Re: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and getting of frame rates

[Steve Longerbeam]

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.

Steve