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SubjectRe: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer
On Wed, Oct 31, 2018 at 12:00:12AM +0200, Laurent Pinchart wrote:
> As discussed before, we're clearly missing a proper non-coherent memory
> allocation API. As much as I would like to see a volunteer for this, I don't
> think it's a reason to block the performance improvement we get from this
> patch.
>
> This being said, I'm a bit concerned about the allocation of 16kB blocks from
> kmalloc(), and believe that the priority of the non-coherent memory allocation
> API implementation should be increased. Christoph, you mentioned in a recent
> discussion on this topic that you are working on removing the existing non-
> coherent DMA allocation API, what is your opinion on how we should gllobally
> solve the problem that this patch addresses ?

I hope to address this on the dma-mapping side for this merge window.
My current idea is to add (back) add dma_alloc_noncoherent-like API
(name to be determindes). This would be very similar to to the
DMA_ATTR_NON_CONSISTENT to dma_alloc_attrs with the following
differences:

- it must actually be implemented by every dma_map_ops instance, no
falling back to dma_alloc_coherent like semantics. For all actually
coherent ops this is trivial as there is no difference in semantics
and we can fall back to the 'coherent' semantics, for non-coherent
direct mappings it also is mostly trivial as we generally can use
dma_direct_alloc. The only instances that will need real work are
IOMMUs that support non-coherent access, but there is only about
a handfull of those.
- instead of using the only vaguely defined dma_cache_sync for
ownership transfers we'll use dma_sync_single_* which are well
defined and available everywhere

I'll try to prioritise this to get done early in the merge window,
but I'll need someone else do the USB side.

> > + dma_sync_single_for_cpu(&urb->dev->dev,
> > + urb->transfer_dma,
> > + urb->transfer_buffer_length,
> > + DMA_FROM_DEVICE);
> > +
>
> As explained before as well, I think we need dma_sync_single_for_device()
> calls, and I know they would degrade performances until we fix the problem on
> the DMA mapping API side. This is not a reason to block the patch either. I
> would appreciate, however, if a comment could be added to the place where
> dma_sync_single_for_device() should be called, to explain the problem.

Yes, as a rule of thumb every dma_sync_single_for_cpu call needs to pair
with a previous dma_sync_single_for_device call. (Exceptions like
selective use of DMA_ATTR_SKIP_CPU_SYNC proove the rule)

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