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

[Russell King - ARM Linux <linux@armlinux.org.uk>]

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@armlinux.org.uk>
> > 
> > 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:

$ 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:

$ 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:

       -  ``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.

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.

-- 
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