linux-kernel - RE: [PATCH v3 0/8] Add dual-LVDS panel support to EK874

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Date:   Mon, 2 Sep 2019 10:01:01 +0000
From:   Fabrizio Castro <fabrizio.castro@...renesas.com>
To:     Rob Herring <robh+dt@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
CC:     Geert Uytterhoeven <geert+renesas@...der.be>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Mark Rutland <mark.rutland@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Simon Horman <horms@...ge.net.au>,
        Magnus Damm <magnus.damm@...il.com>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:MEDIA DRIVERS FOR RENESAS - FCP" 
        <linux-renesas-soc@...r.kernel.org>,
        Chris Paterson <Chris.Paterson2@...esas.com>,
        Biju Das <biju.das@...renesas.com>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        "xu_shunji@...erun.com" <xu_shunji@...erun.com>,
        "ebiharaml@...linux.co.jp" <ebiharaml@...linux.co.jp>
Subject: RE: [PATCH v3 0/8] Add dual-LVDS panel support to EK874

Hi Rob,

Thank you for your feedback!

> From: Rob Herring <robh+dt@...nel.org>
> Sent: 29 August 2019 16:27
> To: Fabrizio Castro <fabrizio.castro@...renesas.com>
> Subject: Re: [PATCH v3 0/8] Add dual-LVDS panel support to EK874
> 
> On Wed, Aug 28, 2019 at 1:36 PM Fabrizio Castro
> <fabrizio.castro@...renesas.com> wrote:
> >
> > Dear All,
> >
> > this series adds support for dual-LVDS panel IDK-2121WR
> > from Advantech:
> > https://buy.advantech.eu/Displays/Embedded-LCD-Kits-High-Brightness/model-IDK-2121WR-K2FHA2E.htm
> >
> > V3 approaches the problem in a completely different way, we now
> > have two new properties to mark the ports in the DT as receiving
> > even pixels and odd pixels: dual-lvds-even-pixels and dual-lvds-odd-pixels,
> > which means device drivers should not use bridge specific or panel
> > specific dual_link flags. Also, in this case the DT describes the
> > connection fully.
> >
> > In order for the solution to be generic, I have exported a new helper
> > (drm_of_lvds_get_dual_link_configuration) to walk the device tree,
> > and figure out if the connection is dual-LVDS. The same helper gives
> > information about the configuration of the connection. If Px is connected
> > to a port expecting even pixels and Py is connected to a port expecting
> > odd pixels, then the helper returns DRM_LVDS_DUAL_LINK_EVEN_ODD_PIXELS
> > (like in the example below), otherwise it returns
> > DRM_LVDS_DUAL_LINK_ODD_EVEN_PIXELS.
> >
> >
> >  --------            dual-lvds-even-pixels  --------
> > |        |----                         ----|        |
> > |        | Px |---------------------->| Pn |        |
> > |        |----                         ----|        |
> > | SOURCE |           dual-lvds-odd-pixels  |  SINK  |
> > |        |----                         ----|        |
> > |        | Py |---------------------->| Pm |        |
> > |        |----                         ----|        |
> >  --------                                   --------
> >
> > The device driver for the encoder then will work out if with the current
> > wiring the pixels need swapping or not.
> >
> > The same solution works for both panels and bridges.
> >
> > Since the DT describes the connection fully, driver
> > drivers/gpu/drm/panel/panel-lvds.c works out-of-the-box, no changes
> > required, however, this implementation opens up a problem with the
> > dt-bindings.
> > Driver drivers/gpu/drm/panel/panel-lvds.c can still be pleased by
> > a port node, but also by a ports node.
> > I have created Documentation/devicetree/bindings/display/bus-timings/lvds.yaml
> > with the idea of including it from panels and bridges dt-bindings
> > supporting dual-LVDS (and of course the dt-bindings for the specific
> > devices should say which port should be marked as what), but file
> > Documentation/devicetree/bindings/display/panel/lvds.yaml formally
> > requires property "port", while with this implementation it should require
> > OneOf "port" and "ports", and unfortunately I can't seem to find a neat way
> > aroud that, other than creating a new compatible string
> 
> Just add 'ports' and drop 'port' from being required in the common
> binding. Then it is up to the panel specific bindings to define which
> one is required. Or we just leave it to allow either form which the
> graph code can handle.
> 
> We could have this in the common binding:
> 
> oneOf:
>  - required: [ports]
>  - required: [port]


Thank you for Rob for looking into this. I will wait for a feedback from Laurent
on the code before sending out v4.

Thanks,
Fab

> 
> Rob