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
