drm/exynos: when to call dma_map_sg() on a GEM object?

[Inki Dae]

Hi Tobias,

2015년 11월 02일 19:29에 Tobias Jakobi 이(가) 쓴 글:
> Hello Joonyoung,
> 
> 
> Joonyoung Shim wrote:
>> On 10/31/2015 05:44 AM, Tobias Jakobi wrote:
>>> Hey there,
>>>
>>> this question arose during some discussion with someone concerning the
>>> Exynos mixer and G2D.
>>>
>>> The question is the following. Consider the Exynos mixer when run under
>>> the IOMMU (that's sysmmu_tv IIRC). What exactly does setup the IOMMU
>>> mapping so that the mixer can scanout the framebuffer?
>>>
>>
>> IOMMU mapping is in dma_alloc_attrs(). Already IOMMU was integrated in
>> the DMA mapping API on ARM arch.
> Thanks, I'm going to take a closer look at dma_alloc_attrs() then.
> 
> 
>>> There is exynos_gem_map_sgt_with_dma() in the Exynos GEM code, but it's
>>> currently exclusively used for the G2D and only when dealing with
>>> userptr (and not GEM) there.
>>>
>>> I was looking at exynos_drm_alloc_buf() since this called when
>>> allocating a BO to be used as scanout.
>>>
>>> I see dma_alloc_attrs() being called, which also sets the DMA address
>>> that is later used in the mixer code. But DMA_ATTR_NO_KERNEL_MAPPING is
>>> passed, so no mapping is done at this point.
>>>
>>
>> Did you read Documentation/DMA-attributes.txt document?
> I did, but I guess I should read it again...
> 
> 
>> DMA_ATTR_NO_KERNEL_MAPPING is just to avoid creating a kernel virtual
>> mapping for the allocated buffer on ARM arch.
> Ah OK, so the IOMMU mapping is always established.
> 
> 
>>> Is the mapping done somewhere else, or is it simply not necessary here?
>>>
>>
>> What is the mapping you mean? As you know, the DMA address of the
>> memory gets from dma_alloc_attrs() and it can be used by device. There
>> is no reason to use dma_map_*().
>>From my understanding the mixer doesn't directly access the physical
> memory when running under IOMMU. It accesses memory through sysmmu_tv,
> so my question was who does the setup so that sysmmu_tv "knows" how to
> translate memory accesses.

That is done by dma-mapping API, dma_alloc_attrs function in case of Exynos DRM.

This dma mapping API calls internally iommu_map function of iommu framework
after allocating physical pages and iova space, and the iova space will be
mapped with physical pages.

Please, look into the dma mapping framework - arch/arm/mm/dma-mapping.c -
if you want to understand. I think you could understand it easily if you
tried to look into exynos_drm_alloc_buf function before asking for it.

Thanks,
Inki Dae

> 
> 
> With best wishes,
> Tobias
> 
> 
>>
>> Thanks.
>>
> 
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