On Wed August 1 2012 22:49:57 Rémi Denis-Courmont wrote: > Le mercredi 1 août 2012 14:35:03 Laurent Pinchart, vous avez écrit : > > > But in general, the V4L element in the pipeline does not know how fast > > > the downstream element(s) will consume the buffers. Thus it has to copy > > > from the MMAP buffers into anonymous user memory pending processing. > > > Then any dequeued buffer can be requeued as soon as possible. In theory, > > > it might also be that, even though the latency is known, the number of > > > required buffers exceeds the maximum MMAP buffers count of the V4L > > > device. Either way, user space ends up doing memory copy from MMAP to > > > custom buffers. > > > > > > This problem does not exist with USERBUF - the V4L2 element can simply > > > allocate a new buffer for each dequeued buffer. > > > > What about using the CREATE_BUFS ioctl to add new MMAP buffers at runtime ? > > Does CREATE_BUFS always work while already streaming has already started? If > it depends on the driver, it's kinda helpless. Yes, it does. It's one of the reasons it exists in the first place. But there are currently only a handful of drivers that implement it. I hope that as more and more drivers are converted to vb2 that the availability of create_bufs will increase. > What's the guaranteed minimum buffer count? It seems in any case, MMAP has a > hard limit of 32 buffers (at least videobuf2 has), though one might argue this > should be more than enough. Minimum or maximum? The maximum is 32, that's hardcoded in the V4L2 core. Although drivers may force a lower maximum if they want. I have no idea whether there are drivers that do that. There probably are. The minimum is usually between 1 and 3, depending on hardware limitations. 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
