linux-kernel - Re: [PATCH v1 2/8] dt-bindings: display: panel: mipi-dbi-spi: add spi-3wire property

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Date:   Wed, 7 Jun 2023 16:58:48 -0600
From:   Rob Herring <robh@...nel.org>
To:     Noralf Trønnes <noralf@...nnes.org>
Cc:     Leonard Göhrs <l.goehrs@...gutronix.de>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, kernel@...gutronix.de,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/8] dt-bindings: display: panel: mipi-dbi-spi: add
 spi-3wire property

On Wed, Jun 07, 2023 at 09:59:47PM +0200, Noralf Trønnes wrote:
> 
> 
> On 6/7/23 13:55, Leonard Göhrs wrote:
> > Some MIPI DBI panels support a three wire mode (clock, chip select,
> > bidirectional data) that can be used to ask the panel if it is already set
> > up by e.g. the bootloader and can thus skip the initialization.
> > This enables a flicker-free boot.
> > 
> > Signed-off-by: Leonard Göhrs <l.goehrs@...gutronix.de>
> > ---
> >  .../devicetree/bindings/display/panel/panel-mipi-dbi-spi.yaml   | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/display/panel/panel-mipi-dbi-spi.yaml b/Documentation/devicetree/bindings/display/panel/panel-mipi-dbi-spi.yaml
> > index c07da1a9e6288..2f0238b770eba 100644
> > --- a/Documentation/devicetree/bindings/display/panel/panel-mipi-dbi-spi.yaml
> > +++ b/Documentation/devicetree/bindings/display/panel/panel-mipi-dbi-spi.yaml
> > @@ -87,6 +87,8 @@ properties:
> >        Logic level supply for interface signals (Vddi).
> >        No need to set if this is the same as power-supply.
> >  
> > +  spi-3wire: true
> > +
> 
> I don't think this should be added here. spi-cpha and spi-cpol are also
> supported but they are not mentioned. Instead those are documented in
> bindings/spi/spi-controller.yaml. Why they're not documented in
> bindings/spi/spi-peripheral-props.yaml instead which this binding has a
> ref to, I have no idea.

spi-peripheral-props.yaml are properties of the controller in the 
peripheral nodes. spi-cpha and spi-cpol are properties of the device 
which are completely invalid on some devices. We can only check that by 
documenting where they are valid. I think spi-3wire is similar. There 
should be more explanation in the spi-peripheral-props.yaml commit 
history.

Rob