linux-kernel - Re: [PATCH 1/8] dt-bindings: display: ti,am65x-dss: Add port properties for DSS

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Date:   Thu, 28 Jul 2022 14:16:50 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Aradhya Bhatia <a-bhatia1@...com>, Jyri Sarha <jyri.sarha@....fi>,
        Rob Herring <robh+dt@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Cc:     Darren Etheridge <detheridge@...com>, Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Rahul T R <r-ravikumar@...com>,
        Krunal Bhargav <k-bhargav@...com>,
        Devarsh Thakkar <devarsht@...com>,
        DRI Development List <dri-devel@...ts.freedesktop.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] dt-bindings: display: ti,am65x-dss: Add port
 properties for DSS



On 19/07/2022 11:08, Aradhya Bhatia wrote:
> Add "ti,oldi-mode" property to indicate the tidss driver the OLDI output
> mode. The 2 OLDI TXes on am625-dss allow a 3 different types of panel
> connections with the board.
> 
> 1. Single Link / Single Mode on OLDI TX 0 OR 1.
> 2. Single Link / Duplicate Mode on OLDI TX 0 and 1.
> 3. Dual Link / Single Mode on OLDI TX 0 and 1.
> 
> Add "ti,rgb565-to-888" property to override 16bit output from a videoport
> for a bridge that only accepts 24bit RGB888 DPI input.
> 
> On some boards the HDMI bridge takes a 24bit DPI input, but only 16 data
> pins are actually enabled from the SoC.  This new property forces the
> output to be RGB565 on a specific video port if the bridge requests a
> 24bit RGB color space.
> 
> This assumes that the video port is connected like so:
> 
> SoC : Bridge
> R0 ->   R3
> R1 ->   R4
> R2 ->   R5
> R3 ->   R6
> R4 ->   R7
> G0 ->   G2
> G1 ->   G3
> G2 ->   G4
> G3 ->   G5
> G4 ->   G6
> G5 ->   G7
> B0 ->   B3
> B1 ->   B4
> B2 ->   B5
> B3 ->   B6
> B4 ->   B7
> 
> On the bridge side R0->R2, G0->G1, B0->B2 would be tied to ground.
> The bridge sees 24bits of data,  but the lsb's are always zero.
> 
> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
> ---
>   .../bindings/display/ti/ti,am65x-dss.yaml     | 25 +++++++++++++++++--
>   1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/display/ti/ti,am65x-dss.yaml b/Documentation/devicetree/bindings/display/ti/ti,am65x-dss.yaml
> index 6bbce921479d..11d9b3821409 100644
> --- a/Documentation/devicetree/bindings/display/ti/ti,am65x-dss.yaml
> +++ b/Documentation/devicetree/bindings/display/ti/ti,am65x-dss.yaml
> @@ -80,15 +80,35 @@ properties:
>   
>       properties:
>         port@0:
> -        $ref: /schemas/graph.yaml#/properties/port
> +        $ref: /schemas/graph.yaml#/$defs/port-base
> +        unevaluatedProperties: false
>           description:
>             The DSS OLDI output port node form video port 1
>   
> +        properties:
> +          ti,oldi-mode:
> +            description: TI specific property to indicate the mode the OLDI TXes
> +              and the display panel are connected in.
> +              0 -> OLDI TXes OFF (driver default for am625-dss)
> +              1 -> Single link, Single Mode (OLDI0) (driver default for am65x-dss)
> +              2 -> Single link, Single Mode (OLDI1)
> +              3 -> Single link, Duplicate Mode
> +              4 -> Dual link (Only Single Mode)
> +            $ref: /schemas/types.yaml#/definitions/uint32
> +            enum: [0, 1, 2, 3, 4]
> +
>         port@1:
> -        $ref: /schemas/graph.yaml#/properties/port
> +        $ref: /schemas/graph.yaml#/$defs/port-base
> +        unevaluatedProperties: false
>           description:
>             The DSS DPI output port node from video port 2
>   
> +        properties:
> +          ti,rgb565-to-888:
> +            description:
> +              property to override DPI output to 16bit for 24bit bridge
> +            type: boolean

So you have a board with 16 DSS pins connected, going to a bridge/panel 
that only supports 24 bit input. I don't think there's anything TI 
specific here, sounds like a common situation.

"rgb565-to-888" sounds like there's some kind of conversion happening, 
but all this does is set the videoport width to 16 bits.

I'm not sure if there's a better solution but in the OMAP DSS we have 
"data-lines" property for an endpoint, which tells the driver the width 
of the bus.

  Tomi