Re: [PATCH v3 0/5] enhance DMA CMA on x86

From: Peter Hurley
Date: Tue Sep 30 2014 - 10:34:50 EST

On 09/29/2014 10:32 AM, Akinobu Mita wrote:
> 2014-09-29 21:09 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>> On 09/27/2014 08:31 PM, Akinobu Mita wrote:
>>> 2014-09-27 23:30 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>>>> On 04/15/2014 09:08 AM, Akinobu Mita wrote:
>>>>> This patch set enhances the DMA Contiguous Memory Allocator on x86.
>> [...]
>>>> What this patchset does is restrict all iommu configurations which can
>>>> map all of system memory to one _very_ small physical region, thus disabling
>>>> the whole point of an iommu.
>>>> Now I know why my GPU is causing paging to disk! And why my RAID controller
>>>> stalls for ages when I do a git log at the same time as a kernel build!
>>> The solution I have for this is that instead of trying to
>>> dma_alloc_from_contiguous() firstly, call alloc_pages() in dma_alloc_coherent().
>>> dma_alloc_from_contiguous() should be called only when alloc_pages() is failed
>>> or DMA_ATTR_FORCE_CONTIGUOUS is specified in dma_attr.
>> Why is all this extra complexity being added when there are no X86 users
> I misunderstood DMA_ATTR_FORCE_CONTIGUOUS. It is specified to request
> that underlaying DMA mapping span physically contiguous with IOMMU.
> But current alloc_dma_coherent() for intel-iommu always returns
> physically contiguous memory, so it is ignored on x86.
>>>> And the apparent goal of this patchset is to enable DMA allocation below
>>>> 4GB, which is already supported in the existing page allocator with the
>>>> GFP_DMA32 flag?!
>>> The goal of this patchset is to enable huge DMA allocation which
>>> alloc_pages() can't (> MAX_ORDER) for the devices that require it.
>> What x86 devices need > MAX_ORDER DMA allocation and why can't they allocate
>> directly from dma_alloc_from_contiguous()?
> I need this for UFS unified memory extension which is apparently not in
> mainline for now.
> But there must be some other use cases on x86, too. Because I have
> received several emails privately from developers who care its status.
> And allocating directly from dma_alloc_from_contiguous() in the driver
> doesn't work with IOMMU, as it just returns memory regoin and doesn't
> create DMA mapping.

I read the UFS Unified Memory Extension v1.0 (JESD220-1) specification and
it is not clear to me that using DMA mapping is the right approach to
supporting UM, at least on x86.

And without a mainline user, the merits of this approach are not evident.
I cannot even find a production x86 UFS controller, much less one that
supports UME.

The only PCI UFS controller I could find (and that mainline supports) is
Samsung's x86 FPGA-based test unit for developing UFS devices in a x86 test
environment, and not a production x86 design.

Samsung's own roadmap (
mentions nothing about bringing UFS to x86 designs.

Unless there's something else I've missed, I don't think these patches
belong in mainline.

Peter Hurley

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