RE: [PATCH v4 7/7] media: open.rst: add a notice about subdev-API on vdev-centric

[Ramesh Shanmugasundaram]

Hi Hans,

> On 28/08/17 12:30, Mauro Carvalho Chehab wrote:
> > Em Mon, 28 Aug 2017 12:05:06 +0200
> > Hans Verkuil <hverkuil@xxxxxxxxx> escreveu:
> >
> >> On 26/08/17 13:53, Mauro Carvalho Chehab wrote:
> >>> The documentation doesn't mention if vdev-centric hardware control
> >>> would have subdev API or not.
> >>>
> >>> Add a notice about that, reflecting the current status, where three
> >>> drivers use it, in order to support some subdev-specific controls.
> >>
> >> I posted a patch removing v4l-subdevX support for cobalt. It's only
> >> used within Cisco, so this is safe to do and won't break any userspace
> support.
> >
> > OK.
> >
> >> atmel-isc is another driver that creates subdev nodes. Like cobalt,
> >> this is unnecessary. There are no sensors that use private controls.
> >
> > The question is not if the driver has private controls. Private
> > controls can be V4L2 device node oriented.
> >
> > The real question is if userspace applications use subdevs or not in
> > order to set something specific to a subdev, on a pipeline where
> > multiple subdevs could use the same control.
> >
> > E. g. even on a simple case where the driver would have something like:
> >
> > sensor -> processing -> DMA
> >
> > both "sensor" and "processing" could provide the same control (bright,
> > contrast, gain, or whatever). Only by exposing such control via subdev
> > is possible to pinpoint what part of the hardware pipeline would be
> > affected when such control is changed.
> 
> In theory, yes. In practice this does not happen for any of the V4L2-
> centric drivers. Including for the three drivers under discussion.
> 
> >
> >> This driver is not referenced anywhere (dts or board file) in the
> kernel.
> >> It is highly unlikely anyone would use v4l-subdevX nodes when there
> >> is no need to do so. My suggestion is to add a kernel option for this
> >> driver to enable v4l-subdevX support, but set it to 'default n'.
> >> Perhaps with a note in the Kconfig description and a message in the
> >> kernel log that this will be removed in the future.
> >>
> >> The final driver is rcar_drif that uses this to set the "I2S Enable"
> >> private control of the max2175 driver.
> >>
> >> I remember that there was a long discussion over this control. I
> >> still think that there is no need to mark this private.
> >
> > The problem with I2S is that a device may have multiple places where
> > I2S could be used. I don't know how the rcar-drif driver uses it, but
> > there are several vdev-centric boards that use I2S for audio.
> >
> > On several of the devices I worked with, the I2S can be enabled, in
> > runtime, if the audio signal would be directed to some digital output,
> > or it can be disabled if the audio signal would be directed to some
> > analog output. Thankfully, on those devices, I2S can be indirectly
> > controlled via either an ALSA mixer or via VIDIOC A/V routing ioctls.
> > Also, there's just one I2S bus on them.
> >
> > However, on a device that have multiple I2S bus, userspace should be
> > able to control each of them individually, as some parts of the
> > pipeline may require it enabled while others may require it disabled.
> > So, I strongly believe that this should be a subdev control on such
> > hardware.
> >
> > That's said, I don't know how rcar_drif uses it. If it has just one
> > I2S bus and it is used only for audio, then VIDIOC A/V routing ioctls
> > and/or an ALSA mixer could replace it. If not, then it should be kept
> > as-is and the driver would need to add support for MC, in order for
> > applications to identify the right sub-devices that are associated
> > with the pipelines where I2S will be controlled.
> 
> Ramesh, do applications using rcar_drif + max2175 have to manually enable
> the i2s? Shouldn't this be part of the device tree description instead?
> 

Yes, applications have to control this explicitly. It is not only enable but also disable control is used at run time and hence DT is not applicable. 

rcar_drif has two registers to write to enable rx on two data pins. It expects a sequence where the master stops output (in this max2175 i2s output - disable) - enable rcar_drif rx and then the master starts output (max2175 i2s output - enable). The application ensures this sequence today. It is one I2S bus and it is not used for audio but raw I/Q samples from max2175 tuner. 

The v4l2_subdev_tuner_ops does not have .s_stream api as in v4l2_subdev_video_ops and v4l2_subdev_audio_ops. If we plan to have one this functionality may be hidden inside it and no need for an explicit control. I too do not like a private control option.

Thanks,
Ramesh