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

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Date:	Mon, 13 Jun 2016 19:21:26 +0900
From:	Tomasz Figa <tfiga@...gle.com>
To:	shunqian.zheng@...il.com
Cc:	Shunqian Zheng <zhengsq@...k-chips.com>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org,
	Heiko Stübner <heiko@...ech.de>,
	David Airlie <airlied@...ux.ie>,
	Joerg Roedel <joro@...tes.org>, linux@...linux.org.uk,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"open list:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Rob Herring
	<robh+dt@...nel.org>, simon xue <xxm@...k-chips.com>,
	linux-arm-kernel@...ts.infradead.org" <mark.yao@...k-chips.com>
Subject: Re: [PATCH v2 6/7] iommu/rockchip: use DMA API to map, to flush cache

On Mon, Jun 13, 2016 at 6:56 PM, Shunqian Zheng
<shunqian.zheng@...il.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@...k-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,
Tomasz