On 06/01/2015 11:44 AM, 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 > #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. > > These flags can be set for every vb2 dma implementation: > > 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. > > The flags field can be extended with things like OPAQUE if the buffers cannot be > mmapped (since they are in protected memory). > > Comments? Alternative solutions? I realized that we need this information for each plane. v4l2_requestbuffers does not have enough room for that so for now I will do a test implementation using v4l2_create_buffers only. Perhaps later requestbuffers can just expose the 'worst case' alignment requirements which would be fine in most (and currently AFAIK all) cases. Regards, Hans -- 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
