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

From: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
To: Joonyoung Shim <jy0922.shim@samsung.com>,
	ML dri-devel <dri-devel@lists.freedesktop.org>,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: drm/exynos: when to call dma_map_sg() on a GEM object?
Date: Mon, 2 Nov 2015 11:29:31 +0100	[thread overview]
Message-ID: <56373B0B.2050902@math.uni-bielefeld.de> (raw)
In-Reply-To: <563713C4.2050309@samsung.com>

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.

With best wishes,
Tobias

> 
> Thanks.
> 

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