Re: [Linaro-mm-sig] [PATCH 0/4] ARM: replace custom consistent dma region with vmalloc

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Apr 23, 2012 at 09:02:25PM +0900, Abhinav Kochhar wrote:
> Hi,
> 
> I see a bottle-neck with the current dma-mapping framework.
> Issue seems to be with the Virtual memory allocation for access in kernel
> address space.
> 
> 1. In "arch/arm/mm/dma-mapping.c" there is a initialization call to
> "consistent_init". It reserves size 32MB of Kernel Address space.
> 2. "consistent_init" allocates memory for kernel page directory and page
> tables.
> 
> 3. "__iommu_alloc_remap" function allocates virtual memory region in kernel
> address space reserved in step 1.
> 
> 4. "__iommu_alloc_remap" function then maps the allocated pages to the
> address space reserved in step 3.
> 
> Since the virtual memory area allocated for mapping these pages in kernel
> address space is only 32MB,
> 
> eventually the calls for allocation and mapping new pages into kernel
> address space are going to fail once 32 MB is exhausted.
> 
> e.g., For Exynos 5 platform Each framebuffer for 1280x800 resolution
> consumes around 4MB.
> 
> We have a scenario where X11 DRI driver would allocate Non-contig pages for
> all "Pixmaps" through arm_iommu_alloc_attrs" function which will follow the
> path given above in steps 1 - 4.
> 
> Now the problem is the size limitation of 32MB. We may want to allocate
> more than 8 such buffers when X11 DRI driver is integrated.
> Possible solutions:
> 
> 1. Why do we need to create a kernel virtual address space? Are we going to
> access these pages in kernel using this address?
> 
> If we are not going to access anything in kernel then why do we need to map
> these pages in kernel address space?. If we can avoid this then the problem
> can be solved.
> 
> OR
> 
> 2 Is it used for only book-keeping to retrieve "struct pages" later on for
> passing/mapping to different devices?
> 
> If yes, then we have to find another way.
> 
> For "dmabuf" framework one solution could be to add a new member variable
> "pages" in the exporting driver's local object and use that for
> passing/mapping to different devices.
> 
> Moreover, even if we increase to say 64 MB that would not be enough for our
> use, we never know how many graphic applications would be spawned by the
> user.
> Let me know your opinion on this.

This is more or less the reason I'm so massively opposed to adding vmap to
dma-buf - you _really_ burn through the vmap space ridiculously quickly on
32bit platforms with too much memory (i.e. everything with more than 1 G).

You need to map/unmap everything page-by-page with all the usual kmap apis
the kernel provides.
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48