[Linaro-mm-sig] [PATCH/RFC 0/8] ARM: DMA-mapping framework redesign

Subash Patel subashrp at gmail.com
Wed Jun 22 04:53:15 EDT 2011

On 06/22/2011 12:29 PM, Marek Szyprowski wrote:
> Hello,
> On Wednesday, June 22, 2011 6:53 AM Subash Patel wrote:
>> On 06/20/2011 01:20 PM, Marek Szyprowski wrote:
>>> Hello,
>>> This patch series is a continuation of my works on implementing generic
>>> IOMMU support in DMA mapping framework for ARM architecture. Now I
>>> focused on the DMA mapping framework itself. It turned out that adding
>>> support for common dma_map_ops structure was not that hard as I initally
>>> thought. After some modification most of the code fits really well to
>>> the generic dma_map_ops methods.
>>> The only change required to dma_map_ops is a new alloc function. During
>>> the discussion on Linaro Memory Management meeting in Budapest we got
>>> the idea that we can have only one alloc/free/mmap function with
>>> additional attributes argument. This way all different kinds of
>>> architecture specific buffer mappings can be hidden behind the
>>> attributes without the need of creating several versions of dma_alloc_
>>> function. I also noticed that the dma_alloc_noncoherent() function can
>>> be also implemented this way with DMA_ATTRIB_NON_COHERENT attribute.
>>> Systems that just defines dma_alloc_noncoherent as dma_alloc_coherent
>>> will just ignore such attribute.
>>> Another good use case for alloc methods with attributes is the
>>> possibility to allocate buffer without a valid kernel mapping. There are
>>> a number of drivers (mainly V4L2 and ALSA) that only exports the DMA
>>> buffers to user space. Such drivers don't touch the buffer data at all.
>>> For such buffers we can avoid the creation of a mapping in kernel
>>> virtual address space, saving precious vmalloc area. Such buffers might
>>> be allocated once a new attribute DMA_ATTRIB_NO_KERNEL_MAPPING.
>> Are you trying to say here, that the buffer would be allocated in the
>> user space, and we just use it to map it to the device in DMA+IOMMU
>> framework?
> Nope. I proposed an extension which would allow you to allocate a buffer
> without creating the kernel mapping for it. Right now dma_alloc_coherent()
> performs 3 operations:
> 1. allocates memory for the buffer
> 2. creates coherent kernel mapping for the buffer
> 3. translates physical buffer address to DMA address that can be used by
> the hardware.
> dma_mmap_coherent makes additional mapping for the buffer in user process
> virtual address space.
> I want make the step 2 in dma_alloc_coherent() optional to save virtual
> address space: it is really limited resource. I really want to avoid
> wasting it for mapping 128MiB buffers just to create full-HD processing
> hardware pipeline, where no drivers will use kernel mapping at all.

I think by (2) above, you are referring to 
__dma_alloc_remap()->arm_vmregion_alloc() to allocate the kernel virtual 
address for the drivers use. That makes sense now.

I have a query in similar lines, but related to user virtual address 
space. Is it feasible to extend these DMA interfaces(and IOMMU), to map 
a user allocated buffer into the hardware?

> Best regards


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