On Fri, Dec 24, 2010 at 5:32 PM, Hans Verkuil <hverk...@xs4all.nl> wrote: > On Friday, December 24, 2010 22:29:37 Rob Clark wrote: >> Hi all, >> >> The request has come up on OMAP4 to support non-page-aligned v4l2 >> buffers. (This is in context of v4l2 display, but the same reasons >> would apply for a camera.) For most common resolutions, this would >> help us get much better memory utilization for a range of memory (or >> rather address space) used for YUV buffers. > > Can you explain this in more detail? I don't really see how non-page > aligned buffers would lead to 'much better' memory usage. I would expect > that the best savings you could achieve would be PAGE_SIZE-1 per buffer. >
Due to how the buffers are mapped, the savings is actually quite substantial. What actually happens is the region of memory that the buffers are allocated from has a stride of 16kb or 32kb. (For NV12, Y has a 16kb stride, and UV is disjoint is a 32kb stride.) To keep things somewhat sane for userspace, the Y followed by UV gets mmap'd into consecutive 4kb pages. So we are actually loosing 1.5 * (4kb - width) per buffer by forcing page alignment. With non page-aligned buffers we can pack buffers next to each other, ie. so one buffer may exist within the stride of another buffer. BR, -R > Regards, > > Hans > >> However it would require >> a small change in the client application, since most (all) v4l2 apps >> that I have seen are assuming the offsets they are given to mmap are >> page aligned. >> >> I am curious if anyone has any suggestions about how to enable this. >> Ideally it would be some sort of opt-in feature to avoid breaking apps >> that are not aware the the offsets to mmap may not be page aligned. >> >> BR, >> -R >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-media" in >> the body of a message to majord...@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > -- > Hans Verkuil - video4linux developer - sponsored by Cisco > -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
