Re: [V9, 1/2] media: dt-bindings: media: i2c: Document OV02A10 bindings

[Dongchun Zhu <dongchun.zhu@mediatek.com>]

Hi Sakari,

On Thu, 2020-05-28 at 10:23 +0300, Sakari Ailus wrote:
> Hi Dongchun,
> 
> On Thu, May 28, 2020 at 11:34:42AM +0800, Dongchun Zhu wrote:
> > Hi Sakari, Rob,
> > 
> > On Thu, 2020-05-28 at 00:16 +0300, Sakari Ailus wrote:
> > > Hi Rob, Dongchun,
> > > 
> > > On Wed, May 27, 2020 at 09:27:22AM -0600, Rob Herring wrote:
> > > > > > > +    properties:
> > > > > > > +      endpoint:
> > > > > > > +        type: object
> > > > > > > +        additionalProperties: false
> > > > > > > +
> > > > > > > +        properties:
> > > > >
> > > > > Actually I wonder whether we need to declare 'clock-lanes' here?
> > > > 
> > > > Yes, if you are using it.
> > > 
> > > Dongchun, can you confirm the chip has a single data and a single clock
> > > lane and that it does not support lane reordering?
> > > 
> > 
> > From the datasheet, 'MIPI inside the OV02A10 provides one single
> > uni-directional clock lane and one bi-directional data lane solution for
> > communication links between components inside a mobile device.
> > The data lane has full support for HS(uni-directional) and
> > LP(bi-directional) data transfer mode.'
> > 
> > The sensor doesn't support lane reordering, so 'clock-lanes' property
> > would not be added in next release.
> > 
> > > So if there's nothing to convey to the driver, also the data-lanes should
> > > be removed IMO.
> > > 
> > 
> > However, 'data-lanes' property may still be required.
> > It is known that either data-lanes or clock-lanes is an array of
> > physical data lane indexes. Position of an entry determines the logical
> > lane number, while the value of an entry indicates physical lane, e.g.,
> > for 1-lane MIPI CSI-2 bus we could have "data-lanes = <1>;", assuming
> > the clock lane is on hardware lane 0.
> > 
> > As mentioned earlier, the OV02A10 sensor supports only 1C1D and does not
> > support lane reordering, so here we shall use 'data-lanes = <1>' as
> > there is only a clock lane for OV02A10.
> > 
> > Reminder:
> > If 'data-lanes' property is not present, the driver would assume
> > four-lane operation. This means for one-lane or two-lane operation, this
> > property must be present and set to the right physical lane indexes.
> > If the hardware does not support lane reordering, monotonically
> > incremented values shall be used from 0 or 1 onwards, depending on
> > whether or not there is also a clock lane.
> 
> How can the driver use four lanes, considering the device only supports a
> single lane??
> 

I understood your meaning.
If we omit the property 'data-lanes', the sensor should work still.
But then what's the meaning of the existence of 'data-lanes'?
If this property 'data-lanes' is always optional, then why dt-bindings
provide the interface?

In the meantime, if omitting 'data-lanes' for one sensor(transmitter)
that has only one physical data lane, MIPI receiver(e.g., MIPI CSI-2)
shall enable four-lane configuration, which may increase consumption of
both power and resource in the process of IIC communication.
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