Re: For the problem when using swiotlb

From: Catalin Marinas
Date: Fri Nov 21 2014 - 06:06:35 EST

On Wed, Nov 19, 2014 at 03:56:42PM +0000, Arnd Bergmann wrote:
> On Wednesday 19 November 2014 15:46:35 Catalin Marinas wrote:
> > Going back to original topic, the dma_supported() function on arm64
> > calls swiotlb_dma_supported() which actually checks whether the swiotlb
> > bounce buffer is within the dma mask. This transparent bouncing (unlike
> > arm32 where it needs to be explicit) is not always optimal, though
> > required for 32-bit only devices on a 64-bit system. The problem is when
> > the driver is 64-bit capable but forgets to call
> > dma_set_mask_and_coherent() (that's not the only question I got about
> > running out of swiotlb buffers).
> I think it would be nice to warn once per device that starts using the
> swiotlb. Really all 32-bit DMA masters should have a proper IOMMU
> attached.

It would be nice to have a dev_warn_once().

I think it makes sense on arm64 to avoid swiotlb bounce buffers for
coherent allocations altogether. The __dma_alloc_coherent() function
already checks coherent_dma_mask and sets GFP_DMA accordingly. If we
have a device that cannot even cope with a 32-bit ZONE_DMA, we should
just not support DMA at all on it (without an IOMMU). The arm32
__dma_supported() has a similar check.

Swiotlb is still required for the streaming DMA since we get bouncing
for pages allocated outside the driver control (e.g. VFS layer which
doesn't care about GFP_DMA), hoping a 16M bounce buffer would be enough.

Ding seems to imply that CMA fixes the problem, which means that the
issue is indeed coherent allocations.

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