Hi Hans, Thanks for the RFC. On Mon, Jun 01, 2015 at 11:44:51AM +0200, Hans Verkuil wrote: > One of the things that is really irritating is the fact that drivers that > use contig-dma sometimes want to support USERPTR, allowing applications to > pass pointers to the driver that point to physically contiguous memory that > was somehow obtained, and that userspace has no way of knowing whether the > driver has this requirement or not. > > A related issue is that, depending on the DMA engine, the user pointer might > have some alignment requirements (page aligned, or at minimum 16 bytes aligned) > that userspace has no way of knowing. > > The same alignment issue is present also for dma-buf. > > I propose to take one reserved field from struct v4l2_create_buffers and > from struct v4l2_requestbuffers and change it to this: > > __u32 flags; > > #define V4L2_REQBUFS_FL_ALIGNMENT_MSK 0x3f How about V4L2_REQBUFS_FL_ALIGN_MASK instead? It's shorter, and that msk part looks odd to me. > #define V4L2_REQBUFS_FL_PHYS_CONTIG (1 << 31) > > Where the alignment is a power of 2 (and if 0 the alignment is unknown). The max > is 2^63, which should be enough for the foreseeable future :-) > > If the physically contiguous flag is set, then the buffer must be physically > contiguous. Both only apply to userptr buffers. I guess saying this in documentation only is enough. The approach looks good to me. > dma-contig: the PHYS_CONTIG flag is always set and the alignment is (unless overridden > by the driver) page aligned. > > dma-sg: the PHYS_CONTIG flag is 0 and the alignment will depend on the driver DMA > implementation. Note: malloc will align the buffer to 8 bytes on a 32 bit OS and 16 bytes > on a 64 bit OS. > > vmalloc: PHYS_CONFIG is 0 and the alignment should be 3 (2^3 == 8) for 32 bit and > 4 (2^4=16) for 64 bit OS. This matches malloc() which will align the buffer to > 8 bytes on a 32 bit OS and 16 bytes on a 64 bit OS. Ack. Many dma-sg drivers actually can handle physically contiguous memory since they're behind an IOMMU; the drivers can then set the flag if needed. -- Regards, Sakari Ailus e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
