Re: [PATCH 08/13] dt-bindings: media: ov5640: Remove data-shift

[Sakari Ailus <sakari.ailus@iki.fi>]

On Sun, Jul 26, 2020 at 12:31:25AM +0300, Laurent Pinchart wrote:
> Hi Sakari,
> 
> On Sun, Jul 26, 2020 at 12:18:33AM +0300, Sakari Ailus wrote:
> > On Fri, Jul 24, 2020 at 02:15:49AM +0300, Laurent Pinchart wrote:
> > > On Fri, Jul 24, 2020 at 01:22:59AM +0300, Sakari Ailus wrote:
> > > > On Fri, Jul 17, 2020 at 11:57:22PM +0300, Laurent Pinchart wrote:
> > > > > Hi Jacopo,
> > > > > 
> > > > > (CC'ing Sakari)
> > > > > 
> > > > > Thank you for the patch.
> > > > > 
> > > > > On Fri, Jul 17, 2020 at 03:28:54PM +0200, Jacopo Mondi wrote:
> > > > > > The value of the data-shift property solely depend on the selected
> > > > > > bus width and it's not freely configurable.
> > > > > > 
> > > > > > Remove it from the bindings document and update its users accordingly.
> > > > > 
> > > > > Hmmmm that's an interesting one. Sakari, what do you think ?
> > > > > 
> > > > > > Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> > > > > > ---
> > > > > >  Documentation/devicetree/bindings/media/i2c/ov5640.yaml | 9 ---------
> > > > > >  arch/arm/boot/dts/stm32mp157c-ev1.dts                   | 1 -
> > > > > >  2 files changed, 10 deletions(-)
> > > > > > 
> > > > > > diff --git a/Documentation/devicetree/bindings/media/i2c/ov5640.yaml b/Documentation/devicetree/bindings/media/i2c/ov5640.yaml
> > > > > > index 5e1662e848bd..ab700a1830aa 100644
> > > > > > --- a/Documentation/devicetree/bindings/media/i2c/ov5640.yaml
> > > > > > +++ b/Documentation/devicetree/bindings/media/i2c/ov5640.yaml
> > > > > > @@ -92,12 +92,6 @@ properties:
> > > > > >                parallel bus.
> > > > > >              enum: [8, 10]
> > > > > > 
> > > > > > -          data-shift:
> > > > > > -            description: |
> > > > > > -              Shall be set to <2> for 8 bits parallel bus (lines 9:2 are used) or
> > > > > > -              <0> for 10 bits parallel bus.
> > > > > > -            enum: [0, 2]
> > > > > 
> > > > > Should you document in the description of bus-width that data-shift is
> > > > > implied ?
> > > > 
> > > > The purpose of the datas-shift property is to convey how the parallel bus
> > > > lines are connected for a given bus width for devices where it is
> > > > configurable. As this device does not not support that, then indeed this
> > > > property is not relevant for the device IMO.
> > > 
> > > Could you elaborate on this ? I believe the case that Jacopo is
> > > describing connects D[9:2] from the sensor to D[7:0] of the receiver
> > > (Jacopo, could you confirm ?). Isn't that what data-shift is for ?
> > 
> > Yes, it is. But in this case what data-shift configures is not configurable
> > as such but defined by another configuration, making the data-shift
> > property redundant. We generally haven't documented redundant things in DT
> > bindings --- for instance data-lanes is documented in bindings only if it
> > is configurable.
> 
> Then I think we share the same understanding. I believe the
> documentation in video-interfaces.txt needs to be expanded, as it's
> quite terse and not very clear.

The DT spec states that:

	A DTSpec-compliant devicetree describes device information in a
	system that cannot necessarily be dynamically detected by a client
	program. For example, the architecture of PCI enables a client to
	probe and detect attached devices, and thus devicetree nodes
	describing PCI devices might not be required. However, a device
	node is required to describe a PCI host bridge device in the system
	if it cannot be detected by probing.

I'd read that as there's no need to specify properties that do not provide
additional information to software. As some properties are dependent on
others and and this depends on hardware features, I don't think we can in
general case take this account in generic binding documentation, but device
specific ones.

Of course we could add this to data-shift documentation, but then I wonder
how many other similar cases there are where in hardware the configuration
defined by one property determines the value of another?

-- 
Regards,

Sakari Ailus