RE: [PATCH v3 1/8] dt-bindings: display: Add bindings for LVDS bus-timings

[Fabrizio Castro]

Hi Laurent,

Thank you for your feedback!

> From: devicetree-owner@xxxxxxxxxxxxxxx <devicetree-owner@xxxxxxxxxxxxxxx> On Behalf Of Laurent Pinchart
> Sent: 07 November 2019 18:01
> Subject: Re: [PATCH v3 1/8] dt-bindings: display: Add bindings for LVDS bus-timings
> 
> Hello Fabrizio,
> 
> On Thu, Aug 29, 2019 at 02:38:06PM +0000, Fabrizio Castro wrote:
> > On 29 August 2019 15:03 Rob Herring wrote:
> > > On Wed, Aug 28, 2019 at 1:36 PM Fabrizio Castro wrote:
> > >>
> > >> Dual-LVDS connections need markers in the DT, this patch adds
> > >> some common documentation to be referenced by both panels and
> > >> bridges.
> > >>
> > >> Signed-off-by: Fabrizio Castro <fabrizio.castro@xxxxxxxxxxxxxx>
> > >>
> > >> ---
> > >> v2->v3:
> > >> * new patch
> > >> ---
> > >>  .../bindings/display/bus-timings/lvds.yaml         | 38 ++++++++++++++++++++++
> > >>  1 file changed, 38 insertions(+)
> > >>  create mode 100644 Documentation/devicetree/bindings/display/bus-timings/lvds.yaml
> > >>
> > >> diff --git a/Documentation/devicetree/bindings/display/bus-timings/lvds.yaml
> b/Documentation/devicetree/bindings/display/bus-timings/lvds.yaml
> > >> new file mode 100644
> > >> index 0000000..f35b55a
> > >> --- /dev/null
> > >> +++ b/Documentation/devicetree/bindings/display/bus-timings/lvds.yaml
> > >> @@ -0,0 +1,38 @@
> > >> +# SPDX-License-Identifier: GPL-2.0
> > >
> > > (GPL-2.0-only OR BSD-2-Clause) is preferred for new bindings.
> > >
> > >> +%YAML 1.2
> > >> +---
> > >> +$id: http://devicetree.org/schemas/display/bus-timings/lvds.yaml#
> > >> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > >> +
> > >> +title: Common Properties for bus timings of LVDS interfaces
> > >> +
> > >> +maintainers:
> > >> +  - Thierry Reding <thierry.reding@xxxxxxxxx>
> > >> +  - Fabrizio Castro <fabrizio.castro@xxxxxxxxxxxxxx>
> > >> +
> > >> +description: |
> > >> +  This document defines device tree properties common to LVDS and dual-LVDS
> > >> +  interfaces, where a dual-LVDS interface is a dual-link connection with even
> > >> +  pixels traveling on one connection, and with odd pixels traveling on the other
> > >> +  connection.
> 
> As you define a dual-LVDS interface as "a dual-link connection", should
> this be "even pixels traveling on one link, and with odd pixels
> traveling on the other link" ?

Will change.

Thanks,
Fab

> 
> > >> +  This document doesn't constitue a device tree binding specification by itself
> > >
> > > typo: constitute
> >
> > Well spotted!
> >
> > >> +  but is meant to be referenced by device tree bindings.
> > >> +  When referenced from panel or bridge device tree bindings, the properties
> > >> +  defined in this document are defined as follows. The panel and bridge device
> > >> +  tree bindings are responsible for defining whether each property is required
> > >> +  or optional.
> > >> +
> > >> +properties:
> > >> +  dual-lvds-even-pixels:
> > >> +    type: boolean
> > >> +    description:
> > >> +      This property is specific to an input port of a sink device. When
> > >
> > > The schema should define what nodes these go in. The description seems
> > > to indicate in 'port' nodes (or endpoint?), but your use in the panel
> > > binding puts them in the parent.
> >
> > Did you manage to read this?
> > https://patchwork.kernel.org/cover/11119607/
> >
> > Could you please advice on how to do this properly?
> >
> > >> +      specified, it marks the port as recipient of even-pixels.
> > >> +
> > >> +  dual-lvds-odd-pixels:
> > >> +    type: boolean
> > >> +    description:
> > >> +      This property is specific to an input port of a sink device. When
> > >> +      specified, it marks the port as recipient of odd-pixels.
> > >
> > > However, I don't think you even need these. A panel's port numbers are
> > > fixed can imply even or odd. For example port@0 can be even and port@1
> > > can be odd. The port numbering is typically panel specific, but we may
> > > be able to define the numbering generically if we don't already have
> > > panels with multiple ports.
> > >
> > > Also, aren't there dual link DSI panels?
> >
> > This is the result of a discussion on here:
> > https://patchwork.kernel.org/patch/11095547/
> >
> > Have you come across it?
> 
> Let me repeat my comments from that thread for Rob in order to
> centralize the discussion here.
> 
> > Taking one step back to look at the big picture, what we need is for the
> > source to know what pixel (odd or even) to send on each LVDS output. For
> > that it needs to know what pixel is expected by the sink the the inputs
> > connected to the source's outputs. From each source output we can easily
> > locate the DT nodes corresponding to the connected sink's input ports,
> > but that doesn't give us enough information. From there, we could
> >
> > - Infer the odd/even pixel expected by the source based on the port
> >   numbers. This would require common DT bindings for all dual-link LVDS
> >   sinks that specify the port ordering (for instance the bindings could
> >   mandate that lowest numbered port correspond to even pixels).
> >
> > - Read the odd/even pixel expected by the source from an explicit DT
> >   property, as proposed above. This would also require common DT
> >   bindings for all dual-link LVDS sinks that define these new
> >   properties. This would I think be better than implicitly infering it
> >   from DT port numbers.
> >
> > - Retrieve the information from the sink drm_bridge at runtime. This
> >   would require a way to query the bridge for the mapping from port
> >   number to odd/even pixel. The DRM_LINK_DUAL_LVDS_ODD_EVEN flag could
> >   be used for that, and would then be defined as "the lowest numbered
> >   port corresponds to odd pixels and the highest numbered port
> >   corresponds to even pixels".
> >
> > The second and third options would both work I think. The third one is
> > roughly what you've implemented, except that I think the flag
> > description should be clarified.
> 
> Rob, what's your opinion ? We could certainly, in the context of a
> panel, decide of a fixed mapping of port numbers to odd/even pixels, but
> the issue is that sources need to know which pixels to send on which
> link (assuming of course that this can be configured on the source
> side). We thus need a way for the source to answer, at runtime, the
> question "which of ports A and B of the sink correspond to even and odd
> pixels ?". This can't be inferred by the source from the sink port
> numbers, as the mapping between port number and odd/even pixels is
> specific to each sink. We thus need to either store that property in the
> DT node of the sink (option 2) or query it at runtime from the sink
> (option 3).
> 
> I like option 2 as it's more explicit, but option 3 minimizes the
> required properties in DT, which is always good. Patch 3/8 introduces a
> helper that abstracts this from a sink point of view (which I think is a
> very good idea), so once we decide which option to use, only 3/8 may
> need to be adapted, the other patches should hopefully not require
> rework.
> 
> --
> Regards,
> 
> Laurent Pinchart