From mboxrd@z Thu Jan  1 00:00:00 1970
From: catalin.marinas@arm.com (Catalin Marinas)
Date: Fri, 21 Nov 2014 11:06:10 +0000
Subject: For the problem when using swiotlb
In-Reply-To: <3918100.vD6471iH9k@wuerfel>
References: <5469E26B.2010905@huawei.com> <1497289.TPKf3qDiKk@wuerfel>
 <20141119154634.GE7156@e104818-lin.cambridge.arm.com>
 <3918100.vD6471iH9k@wuerfel>
Message-ID: <20141121110609.GC19783@e104818-lin.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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