linux-kernel - Re: [PATCH v2 14/31] arm64: DMA mapping API

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Message-ID: <20120821125948.GC12708@arm.com>
Date:	Tue, 21 Aug 2012 13:59:48 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 14/31] arm64: DMA mapping API

On Wed, Aug 15, 2012 at 05:16:00PM +0100, Arnd Bergmann wrote:
> On Tuesday 14 August 2012, Catalin Marinas wrote:
> > +static struct dma_map_ops arm64_swiotlb_dma_ops = {
> > +       .alloc = arm64_swiotlb_alloc_coherent,
> > +       .free = arm64_swiotlb_free_coherent,
> > +       .map_page = arm64_swiotlb_map_page,
> > +       .unmap_page = arm64_swiotlb_unmap_page,
> > +       .map_sg = arm64_swiotlb_map_sg_attrs,
> > +       .unmap_sg = arm64_swiotlb_unmap_sg_attrs,
> > +       .sync_single_for_cpu = arm64_swiotlb_sync_single_for_cpu,
> > +       .sync_single_for_device = arm64_swiotlb_sync_single_for_device,
> > +       .sync_sg_for_cpu = arm64_swiotlb_sync_sg_for_cpu,
> > +       .sync_sg_for_device = arm64_swiotlb_sync_sg_for_device,
> > +       .dma_supported = swiotlb_dma_supported,
> > +       .mapping_error = swiotlb_dma_mapping_error,
> > +};
> > +
> > +void __init swiotlb_init_with_default_size(size_t default_size, int verbose);
> > +
> > +void __init arm64_swiotlb_init(size_t max_size)
> > +{
> > +       dma_ops = &arm64_swiotlb_dma_ops;
> > +       swiotlb_init_with_default_size(min((size_t)SZ_64M, max_size), 1);
> > +}
> 
> Why is swiotlb the default? I would expect that most devices can in fact
> use the entire 64 bit address space, so you can use a simple linear
> implementation for those.

That was my worry, devices not capable of accessing the full 64-bit
address space. We can hope that those SoCs would have an IOMMU but I
can't tell for sure at this stage.

The default implementation could be simpler. I can even drop it
altogether from the initial patchset given that no SoC makes use of it
yet.

-- 
Catalin
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