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

[Takashi Iwai]

On Thu, 12 Apr 2018 10:03:56 +0200,
Takashi Iwai wrote:
> 
> On Thu, 12 Apr 2018 08:02:27 +0200,
> Christoph Hellwig wrote:
> > 
> > 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?
> 
> Below is a quick attempt, totally untested.  Actually the retry with
> GFP_DMA is superfluous for archs without it, so the first patch
> corrects it.

Gah, scratch this, it doesn't work.  A different check is needed...


Takashi