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

[Marc Gonzalez]

On 24/07/2024 19:25, Dmitry Baryshkov wrote:
> 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 at 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 at freebox.fr>
>>>>>>>> +  - Pierre-Hugues Husson <phhusson at 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 at 0:
>>>>>>>> +        $ref: /schemas/graph.yaml#/properties/port
>>>>>>>> +        description: Bridge input
>>>>>>>> +
>>>>>>>> +      port at 1:
>>>>>>>> +        $ref: /schemas/graph.yaml#/properties/port
>>>>>>>> +        description: Bridge output
>>>>>>>> +
>>>>>>>> +    required:
>>>>>>>> +      - port at 0
>>>>>>>> +      - port at 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.

OK.
How does the proposed binding say
"I2C device shouldn't be accessed through I2C" ?

The tfp410 has the same property that it can be configured
either through pin straps or via I2C.