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

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On Wed, Jul 24, 2024 at 07:18:39PM GMT, Marc Gonzalez wrote:
> On 15/07/2024 16:42, Maxime Ripard wrote:
> > 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@xxxxxxxxxx>
> >>>>>> ---
> >>>>>>  .../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@xxxxxxxxxx>
> >>>>>> +  - Pierre-Hugues Husson <phhusson@xxxxxxxxxx>
> >>>>>> +
> >>>>>> +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.
> 
> Why do you say:
> "current binding is for a I2C device that
> shouldn't be accessed through I2C" ?
> 
> As a matter of fact, my debug code queries the device ID using
> regmap_read() to make sure I set the correct I2C address.

Please note: bingdings do not cover the driver at all. The driver might
do whatever it wants. The bindings describe the hardware.

> 
> It's not that the device "SHOULD NOT" be accessed.
> 
> It's just that the driver DOES NOT NEED TO access the device,
> simply because the default settings work fine.

-- 
With best wishes
Dmitry




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