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

[Rob Herring]

On Sun, Oct 9, 2016 at 11:33 AM, Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
> Hi Rob,
>
> 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

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.


>> > +- 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.

>> > +- 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.

>
>> > +- 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".

Rob