On 03/22/2012 03:52 PM, Subash Patel wrote: > Hi Laurent, > > On 03/22/2012 08:12 PM, Laurent Pinchart wrote: >> Hi Tomasz, >> >> On Thursday 22 March 2012 14:36:33 Tomasz Stanislawski wrote: >>> Hi Laurent, >>> Thank you very much for your comments and question. >>> They were very useful. >> >> You're welcome. >> >>> Please refer to the comments below. >>> >>> On 03/22/2012 11:50 AM, Laurent Pinchart wrote: >>>> On Thursday 22 March 2012 11:02:23 Laurent Pinchart wrote: >>>>> From: Tomasz Stanislawski<t.stanislaws@xxxxxxxxxxx> >>>>> [snip] >>> The physical address is already present in buf->dma_addr, but it is only >>> valid if the device has no MMU. Notice that vb2-dma-contig possess no >>> knowledge if MMU is present for a given device. >> >> That's why buf->dma_addr can't be considered as a physical address. It's only >> useful in the device context. >> >>> The sg list is not going to be single-entry if the device is provided with >>> its own MMU. >> >> There's something I don't get then. vb2-dma-contig deals with physically >> contiguous buffers. The buffer is backed by physically contiguous pages, so >> the sg list should have a single entry. >> > I think at present, vb2-dma-contig is abused for any kind of memory allocation (continuous or not). Wouldnt it be good to have a > proper working setup for videobuf2-dma-sg instead? Driver which chooses to use continuous, shall assign vb2_queue->mem_ops = > vb2_dma_contig_memops. The devices which know they have a MMU backing can assign the same to vb2_dma_sg_memops. But as of now, > we try to use vb2_dma_contig_memops for all kind of operation. I have also done this mistake, and wish I repaired it and posted > it before :( > Hi Subash, At first, I do not think that vb2-dma-contig is abused for any kind of allocation. It is used only for DMA coherent contiguous mapping what is very close to vb2-dma-contig's original purpose. One thing has to be said loudly: "The driver does not need to and should not know if IOMMU is present or not." The DMA framework must know if device uses IOMMU or not. Reason: the memory is allocated, free, flushed, mapped only by DMA framework. Usage of vb2-dma-contig or vb2-dma-sg depends only on the way how the memory is configured into the device. Most of embedded devices use only buffer's start address and buffer size (often indirectly specified by width/height and format). It mean that device needs a buffer that is contiguous for its DMA engine. In such a case, the driver should ALWAYS use vb2-dma-contig. The DMA framework hidden deep in dma_alloc_coherent internals will deal with IOMMU configuration. I hope you find this information useful. Regards, Tomasz Stanislawski _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
