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

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Date:   Wed, 11 Apr 2018 09:28:54 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     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 Tue, 10 Apr 2018 20:10:20 +0200,
Christoph Hellwig wrote:
> 
> On Tue, Apr 10, 2018 at 06:50:04PM +0100, Robin Murphy wrote:
> > In the first one, the machine appears to have enough RAM that most of it is 
> > beyond the device's 36-bit DMA mask, thus it's fairly likely for the 
> > initial direct alloc to come back with something unsuitable.
> 
> 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.

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...

thanks,

Takashi