Re: [PATCH v3 1/2] dt-bindings: display: bridge: add TI TDP158

[Maxime Ripard <mripard@kernel.org>]

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On Mon, Jul 08, 2024 at 11:29:46PM GMT, Dmitry Baryshkov wrote:
> On Mon, Jul 08, 2024 at 04:59:23PM GMT, Maxime Ripard wrote:
> > On Mon, Jul 01, 2024 at 05:36:18PM GMT, Marc Gonzalez wrote:
> > > On 01/07/2024 15:50, Maxime Ripard wrote:
> > > 
> > > > On Thu, Jun 27, 2024 at 01:13:03PM GMT, Marc Gonzalez wrote:
> > > >
> > > >> TDP158 is an AC-coupled DVI / HDMI to TMDS level shifting Redriver.
> > > >> It supports DVI 1.0, HDMI 1.4b and 2.0b.
> > > >> It supports 4 TMDS channels, HPD, and a DDC interface.
> > > >> It supports dual power supply rails (1.1V on VDD, 3.3V on VCC)
> > > >> for power reduction. Several methods of power management are
> > > >> implemented to reduce overall power consumption.
> > > >> It supports fixed receiver EQ gain using I2C or pin strap to
> > > >> compensate for different lengths input cable or board traces.
> > > >>
> > > >> Features
> > > >>
> > > >> - AC-coupled TMDS or DisplayPort dual-mode physical layer input
> > > >> to HDMI 2.0b TMDS physical layer output supporting up to 6Gbps
> > > >> data rate, compatible with HDMI 2.0b electrical parameters
> > > >> - DisplayPort dual-mode standard version 1.1
> > > >> - Programmable fixed receiver equalizer up to 15.5dB
> > > >> - Global or independent high speed lane control, pre-emphasis
> > > >> and transmit swing, and slew rate control
> > > >> - I2C or pin strap programmable
> > > >> - Configurable as a DisplayPort redriver through I2C
> > > >> - Full lane swap on main lanes
> > > >> - Low power consumption (200 mW at 6Gbps, 8 mW in shutdown)
> > > >>
> > > >> https://www.ti.com/lit/ds/symlink/tdp158.pdf
> > > >>
> > > >> Signed-off-by: Marc Gonzalez <mgonzalez@freebox.fr>
> > > >> ---
> > > >>  .../bindings/display/bridge/ti,tdp158.yaml         | 51 ++++++++++++++++++++++
> > > >>  1 file changed, 51 insertions(+)
> > > >>
> > > >> diff --git a/Documentation/devicetree/bindings/display/bridge/ti,tdp158.yaml b/Documentation/devicetree/bindings/display/bridge/ti,tdp158.yaml
> > > >> new file mode 100644
> > > >> index 0000000000000..21c8585c3bb2d
> > > >> --- /dev/null
> > > >> +++ b/Documentation/devicetree/bindings/display/bridge/ti,tdp158.yaml
> > > >> @@ -0,0 +1,51 @@
> > > >> +# SPDX-License-Identifier: GPL-2.0-only
> > > >> +%YAML 1.2
> > > >> +---
> > > >> +$id: http://devicetree.org/schemas/display/bridge/ti,tdp158.yaml#
> > > >> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > >> +
> > > >> +title: TI TDP158 HDMI to TMDS Redriver
> > > >> +
> > > >> +maintainers:
> > > >> +  - Arnaud Vrac <avrac@freebox.fr>
> > > >> +  - Pierre-Hugues Husson <phhusson@freebox.fr>
> > > >> +
> > > >> +properties:
> > > >> +  compatible:
> > > >> +    const: ti,tdp158
> > > >> +
> > > >> +  reg:
> > > >> +    description: I2C address of the device
> > > >> +
> > > >> +  enable-gpios:
> > > >> +    description: GPIO controlling bridge enable
> > > >> +
> > > >> +  vcc-supply:
> > > >> +    description: Power supply 3.3V
> > > >> +
> > > >> +  vdd-supply:
> > > >> +    description: Power supply 1.1V
> > > >> +
> > > >> +  ports:
> > > >> +    $ref: /schemas/graph.yaml#/properties/ports
> > > >> +
> > > >> +    properties:
> > > >> +      port@0:
> > > >> +        $ref: /schemas/graph.yaml#/properties/port
> > > >> +        description: Bridge input
> > > >> +
> > > >> +      port@1:
> > > >> +        $ref: /schemas/graph.yaml#/properties/port
> > > >> +        description: Bridge output
> > > >> +
> > > >> +    required:
> > > >> +      - port@0
> > > >> +      - port@1
> > > > 
> > > > The device supports DVI, HDMI or DP input, with various requirements and
> > > > capabilities depending on the input. Your binding doesn't address that.
> > > > 
> > > > Similarly, it can do lane-swapping, so we should probably have a
> > > > property to describe what mapping we want to use.
> > > > 
> > > > The i2c register access (and the whole behaviour of the device) is
> > > > constrained on the I2C_EN pin status, and you can't read it from the
> > > > device, so it's also something we need to have in the DT.
> > > 
> > > We are using the device in its default configuration.
> > > (Power on via OE, then it works as expected)
> > 
> > I know, but that doesn't really matter for a binding.
> > 
> > > Can we leave any additional properties to be defined by whomever needs
> > > them in the future?
> > 
> > If you can guarantee that doing so would be backward compatible, sure.
> > But that means being able to answer those questions with a reasonable
> > plan.
> 
> I think proposed bindings are generic enough to cover other possible
> usecases in future.

I don't think it is. The current binding is for a I2C device that
shouldn't be accessed through I2C.

It's working for now because the driver doesn't do any access, so it's
all great, but as soon as we add support for the other case, then we'll
have to add a property that states that while it's an i2c device, it
shouldn't be accessed.

And adding such a property is a compatibility-breaking change.

Maxime

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