Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

[Christoph Hellwig <hch@infradead.org>]

On Tue, Dec 18, 2018 at 04:22:43PM +0900, Tomasz Figa wrote:
> It kind of limits the usability of this API, since it enforces
> contiguous allocations even for big sizes even for devices behind
> IOMMU (contrary to the case when DMA_ATTR_NON_CONSISTENT is not set),
> but given that it's just a temporary solution for devices like these
> USB cameras, I guess that's fine.

The problem is that you can't have flexibility and simplicity at the
same time.  Once you use kernel virtual address remapping you need to
be prepared to have multiple segments.

So as I said you can call dma_alloc_attrs with DMA_ATTR_NON_CONSISTENT
in a loop with a suitably small chunk size, then stuff the results into
a scatterlist and map that again for the device share with if you don't
want a single contigous region.  You just have to either deal with
non-contigous access from the kernel or use vmap and the right vmap
cache flushing helpers.

> Note that in V4L2 we use the DMA API extensively, so that we don't
> need to embed any device-specific or integration-specific knowledge in
> the framework. Right now we're using dma_alloc_attrs() with
> driver-provided attrs [1], but current driver never request
> non-consistent memory. We're however thinking about making it possible
> to allocate non-consistent memory. What would you suggest for this?
> 
> [1] https://elixir.bootlin.com/linux/v4.20-rc7/source/drivers/media/common/videobuf2/videobuf2-dma-contig.c#L139

I would advice against new non-consistent users until this series
goes through, mostly because dma_cache_sync is such an amazing bad
API.  Otherwise things will just work at the allocation side, you'll
just need to be careful to transfer ownership between the cpu and
the device(s) carefully using the dma_sync_* APIs.