[PATCH v2 6/7] iommu/rockchip: use DMA API to map, to flush cache

Tomasz Figa tfiga at google.com
Mon Jun 13 03:21:26 PDT 2016

On Mon, Jun 13, 2016 at 6:56 PM, Shunqian Zheng
<shunqian.zheng at gmail.com> wrote:
> Hi
> On 2016年06月10日 17:10, Tomasz Figa wrote:
>> Hi,
>> On Wed, Jun 8, 2016 at 10:26 PM, Shunqian Zheng <zhengsq at rock-chips.com>
>> wrote:
>>> Use DMA API instead of architecture internal functions like
>>> __cpuc_flush_dcache_area() etc.
>>> To support the virtual device like DRM the virtual slave iommu
>>> added in the previous patch, attaching to which the DRM can use
>>> it own domain->dev for dma_map_*(), dma_sync_*() even VOP is disabled.
>>> With this patch, this driver is available for ARM64 like RK3399.
>> Could we instead simply allocate coherent memory for page tables using
>> dma_alloc_coherent() and skip any flushing on CPU side completely? If
>> I'm looking correctly, the driver only reads back the page directory
>> when checking if there is a need to allocate new page table, so there
>> shouldn't be any significant penalty for disabling the cache.
> I try to use dma_alloc_coherent() to replace the dma_map_single(),
> but it doesn't work for me properly.
> Because the DRM uses the iommu_dma_ops instead the swiotlb_dma_ops after
> attaching
> to iommu, so when the iommu domain need to alloc a new page in
> rk_iommu_map(),
> it would call:
> rk_iommu_map()  --> dma_alloc_coherent()  --> ops->alloc()  --> iommu_map()
> --> rk_iommu_map()

It shouldn't call iommu_map(), because the IOMMU is not behind another
IOMMU. Are you sure you called dma_alloc_coherent() on behalf of the
IOMMU struct device and not the DRM device?

Best regards,

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