linux-kernel - Re: [PATCH v1 3/6] dt-bindings: display: bridge: toshiba,tc358768: add parallel input mode

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Date:   Mon, 3 Apr 2023 23:10:25 +0200
From:   Francesco Dolcini <francesco@...cini.it>
To:     Rob Herring <robh@...nel.org>
Cc:     Francesco Dolcini <francesco@...cini.it>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        dri-devel@...ts.freedesktop.org,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        devicetree@...r.kernel.org,
        Francesco Dolcini <francesco.dolcini@...adex.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/6] dt-bindings: display: bridge: toshiba,tc358768:
 add parallel input mode

On Mon, Apr 03, 2023 at 04:01:17PM -0500, Rob Herring wrote:
> On Fri, Mar 31, 2023 at 11:40:01AM +0200, Francesco Dolcini wrote:
> > On Fri, Mar 31, 2023 at 10:48:15AM +0200, Krzysztof Kozlowski wrote:
> > > On 30/03/2023 11:59, Francesco Dolcini wrote:
> > > > From: Francesco Dolcini <francesco.dolcini@...adex.com>
> > > > 
> > > > Add new toshiba,input-rgb-mode property to describe the actual signal
> > > > connection on the parallel RGB input interface.
> > > > 
> > > > Signed-off-by: Francesco Dolcini <francesco.dolcini@...adex.com>
> > > > ---
> > > >  .../bindings/display/bridge/toshiba,tc358768.yaml | 15 +++++++++++++++
> > > >  1 file changed, 15 insertions(+)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/display/bridge/toshiba,tc358768.yaml b/Documentation/devicetree/bindings/display/bridge/toshiba,tc358768.yaml
> > > > index 8f22093b61ae..2638121a2223 100644
> > > > --- a/Documentation/devicetree/bindings/display/bridge/toshiba,tc358768.yaml
> > > > +++ b/Documentation/devicetree/bindings/display/bridge/toshiba,tc358768.yaml
> > > > @@ -42,6 +42,21 @@ properties:
> > > >    clock-names:
> > > >      const: refclk
> > > >  
> > > > +  toshiba,input-rgb-mode:
> > > > +    description: |
> > > > +      Parallel Input (RGB) Mode.
> > > > +
> > > > +      RGB inputs (PD[23:0]) color arrangement as documented in the datasheet
> > > > +      and in the table below.
> > > > +
> > > > +      0 = R[7:0], G[7:0], B[7:0]
> > > 
> > > RGB888?
> > 
> > Or anything else - like a RGB666 - just connecting to GND the unused
> > pins.
> 
> If the bridge is configured for RGB666, then that's fine. If not, the 
> unused pins should be driven with either the MSB of each component. 
> Otherwise, you'd can't fully saturate the colors.

> > > > +      1 = R[1:0], G[1:0], B[1:0], R[7:2], G[7:2], B[7:2]
> > > > +      2 = 8’b0, R[4:0], G[5:0], B[4:0]
> > > 
> > > Isn't this RGB565?
> > > 
> > > Don't we have already properties like this? e.g. colorspace?
> > 
> > It's not really the colorspace this property.
> > 
> > tc358768 is a parallel RGB to DSI bridge, it has 24 bit parallel input
> > line.
> > 
> > The way this lines are connected is configurable with this parameter, if you
> > look at mode 0 and 1 they all allow to have a RGB888 or a RGB666 or a
> > RGB565 mapping. This just configure some internal mux, it's not strictly
> > about the RGB mode.
> 
> This is the same as other cases. There's a need for describing the 
> interface. It keeps coming up and I keep saying to go create something 
> common.

I am not aware of other discussion on the topic, do you have any pointer
I can look at?

What I'd like to re-iterate here once more is that this configuration is
about how the external 24-bit parallel RGB lines are mapped withing this
bridge.

It's not mapping the linux media bus format (e.g. not
MEDIA_BUS_FMT_RBG888_1X24 or alike).

This bridge allow for a limited set of combination (3) as described in
this binding.

Francesco