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

[Russell King - ARM Linux]

On Tue, Feb 21, 2017 at 02:37:57PM +0200, Sakari Ailus wrote:
> Hi Russell,
> 
> On Tue, Feb 21, 2017 at 12:13:32AM +0000, Russell King - ARM Linux wrote:
> > On Tue, Feb 21, 2017 at 12:04:10AM +0200, Sakari Ailus wrote:
> > > On Wed, Feb 15, 2017 at 06:19:31PM -0800, Steve Longerbeam wrote:
> > > > From: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
> > > > 
> > > > Setting and getting frame rates is part of the negotiation mechanism
> > > > between subdevs.  The lack of support means that a frame rate at the
> > > > sensor can't be negotiated through the subdev path.
> > > 
> > > Just wondering --- what do you need this for?
> > 
> > The v4l2 documentation contradicts the media-ctl implementation.
> > 
> > While v4l2 documentation says:
> > 
> >   These ioctls are used to get and set the frame interval at specific
> >   subdev pads in the image pipeline. The frame interval only makes sense
> >   for sub-devices that can control the frame period on their own. This
> >   includes, for instance, image sensors and TV tuners. Sub-devices that
> >   don't support frame intervals must not implement these ioctls.
> > 
> > However, when trying to configure the pipeline using media-ctl, eg:
> > 
> > media-ctl -d /dev/media1 --set-v4l2 '"imx219 pixel 0-0010":0[crop:(0,0)/3264x2464]'
> > media-ctl -d /dev/media1 --set-v4l2 '"imx219 0-0010":1[fmt:SRGGB10/3264x2464@1/30]'
> > media-ctl -d /dev/media1 --set-v4l2 '"imx219 0-0010":0[fmt:SRGGB8/816x616@1/30]'
> > media-ctl -d /dev/media1 --set-v4l2 '"imx6-mipi-csi2":1[fmt:SRGGB8/816x616@1/30]'
> > Unable to setup formats: Inappropriate ioctl for device (25)
> > media-ctl -d /dev/media1 --set-v4l2 '"ipu1_csi0_mux":2[fmt:SRGGB8/816x616@1/30]'
> > media-ctl -d /dev/media1 --set-v4l2 '"ipu1_csi0":2[fmt:SRGGB8/816x616@1/30]'
> > 
> > The problem there is that the format setting for the csi2 does not get
> > propagated forward:
> 
> The CSI-2 receivers typically do not implement frame interval IOCTLs as they
> do not control the frame interval. Some sensors or TV tuners typically do,
> so they implement these IOCTLs.

No, TV tuners do not.  The frame rate for a TV tuner is set by the
broadcaster, not by the tuner.  The tuner can't change that frame rate.
The tuner may opt to "skip" fields or frames.  That's no different from
what the CSI block in my example below is capable of doing.

Treating a tuner differently from the CSI block is inconsistent and
completely wrong.

> There are alternative ways to specify the frame rate.

Empty statements (or hand-waving type statements) I'm afraid don't
contribute to the discussion, because they mean nothing to me.  Please
give an example, or flesh out what you mean.

> > $ strace media-ctl -d /dev/media1 --set-v4l2 '"imx6-mipi-csi2":1[fmt:SRGGB8/816x616@1/30]'
> > ...
> > open("/dev/v4l-subdev16", O_RDWR)       = 3
> > ioctl(3, VIDIOC_SUBDEV_S_FMT, 0xbec16244) = 0
> > ioctl(3, VIDIOC_SUBDEV_S_FRAME_INTERVAL, 0xbec162a4) = -1 ENOTTY (Inappropriate
> > ioctl for device)
> > fstat64(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 2), ...}) = 0
> > write(1, "Unable to setup formats: Inappro"..., 61) = 61
> > Unable to setup formats: Inappropriate ioctl for device (25)
> > close(3)                                = 0
> > exit_group(1)                           = ?
> > +++ exited with 1 +++
> > 
> > because media-ctl exits as soon as it encouters the error while trying
> > to set the frame rate.
> > 
> > This makes implementing setup of the media pipeline in shell scripts
> > unnecessarily difficult - as you need to then know whether an entity
> > is likely not to support the VIDIOC_SUBDEV_S_FRAME_INTERVAL call,
> > and either avoid specifying a frame rate:
> 
> You should remove the frame interval setting from sub-devices that do not
> support it.

That means we end up with horribly complex scripts.  This "solution" does
not scale.  Therefore, it is not a "solution".

It's fine if you want to write a script to setup the media pipeline using
media-ctl, listing _each_ media-ctl command individually, with arguments
specific to each step, but as I've already said, that does not scale.

I don't want to end up writing separate scripts to configure the pipeline
for different parameters or setups.  I don't want to teach users how to
do that either.

How are users supposed to cope with this craziness?  Are they expected to
write their own scripts and understand this stuff?

As far as I can see, there are no applications out there at the moment that
come close to understanding how to configure a media pipeline, so users
have to understand how to use media-ctl to configure the pipeline manually.
Are we really expecting users to write scripts to do this, and understand
all these nuances?

IMHO, this is completely crazy, and hasn't been fully thought out.

> > $ strace media-ctl -d /dev/media1 --set-v4l2 '"imx6-mipi-csi2":1[fmt:SRGGB8/816x616]'
> > ...
> > open("/dev/v4l-subdev16", O_RDWR)       = 3
> > ioctl(3, VIDIOC_SUBDEV_S_FMT, 0xbeb1a254) = 0
> > open("/dev/v4l-subdev0", O_RDWR)        = 4
> > ioctl(4, VIDIOC_SUBDEV_S_FMT, 0xbeb1a254) = 0
> > close(4)                                = 0
> > close(3)                                = 0
> > exit_group(0)                           = ?
> > +++ exited with 0 +++
> > 
> > or manually setting the format on the sink.
> > 
> > Allowing the S_FRAME_INTERVAL call seems to me to be more in keeping
> > with the negotiation mechanism that is implemented in subdevs, and
> > IMHO should be implemented inside the kernel as a pad operation along
> > with the format negotiation, especially so as frame skipping is
> > defined as scaling, in just the same way as the frame size is also
> > scaling:
> 
> The origins of the S_FRAME_INTERVAL IOCTL for sub-devices are the S_PARM
> IOCTL for video nodes. It is used to control the frame rate for more simple
> devices that do not expose the Media controller interface. The similar
> S_FRAME_INTERVAL was added for sub-devices as well, and it has been so far
> used to control the frame interval for sensors (and G_FRAME_INTERVAL to
> obtain the frame interval for TV tuners, for instance).
> 
> The pad argument was added there but media-ctl only supported setting the
> frame interval on pad 0, which, coincidentally, worked well for sensor
> devices.
> 
> The link validation is primarily done in order to ensure the validity of the
> hardware configuration: streaming may not be started if the hardware
> configuration is not valid.
> 
> Also, frame interval is not a static property during streaming: it may be
> changed without the knowledge of the other sub-device drivers downstream. It
> neither is a property of hardware receiving or processing images: if there
> are limitations in processing pixels, then they in practice are related to
> pixel rates or image sizes (i.e. not frame rates).

So what about the case where we have a subdev (CSI) that is capable of
frame rate reduction, that needs to know the input frame rate and the
desired output frame rate?  It seems to me that this has not been
thought through...

> >        -  ``MEDIA_ENT_F_PROC_VIDEO_SCALER``
> > 
> >        -  Video scaler. An entity capable of video scaling must have
> >           at least one sink pad and one source pad, and scale the
> >           video frame(s) received on its sink pad(s) to a different
> >           resolution output on its source pad(s). The range of
> >           supported scaling ratios is entity-specific and can differ
> >           between the horizontal and vertical directions (in particular
> >           scaling can be supported in one direction only). Binning and
> >           skipping are considered as scaling.
> > 
> > Although, this is vague, as it doesn't define what it means by "skipping",
> > whether that's skipping pixels (iow, sub-sampling) or whether that's
> > frame skipping.
> 
> Skipping in the context is used to refer to sub-sampling. The term is often
> used in conjunction of sensors. The documentation could certainly be
> clarified here.

It definitely needs to be, it's currently mis-leading.

> > Then there's the issue where, if you have this setup:
> > 
> >  camera --> csi2 receiver --> csi --> capture
> > 
> > and the "csi" subdev can skip frames, you need to know (a) at the CSI
> > sink pad what the frame rate is of the source (b) what the desired
> > source pad frame rate is, so you can configure the frame skipping.
> > So, does the csi subdev have to walk back through the media graph
> > looking for the frame rate?  Does the capture device have to walk back
> > through the media graph looking for some subdev to tell it what the
> > frame rate is - the capture device certainly can't go straight to the
> > sensor to get an answer to that question, because that bypasses the
> > effect of the CSI frame skipping (which will lower the frame rate.)
> > 
> > IMHO, frame rate is just another format property, just like the
> > resolution and data format itself, and v4l2 should be treating it no
> > differently.
> > 
> > In any case, the documentation vs media-ctl create something of a very
> > obscure situation, one that probably needs solving one way or another.
> 
> Before going to solutions I need to ask: what do you want to achieve?

Full and consistent support for the hardware, and a sane and consistent
way to setup a media pipeline that is easy for everyone to understand.

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