Re: [RFC 0/2] ARM: DMA-mapping & IOMMU integration

[KyongHo Cho <pullip.cho@samsung.com>]

Hi.

On Tue, Jun 14, 2011 at 12:07 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> I'm sure that the graphics people will disagree with you on that.
> Having the frame buffer mapped in write-combine mode is rather
> important when you want to efficiently output videos from your
> CPU.
>
I agree with you.
But I am discussing about dma_alloc_writecombine() in ARM.
You can see that only ARM and AVR32 implement it and there are few
drivers which use it.
No function in dma_map_ops corresponds to dma_alloc_writecombine().
That's why Marek tried to add 'alloc_writecombine' to dma_map_ops.

> I can understand that there are arguments why mapping a DMA buffer into
> user space doesn't belong into dma_map_ops, but I don't see how the
> presence of an IOMMU is one of them.
>
> The entire purpose of dma_map_ops is to hide from the user whether
> you have an IOMMU or not, so that would be the main argument for
> putting it in there, not against doing so.
>
I also understand the reasons why dma_map_ops maps a buffer into user space.
Mapping in device and user space at the same time or in a simple
approach may look good.
But I think mapping to user must be and driver-specific.
Moreover, kernel already provides various ways to map physical memory
to user space.
And I think that remapping DMA address that is in device address space
to user space is not a good idea
because DMA address is not same to physical address semantically if
features of IOMMU are implemented.

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