RE: [PATCH v6 3/7] media: i2c: max2175: Add MAX2175 support

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> Em Thu, 8 Jun 2017 09:42:43 +0000
> Ramesh Shanmugasundaram <ramesh.shanmugasundaram@xxxxxxxxxxxxxx> escreveu:
> 
> > > Subject: Re: [PATCH v6 3/7] media: i2c: max2175: Add MAX2175 support
> > >
> > > Em Wed, 31 May 2017 09:44:53 +0100
> > > Ramesh Shanmugasundaram <ramesh.shanmugasundaram@xxxxxxxxxxxxxx>
> escreveu:
> > >
> > > > +++ b/Documentation/media/v4l-drivers/max2175.rst
> > > > @@ -0,0 +1,60 @@
> > > > +Maxim Integrated MAX2175 RF to bits tuner driver
> > > > +================================================
> > > > +
> > > > +The MAX2175 driver implements the following driver-specific
> controls:
> > > > +
> > > > +``V4L2_CID_MAX2175_I2S_ENABLE``
> > > > +-------------------------------
> > > > +    Enable/Disable I2S output of the tuner.
> > > > +
> > > > +.. flat-table::
> > > > +    :header-rows:  0
> > > > +    :stub-columns: 0
> > > > +    :widths:       1 4
> > > > +
> > > > +    * - ``(0)``
> > > > +      - I2S output is disabled.
> > > > +    * - ``(1)``
> > > > +      - I2S output is enabled.
> > >
> > > Hmm... There are other drivers at the subsystem that use I2S (for
> > > audio - not for SDR - but I guess the issue is similar).
> > >
> > > On such drivers, the bridge driver controls it directly, being sure
> > > that I2S is enabled when it is expecting some data coming from the I2S
> bus.
> > >
> > > On some drivers, there are both I2S and A/D inputs at the bridge
> chipset.
> > > On such drivers, enabling/disabling I2S is done via VIDIOC_S_INPUT
> > > (and optionally via ALSA mixer), being transparent to the user if
> > > the stream comes from a tuner via I2S or from a directly connected A/D
> input.
> > >
> > > I don't think it is a good idea to enable it via a control, as, if
> > > the bridge driver is expecting data via I2S, disabling it will cause
> > > timeouts at the videobuf handling.
> >
> > The MAX2175 device is exposed as a v4l2 subdev with tuner ops and can
> interface with an SDR device. When the tuner is configured, the I2S output
> is enabled by default. From an independent tuner device perspective, this
> default behaviour is enough and this control may not be needed/used.
> >
> > However, for the use case here, the R-Car DRIF device acts as the main
> SDR device and the Maxim MAX2175 provides a sub-dev interface with tuner
> ops.
> >
> > +---------------------+                +---------------------+
> > |                     |-----SCK------->|CLK                  |
> > |       Master        |-----SS-------->|SYNC  DRIFn (slave)  |
> > |      (MAX2175)      |-----SD0------->|D0                   |
> > |                     |-----SD1------->|D1                   |
> > +---------------------+                +---------------------+
> >
> > The DRIF device design is such that it involves separate register writes
> to enable Rx on each of the data line. To keep both the data lines in sync
> it expects the master device to enable output after both the data line Rx
> are enabled.
> >
> > This level of control is exposed as a feature in the MAX2175 using this
> control. When interfaced with DRIF this control is used to achieve the
> desired functionality. When not interfaced with DRIF, the MAX2175 default
> behaviour does not have to change because of DRIF and hence this I2S
> control may be unused. Like MAX2175, DRIF is also an independent device
> and can interface with a different third party tuner.
> >
> > Hence, this I2S enable/disable is exposed as a user control. The end
> user application (knowing both these devices) is expected to use these
> controls appropriately. Please let me know if I need to explain anything
> in further detail.
> 
> 
> The usecase is clear. That's exactly what other drivers with I2S do,
> except that, on those other drivers, they pass I2S control info via
> platform_data (they're not platform drivers).
> 
> With those drivers, generic applications work as-is via the standard
> video, radio or sdr devnodes, without knowing about I2S.
> 
> The main difference here is that you're requiring an specialized
> application for this device to work, 

Specialized application may be needed for this specific combination of devices (DRIF + MAX2175) only. For MAX2175 alone, a generic application itself would be enough. The MAX2175 will have I2S output enabled by default and the SDR device have to just enable/disable its own RX during .start_streaming/.stop_streaming.

as a generic one won't be aware of
> this device-specific control, and may end by exposing this "internal"
> control to the end user. That is OK for embedded usage, but, as soon as
> this is used on some non-embedded usecase (with is likely, as there are
> several PC consumer products using other chips from Maxim), we'll have
> problems.
> 
> I guess the solution here is to make such control visible only via the
> subdev interface.
> 

Did you mean using v4l2_subdev_tuner_ops? It does not have a method as of today to support this. The video, audio subdev ops supports s_stream method. We may need to create one for tuner ops.

Thanks,
Ramesh
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