On Tue, Jun 22, 2021 at 4:33 PM Christoph Hellwig [off-list ref] wrote:

On Fri, Jun 18, 2021 at 01:44:08PM +0900, Tomasz Figa wrote:

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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.

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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.

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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