For the problem when using swiotlb

Arnd Bergmann
2014-11-21T11:26:45Z

On Friday 21 November 2014 11:06:10 Catalin Marinas wrote:
> 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.

If we ever encounter this case, we may have to add a smaller ZONE_DMA
and use ZONE_DMA32 for the normal dma allocations.

> 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.

I wonder what's going on here, since swiotlb_alloc_coherent() actually
tries a regular __get_free_pages(flags, order) first, and when ZONE_DMA
is set here, it just work without using the pool.

Arnd