Re: [PATCH v2 1/2] dt-bindings: media: Add Cadence MIPI-CSI2 RX Device Tree bindings

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Hi Laurent,

On Wed, Aug 23, 2017 at 12:03:32AM +0300, Laurent Pinchart wrote:
> >>>>> +  - phys: phandle to the external D-PHY
> >>>>> +  - phy-names: must contain dphy, if the implementation uses an
> >>>>> +     external D-PHY
> >>>> 
> >>>> I would move the last two properties in an optional category as
> >>>> they're effectively optional. I think you should also explain a bit more
> >>>> clearly that the phys property must not be present if the phy-names
> >>>> property is not present.
> >>> 
> >>> It's not really optional. The IP has a configuration register that
> >>> allows you to see if it's been synthesized with or without a PHY. If
> >>> the right bit is set, that property will be mandatory, if not, it's
> >>> useless.
> >> 
> >> Just to confirm, the PHY is a separate IP core, right ? Is the CSI-2
> >> receiver input interface different when used with a PHY and when used
> >> without one ? Could a third-party PHY be used as well ? If so, would the
> >> PHY synthesis bit be set or not ?
> > 
> > The PHY (in our case a D-PHY) is a separate entity, it can be from a 3rd
> > party as the IP interface is standard, the SoC integrator would set the bit
> > accordingly based on whether any PHY is present or not. There is also an
> > option of routing digital output from a CSI-TX to a CSI-RX and in such case
> > a PHY would not need to be used (as in the case of our current platform). 
> 
> OK, thank you for the clarification. 
> 
> Maxime mentioned that a bit can be read from a register to notify whether a 
> PHY has been synthesized or not. Does it influence the CSI-2 RX input 
> interface at all, or is the CSI-2 RX IP core synthesized the same way 
> regardless of whether a PHY is present or not ?

So we got an answer to this, and the physical interface remains the
same.

However, the PHY bit is set only when there's an internal D-PHY, which
means we have basically three cases:
  - No D-PHY at all, D-PHY presence bit not set
  - Internal D-PHY, D-PHY presence bit set
  - External D-PHY, D-PHY presence bit not set

I guess that solves our discussion about whether the phys property
should be marked optional or not. It should indeed be optional, and
when it's not there, the D-PHY presence bit will tell whether we have
to program the internal D-PHY or not.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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