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

[Robin Murphy <robin.murphy@arm.com>]

On 27/07/15 16:31, Russell King - ARM Linux wrote:
> 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().

Indeed, as it happens I do have a branch where I prototyped that for the 
long-descriptor io-pgtable-arm code a while ago; this discussion has 
prompted me to dig it up again. Stay tuned, folks...

Robin.