Re: [PATCH v3 5/6] iommu/mediatek: Add mt8173 IOMMU driver

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Mon, Jul 27, 2015 at 02:23:26PM +0100, Robin Murphy wrote:
> On 16/07/15 10:04, Yong Wu wrote:
> >This patch adds support for mediatek m4u (MultiMedia Memory Management
> >Unit).
> >
> >Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> [...]
> >+static void mtk_iommu_flush_pgtable(void *ptr, size_t size, void *cookie)
> >+{
> >+       struct mtk_iommu_domain *domain = cookie;
> >+       unsigned long offset = (unsigned long)ptr & ~PAGE_MASK;
> >+
> >+       dma_map_page(domain->data->dev, virt_to_page(ptr), offset,
> >+                    size, DMA_TO_DEVICE);
> 
> Nit: this looks like it may as well be dma_map_single.
> 
> It would probably be worth following it with a matching unmap too, just to
> avoid any possible leakage bugs (especially if this M4U ever appears in a
> SoC supporting RAM above the 32-bit boundary).

Why not do the job properly?  Take a look at how I implemented the
streaming DMA API on Tegra SMMU (patch set recently sent out earlier
today).

There's no need for hacks like dma_map_page() (and discarding it's
return value) or dma_map_page() followed by dma_unmap_page().

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