Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures

[Christoph Hellwig]

On Wed, Apr 11, 2018 at 09:28:54AM +0200, Takashi Iwai wrote:
> > But we should try a GFP_DMA32 allocation first, so this is a bit
> > surprising.
> 
> Hm, do we really try that?
> Through a quick glance, dma_alloc_coherent_gfp_flags() gives GFP_DMA32
> only when coherent mask <= DMA_BIT_MASK(32); in the case of iwlwifi,
> it's 36bit, so GFP_DMA isn't set.

Oh, yes - it is using an odd dma mask, and amdgpu seems to use an
just as odd 40-bit dma mask.

> We had a fallback allocation with GFP_DMA32 in the past, but this
> seems gone long time ago along with cleanups (commit c647c3bb2d16).
> 
> But I haven't followed about this topic for long time, so I might have
> missed obviously...

I think a fallback would be much better here rather than relying on the
limited swiotlb buffer bool.  dma_direct_alloc (which in 4.17 is also
used for x86) already has a GFP_DMA fallback, so extending this for
GFP_DMA32 as well would seem reasonable.

Any volunteers?