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From: Shuah Khan <>
To: Russell King - ARM Linux <>,
	Marek Szyprowski <>
	Shuah Khan <>
Subject: Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page
Date: Wed, 19 Apr 2017 17:38:10 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Russell, and Marek,

On 04/14/2017 03:46 AM, Russell King - ARM Linux wrote:
> On Fri, Apr 14, 2017 at 09:56:07AM +0200, Marek Szyprowski wrote:
>>>> This would be however quite large task, especially taking into account
>>>> all current users of DMA-buf framework...
>>> Yeah it will be a large task.
>> Maybe once scatterlist are switched to pfns, changing dmabuf internal
>> memory representation to pfn array might be much easier.
> Switching to a PFN array won't work either as we have no cross-arch
> way to translate PFNs to a DMA address and vice versa.  Yes, we have
> them in ARM, but they are an _implementation detail_ of ARM's
> DMA API support, they are not for use by drivers.
> So, the very first problem that needs solving is this:
>   How do we go from a coherent DMA allocation for device X to a set
>   of DMA addresses for device Y.
> Essentially, we need a way of remapping the DMA buffer for use with
> another device, and returning a DMA address suitable for that device.
> This could well mean that we need to deal with setting up an IOMMU
> mapping.  My guess is that this needs to happen at the DMA coherent
> API level - the DMA coherent API needs to be augmented with support
> for this.  I'll call this "DMA coherent remap".
> We then need to think about how to pass this through the dma-buf API.
> dma_map_sg() is done by the exporter, who should know what kind of
> memory is being exported.  The exporter can avoid calling dma_map_sg()
> if it knows in advance that it is exporting DMA coherent memory.
> Instead, the exporter can simply create a scatterlist with the DMA
> address and DMA length prepopulated with the results of the DMA
> coherent remap operation above.

As Russell pointed to armama-drm case, I looked at that closely.
armada-drm is creating sg_table and populating it with DMA-address in
its map_dma_buf ops and unmap_dma_buf ops handles the special case and
doesn't call dma_unmap_sg().

In the case of drm, gem_prime_map_dma_buf interfaces and the common
drm_gem_map_dma_buf() will need modification to not do dma_map_sg()
and create scatterlist with the DMA address and DMA length instead.
We have to get drm_gem_map_dma_buf() info. to have it not do dma_map_sg()
and create scatterlist.

Focusing on drm for now, looks like there are probably about 15 or so
map_dma_buf interfaces will need to handle coherent memory case.

> What the scatterlist can't carry in this case is a set of valid
> struct page pointers, and an importer must not walk the scatterlist
> expecting to get at the virtual address parameters or struct page
> pointers.

Right - importers need handling to not walk the sg_list and handle
it differently. Is there a good example drm you can point me to for
this? aramda-drm seems to special case this in armada_gem_map_import()
if I am not mistaken.

> On the mmap() side of things, remember that DMA coherent allocations
> may require special mapping into userspace, and which can only be
> mapped by the DMA coherent mmap support.  kmap etc will also need to
> be different.  So it probably makes sense for DMA coherent dma-buf
> exports to use a completely separate set of dma_buf_ops from the
> streaming version.

I agree. It would make is easier and also limits the scope of changes.

> I think this is the easiest approach to solving the problem without
> needing massive driver changes all over the kernel.

Anyway this is a quick note to say that I am looking into this and
haven't drooped it :)

-- Shuah

      parent reply	other threads:[~2017-04-19 23:38 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2017-04-05 16:02 ` Shuah Khan
2017-04-05 23:14   ` Russell King - ARM Linux
2017-04-10 14:52     ` Shuah Khan
2017-04-06  4:11   ` kbuild test robot
2017-04-06 12:01   ` Marek Szyprowski
2017-04-10 22:50     ` Shuah Khan
2017-04-14  7:56       ` Marek Szyprowski
2017-04-14  9:46         ` Russell King - ARM Linux
2017-04-17  1:10           ` Shuah Khan
2017-04-17 10:22             ` Russell King - ARM Linux
2017-04-19 23:38           ` Shuah Khan [this message]

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