On Tue, 2017-03-14 at 08:34 +0100, Hans Verkuil wrote: > On 03/13/2017 10:03 PM, Sakari Ailus wrote: > > Hi Steve, > > > > On Mon, Mar 13, 2017 at 11:06:22AM -0700, Steve Longerbeam wrote: > >> > >> > >> On 03/13/2017 06:55 AM, Philipp Zabel wrote: > >>> On Mon, 2017-03-13 at 13:27 +0000, Russell King - ARM Linux wrote: > >>>> On Mon, Mar 13, 2017 at 03:16:48PM +0200, Sakari Ailus wrote: > >>>>> The vast majority of existing drivers do not implement them nor the user > >>>>> space expects having to set them. Making that mandatory would break existing > >>>>> user space. > >>>>> > >>>>> In addition, that does not belong to link validation either: link validation > >>>>> should only include static properties of the link that are required for > >>>>> correct hardware operation. Frame rate is not such property: hardware that > >>>>> supports the MC interface generally does not recognise such concept (with > >>>>> the exception of some sensors). Additionally, it is dynamic: the frame rate > >>>>> can change during streaming, making its validation at streamon time useless. > >>>> > >>>> So how do we configure the CSI, which can do frame skipping? > >>>> > >>>> With what you're proposing, it means it's possible to configure the > >>>> camera sensor source pad to do 50fps. Configure the CSI sink pad to > >>>> an arbitary value, such as 30fps, and configure the CSI source pad to > >>>> 15fps. > >>>> > >>>> What you actually get out of the CSI is 25fps, which bears very little > >>>> with the actual values used on the CSI source pad. > >>>> > >>>> You could say "CSI should ask the camera sensor" - well, that's fine > >>>> if it's immediately downstream, but otherwise we'd need to go walking > >>>> down the graph to find something that resembles its source - there may > >>>> be mux and CSI2 interface subdev blocks in that path. Or we just accept > >>>> that frame rates are completely arbitary and bear no useful meaning what > >>>> so ever. > >>> > >>> Which would include the frame interval returned by VIDIOC_G_PARM on the > >>> connected video device, as that gets its information from the CSI output > >>> pad's frame interval. > >>> > >> > >> I'm kinda in the middle on this topic. I agree with Sakari that > >> frame rate can fluctuate, but that should only be temporary. If > >> the frame rate permanently shifts from what a subdev reports via > >> g_frame_interval, then that is a system problem. So I agree with > >> Phillip and Russell that a link validation of frame interval still > >> makes sense. > >> > >> But I also have to agree with Sakari that a subdev that has no > >> control over frame rate has no business implementing those ops. > >> > >> And then I agree with Russell that for subdevs that do have control > >> over frame rate, they would have to walk the graph to find the frame > >> rate source. > >> > >> So we're stuck in a broken situation: either the subdevs have to walk > >> the graph to find the source of frame rate, or s_frame_interval > >> would have to be mandatory and validated between pads, same as set_fmt. > > > > It's not broken; what we are missing though is documentation on how to > > control devices that can change the frame rate i.e. presumably drop frames > > occasionally. > > > > If you're doing something that hasn't been done before, it may be that new > > documentation needs to be written to accomodate that use case. As we have an > > existing interface (VIDIOC_SUBDEV_[GS]_FRAME_INTERVAL) it does make sense > > to use that. What is not possible, though, is to mandate its use in link > > validation everywhere. > > > > If you had a hardware limitation that would require that the frame rate is > > constant, then we'd need to handle that in link validation for that > > particular piece of hardware. But there really is no case for doing that for > > everything else. > > > > General note: I would strongly recommend that g/s_parm support is removed in > v4l2_subdev in favor of g/s_frame_interval. > > g/s_parm is an abomination... Agreed. Just in this specific case I was talking about G_PARM on the /dev/video node, not the v4l2_subdev nodes. This is currently used by non-subdev-aware userspace to obtain the framerate from the video capture device. > There seem to be only a few i2c drivers that use g/s_parm, so this shouldn't > be a lot of work. > > Having two APIs for the same thing is always very bad. > > Regards, > > Hans > regards Philipp
