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... 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
