linux-kernel - Re: [RFC PATCH 1/6] dt-bindings: display: imx: Add EPDC

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Date:   Wed, 16 Feb 2022 17:52:16 -0600
From:   Rob Herring <robh@...nel.org>
To:     Andreas Kemnade <andreas@...nade.info>
Cc:     p.zabel@...gutronix.de, airlied@...ux.ie, daniel@...ll.ch,
        shawnguo@...nel.org, s.hauer@...gutronix.de, kernel@...gutronix.de,
        festevam@...il.com, linux-imx@....com,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, dri-devel@...ts.freedesktop.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, alistair@...stair23.me,
        samuel@...lland.org, josua.mayer@....eu,
        letux-kernel@...nphoenux.org
Subject: Re: [RFC PATCH 1/6] dt-bindings: display: imx: Add EPDC

On Mon, Feb 14, 2022 at 11:45:17PM +0100, Andreas Kemnade wrote:
> Hi Rob,
> 
> On Fri, 11 Feb 2022 09:46:27 -0600
> Rob Herring <robh@...nel.org> wrote:
> 
> > On Sun, Feb 06, 2022 at 09:00:11AM +0100, Andreas Kemnade wrote:
> > > Add a binding for the Electrophoretic Display Controller found at least
> > > in the i.MX6.  
> > 
> > The first version was in i.MX50 (I helped design the register 
> > interface). Is that version compatible?
> > 
> it has some differences, but that could be detected by EPDC_VERSION
> register. I do not own such a device, so I cannot fully check. I have
> not seen any driver with devicetree for IMX5. For now I am rejecting
> anything which has a EPDC version which I cannot check. 
> 
> > > The timing subnode is directly here to avoid having display parameters
> > > spread all over the plate.
> > > 
> > > Supplies are organized the same way as in the fbdev driver in the
> > > NXP/Freescale kernel forks. The regulators used for that purpose,
> > > like the TPS65185, the SY7636A and MAX17135 have typically a single bit to
> > > start a bunch of regulators of higher or negative voltage with a
> > > well-defined timing. VCOM can be handled separately, but can also be
> > > incorporated into that single bit.
> > > 
> > > Signed-off-by: Andreas Kemnade <andreas@...nade.info>
> > > ---
> > >  .../bindings/display/imx/fsl,mxc-epdc.yaml    | 159 ++++++++++++++++++
> > >  1 file changed, 159 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml
> > > 
> > > diff --git a/Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml b/Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml
> > > new file mode 100644
> > > index 000000000000..7e0795cc3f70
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml
> > > @@ -0,0 +1,159 @@
> > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/display/imx/fsl,mxc-epdc.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: Freescale i.MX6 EPDC
> > > +
> > > +maintainers:
> > > +  - Andreas Kemnade <andreas@...nade.info>
> > > +
> > > +description: |
> > > +  The EPDC is a controller for handling electronic paper displays found in
> > > +  i.MX6 SoCs.
> > > +
> > > +properties:
> > > +  compatible:
> > > +    enum:
> > > +      - fsl,imx6sl-epdc
> > > +      - fsl,imx6sll-epdc  
> > 
> > Not compatible with each other?
> > 
> differences are detectable by EPDC_VERSION register, so probably so
> problem. NXP/Freescale kernel uses
> fsl,imx6dl-epdc
> and 
> fsl,imx7d-epdc (used also by imx6 devices with EPDC_VERSION = 3.0)
> in their drivers.
> 
> fsl,imx6dl-epdc
> fsl,imx6sl-epdc
> fsl,imx6sll-epdc
> fsl,imx7d-epdc
> in their dtsis.
> 
> But the general rule is to use as less as possible compatible strings
> if differences can be probed properly, so only one should be
> sufficient? Which one?

If you can probe all the differences, then just 'fsl,imx-epdc' is 
sufficient. Just document that so the next time around I don't forget 
and tell you it needs to be specific.

Rob