[PATCH 1/2] devicetree/bindings: display: Add bindings for LVDS panels

[Laurent Pinchart]

Hi Rob,

On Friday 14 Oct 2016 07:40:14 Rob Herring wrote:
> On Sun, Oct 9, 2016 at 11:33 AM, Laurent Pinchart wrote:
> > On Saturday 08 Oct 2016 20:29:39 Rob Herring wrote:
> >> On Tue, Oct 04, 2016 at 07:23:29PM +0300, Laurent Pinchart wrote:
> >>> LVDS is a physical layer specification defined in ANSI/TIA/EIA-644-A.
> >>> Multiple incompatible data link layers have been used over time to
> >>> transmit image data to LVDS panels. This binding supports display
> >>> panels compatible with the JEIDA-59-1999, Open-LDI and VESA SWPG
> >>> specifications.
> >>> 
> >>> Signed-off-by: Laurent Pinchart
> >>> <laurent.pinchart+renesas at ideasonboard.com>
> >>> ---
> >>> 
> >>>  .../bindings/display/panel/panel-lvds.txt          | 119 ++++++++++++++
> >>>  1 file changed, 119 insertions(+)
> >>>  create mode 100644
> >>>  Documentation/devicetree/bindings/display/panel/panel-lvds.txt>
> >>> 
> >>> diff --git
> >>> a/Documentation/devicetree/bindings/display/panel/panel-lvds.txt
> >>> b/Documentation/devicetree/bindings/display/panel/panel-lvds.txt
> >>> new file mode 100644
> >>> index 000000000000..250861f2673e
> >>> --- /dev/null
> >>> +++ b/Documentation/devicetree/bindings/display/panel/panel-lvds.txt
> >>> @@ -0,0 +1,119 @@
> >>> +Generic LVDS Panel
> >>> +==================
> >>> +
> >>> +LVDS is a physical layer specification defined in ANSI/TIA/EIA-644-A.
> >>> Multiple
> >>> +incompatible data link layers have been used over time to transmit
> >>> image data
> >>> +to LVDS panels. This bindings supports display panels compatible with
> >>> the
> >>> +following specifications.
> >>> +
> >>> +[JEIDA] "Digital Interface Standards for Monitor", JEIDA-59-1999,
> >>> February
> >>> +1999 (Version 1.0), Japan Electronic Industry Development Association
> >>> (JEIDA)
> >>> +[LDI] "Open LVDS Display Interface", May 1999 (Version 0.95), National
> >>> +Semiconductor
> >>> +[VESA] "VESA Notebook Panel Standard", October 2007 (Version 1.0),
> >>> Video
> >>> +Electronics Standards Association (VESA)
> >>> +
> >>> +Device compatible with those specifications have been marketed under
> >>> the
> >>> +FPD-Link and FlatLink brands.
> >>> +
> >>> +
> >>> +Required properties:
> >>> +- compatible: shall contain "panel-lvds"
> >> 
> >> Maybe as a fallback, but on its own, no way.
> > 
> > Which brings an interesting question: when designing generic DT bindings,
> > what's the rule regarding

Looks like I forgot part of the question. I meant to ask what is the rule 
regarding usage of more precise compatible strings ?

For instance (but perhaps not the best example), the input/rotary-encoder.txt 
bindings define a "rotary-encoder" compatible string, with no other bindings 
defining more precise compatible strings for the exact rotary encoder model. 
When it comes to panels I believe it makes sense to define model-specific 
compatible strings even if they're unused by drivers. I'm however wondering 
what the rule is there, and where those device-specific compatible strings 
should be defined. I'd like to avoid using one file per panel model as done 
today for the simple-panel bindings.

> Call it "simple" so I can easily NAK it. :)
> 
> Define a generic structure, not a single binding trying to serve all.
> 
> >> > +- width-mm: panel display width in millimeters
> >> > +- height-mm: panel display height in millimeters
> >> 
> >> This is already documented for all panels IIRC.
> > 
> > Note that this DT binding has nothing to do with the simple-panel binding.
> > It is instead similar to the panel-dpi and panel-dsi-cm bindings (which
> > currently don't but should specify the panel size in DT). The LVDS panel
> > driver will *not* include any panel-specific information such as size or
> > timings, these are specified in DT.
> 
> The panel bindings aren't really different. The biggest difference was
> location in the tree, but we now generally allow panels to be either a
> child of the LCD controller or connected with OF graph. We probably
> need to work on restructuring the panel bindings a bit. We should have
> an inheritance with a base panel binding of things like size, label,
> graph, backlight, etc, then perhaps an interface specific bindings for
> LVDS, DSI, and parallel, then a panel specific binding. With this the
> panel specific binding is typically just a compatible string and which
> inherited properties apply to it.

That sounds good to me, but we have multiple models for panel bindings.

As you mentioned panels can be referenced through an LCD controller node 
property containing a phandle to the panel node, or through OF graph. That's a 
situation we have today, and we need to keep supporting both (at least for 
existing panels, perhaps not for the new ones).

Another difference is how to express panel data such as size and timings. The 
simple-panel DT bindings don't contain such data and expects the drivers to 
contain a table of panel data for all models supported, while the DPI, DSI and 
now the proposed LVDS panel bindings contain properties for panel data.

How would you like to reconcile all that ?

> >>> +- data-mapping: the color signals mapping order, "jeida-18",
> >>> "jeida-24"
> >>> +  or "vesa-24"
> >> 
> >> Maybe this should be part of the compatible.
> > 
> > I've thought about it, but given that some panels support selecting
> > between multiple modes (through a mode pin that is usually hardwired), I
> > believe a separate DT property makes sense.
> 
> Okay.
> 
> > Furthermore, LVDS data organization is controlled by the combination of
> > both data-mapping and data-mirror. It makes little sense from my point of
> > view to handle one as part of the compatible string and the other one as
> > a separate property.
> > 
> >> > +Optional properties:
> >> > +- label: a symbolic name for the panel
> >> 
> >> Could be for any panel or display connector.
> > 
> > Yes, but I'm not sure to understand how that's relevant :-)
> 
> Meaning it should be a common property.

Sure. So you expect me to reorganize all the panels and connectors DT bindings 
in order to get this one merged ? :-)

> >>> +- avdd-supply: reference to the regulator that powers the panel
> >>> analog supply
> >>> +- dvdd-supply: reference to the regulator that powers the panel
> >>> digital supply
> >> 
> >> Which one has to be powered on first, what voltage, and with what time
> >> in between? This is why "generic" or "simple" bindings don't work.
> > 
> > The above-mentioned specifications also define connectors, pinouts and
> > power supplies, but many LVDS panels compatible with the LVDS physical
> > and data layers use a different connector with small differences in power
> > supplies.
> > 
> > I believe the voltage is irrelevant here, it doesn't need to be controlled
> > by the operating system. Power supplies order and timing is relevant,
> > I'll investigate the level of differences between panels. I'm also fine
> > with dropping those properties for now.
> 
> Whether you have control of the supplies is dependent on the board.
> Dropping them is just puts us in the simple binding trap. The simple
> bindings start out that way and then people keep adding to them.

Damn, you can't be fooled easily ;-)

On a more serious note, I'd like to design the bindings in a way that wouldn't 
require adding device-specific code in the driver for each panel model, given 
that in most cases power supply handling will be generic. What's your opinion 
about a generic power supply model that would be used in the default case, 
with the option to override it with device-specific code when needed ?

> >>> +- data-mirror: if set, reverse the bit order on all data lanes (6 to 0
> >>> instead
> >>> +  of 0 to 6)
> 
> On this one, make the name describe the order. "mirror" requires that
> I know what is normal ordering. Perhaps "data-msb-first".

Sounds good to me, I'll use that.

-- 
Regards,

Laurent Pinchart