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.Regards,
Abhinav
Hi!
Recent changes to ioremap and unification of vmalloc regions on ARM
significantly reduces the possible size of the consistent dma region and
limited allowed dma coherent/writecombine allocations.
This experimental patch series replaces custom consistent dma regions
usage in dma-mapping framework in favour of generic vmalloc areas
created on demand for each coherent and writecombine allocations.
This patch is based on vanilla v3.4-rc2 release.
Best regards
Marek Szyprowski
Samsung Poland R&D Center
Patch summary:
Marek Szyprowski (4):
mm: vmalloc: use const void * for caller argument
mm: vmalloc: export find_vm_area() function
mm: vmalloc: add VM_DMA flag to indicate areas used by dma-mapping
framework
ARM: remove consistent dma region and use common vmalloc range for
dma allocations
arch/arm/include/asm/dma-mapping.h | 2 +-
arch/arm/mm/dma-mapping.c | 220 +++++++-----------------------------
include/linux/vmalloc.h | 10 +-
mm/vmalloc.c | 31 ++++--
4 files changed, 67 insertions(+), 196 deletions(-)
--
1.7.1.569.g6f426
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