
> > I think this could be addressed by having a dma_vmap() helper that
> > does the right thing, whether it's vmap() or dma_common_pages_remap()
> > as appropriate. Or would be this still insufficient for some
> > architectures?
>
> It can't always do the right thing.  E.g. for the case where uncached
> memory needs to be allocated from a special boot time fixed pool.
>

Fair enough. Thanks for elaborating.

> > > And even without that dma_alloc_noncoherent causes less overhead than
> > > dma_alloc_noncontigious if you only need a single contiguous range.
> > >
> >
> > Given that behind the scenes dma_alloc_noncontiguous() would also just
> > call __dma_alloc_pages() for devices that need contiguous pages, would
> > the overhead be basically the creation of a single-entry sgtable?
>
> In the best case: yes.
>
> > > 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.
> >
> > One thing to take into account here is that many drivers use the
> > existing "simple" way, just because there wasn't a viable alternative
> > to do something better. Agreed, though, that we shouldn't optimize for
> > the rare cases.
>
> While that might be true for a few drivers, it is absolutely not true
> for the wide majority.  I think you media people are a little special,
> with only the GPU folks contending for "specialness" :)  (although
> media handles it way better, gpu folks just create local hacks that
> can't work portably).

I don't have the evidence to argue, so let's just leave it at "time
will tell". I think it's great that we have the possibility to do the
more special things and we can see where it goes from now on. :)

Best regards,
Tomasz