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@xxxxxxxx Mobile: +41 (0)79 365 57 48 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>