linux-kernel - Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc

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Date:   Thu, 12 Apr 2018 08:02:27 +0200
From:   Christoph Hellwig <hch@....de>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian König <christian.koenig@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent()
        failures

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?