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

[Tomasz Figa]

On Fri, Dec 14, 2018 at 9:36 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 14, 2018 at 12:12:38PM +0900, Tomasz Figa wrote:
> > > If the buffer always is physically contiguous, as it is in the currently
> > > posted series, we can always map it with a single dma_map_single call
> > > (if the hardware can handle that in a single segment is a different
> > > question, but out of scope here).
> >
> > Are you sure the buffer is always physically contiguous? At least the
> > ARM IOMMU dma_ops [1] and the DMA-IOMMU dma_ops [2] will simply
> > allocate pages without any continuity guarantees and remap the pages
> > into a contiguous kernel VA (unless DMA_ATTR_NO_KERNEL_MAPPING is
> > given, which makes them return an opaque cookie instead of the kernel
> > VA).
> >
> > [1] http://git.infradead.org/users/hch/misc.git/blob/2dbb028e4a3017e1b71a6ae3828a3548545eba24:/arch/arm/mm/dma-mapping.c#l1291
> > [2] http://git.infradead.org/users/hch/misc.git/blob/2dbb028e4a3017e1b71a6ae3828a3548545eba24:/drivers/iommu/dma-iommu.c#l450
>
> We never end up in this allocator for the new DMA_ATTR_NON_CONSISTENT
> case, and that is intentional.

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.

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

Best regards,
Tomasz