linux-kernel - Re: For the problem when using swiotlb

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Message-ID: <20141121110609.GC19783@e104818-lin.cambridge.arm.com>
Date:	Fri, 21 Nov 2014 11:06:10 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ding Tianhong <dingtianhong@...wei.com>
Subject: Re: For the problem when using swiotlb

On Wed, Nov 19, 2014 at 03:56:42PM +0000, Arnd Bergmann wrote:
> On Wednesday 19 November 2014 15:46:35 Catalin Marinas wrote:
> > Going back to original topic, the dma_supported() function on arm64
> > calls swiotlb_dma_supported() which actually checks whether the swiotlb
> > bounce buffer is within the dma mask. This transparent bouncing (unlike
> > arm32 where it needs to be explicit) is not always optimal, though
> > required for 32-bit only devices on a 64-bit system. The problem is when
> > the driver is 64-bit capable but forgets to call
> > dma_set_mask_and_coherent() (that's not the only question I got about
> > running out of swiotlb buffers).
> 
> I think it would be nice to warn once per device that starts using the
> swiotlb. Really all 32-bit DMA masters should have a proper IOMMU
> attached.

It would be nice to have a dev_warn_once().

I think it makes sense on arm64 to avoid swiotlb bounce buffers for
coherent allocations altogether. The __dma_alloc_coherent() function
already checks coherent_dma_mask and sets GFP_DMA accordingly. If we
have a device that cannot even cope with a 32-bit ZONE_DMA, we should
just not support DMA at all on it (without an IOMMU). The arm32
__dma_supported() has a similar check.

Swiotlb is still required for the streaming DMA since we get bouncing
for pages allocated outside the driver control (e.g. VFS layer which
doesn't care about GFP_DMA), hoping a 16M bounce buffer would be enough.

Ding seems to imply that CMA fixes the problem, which means that the
issue is indeed coherent allocations.

-- 
Catalin
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