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Message-ID: <s5htvsc6es9.wl-tiwai@suse.de> Date: Sun, 15 Apr 2018 10:43:18 +0200 From: Takashi Iwai <tiwai@...e.de> To: Robin Murphy <robin.murphy@....com> Cc: Takashi Iwai <tiwai@...e.de>, Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, linux-kernel@...r.kernel.org, Christian K6nig <christian.koenig@....com> Subject: Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures On Thu, 12 Apr 2018 12:32:54 +0200, Robin Murphy wrote: > > On 12/04/18 09:27, Takashi Iwai wrote: > > On Thu, 12 Apr 2018 10:19:05 +0200, > > Takashi Iwai wrote: > >> > >> 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... > > > > The v2 patches are below, replaced with IS_ENABLED(CONFIG_ZONE_DMA). > > That looks pretty reasonable to me - I'd be tempted to try a factoring > out a helper function to avoid the "goto again" logic, but that's > hardly crucial. Right, this change won't make the code flow more complex than before, so I guess it's OK as a first step. I'll submit this second patch as RFC while the first one as a safe fix (in addition to another regression fix I stumbled on). > What I'd really love to see is something like the alloc_pages_mask() > proposal from years ago to come back such that GFP_DMA32 could die > entirely, but I don't know anywhere near enough about the mm layer to > consider taking that on myself :( Yeah, it'd be lovely, but I'm afraid it's hard to fly high... thanks, Takashi
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