Re: [PATCH v4 1/2] dt-bindings: drm/bridge: ti-tmds181: Add TI TMDS181 and SN65DP159 bindings

[Mike Looijmans <mike.looijmans@topic.nl>]

On 03-09-2025 17:25, Dmitry Baryshkov wrote:
> On Wed, Sep 03, 2025 at 08:17:33AM +0200, Mike Looijmans wrote:
>> On 02-09-2025 19:29, Maxime Ripard wrote:
>>> On Mon, Sep 01, 2025 at 04:29:01PM +0200, Mike Looijmans wrote:
>>>> +  ti,retimer-threshold-hz:
>>>> +    minimum: 25000000
>>>> +    maximum: 600000000
>>>> +    default: 200000000
>>>> +    description:
>>>> +      Cross-over point. Up until this pixel clock frequency
>>>> +      the chip remains in the low-power redriver mode. Above
>>>> +      the threshold the chip should operate in retimer mode.
>>> Why should anyone want to tune this at the firmware level?
>> It's a board property. You'd set this based on the hardware you've soldered
>> on. If your clock and serdes are good quality, there's no need for the chip
>> to be in retimer mode (it will consume more power and actually make the
>> signal worse). At higher speeds, that situation may change, hence the need
>> for a way to describe that. The chip has a similar function built in, but
>> with only 2 choices of cross-over point.
>>
>> To tune these parameters (retimer, equalizer), you'll probably have to take
>> your equipment to a test facility (like we did). It's not something that
>> end-users would want to tune themselves.
>>
>> Most of these settings can also be done using pin strapping. I guess it'd be
>> helpful if I added that to the description.
>>
>>
>>>> +  ti,dvi-mode:
>>>> +    type: boolean
>>>> +    description: Makes the DP159 chip operate in DVI mode.
>>> Ditto. Both describe policy, not hardware.
>> I would set this flag if I've soldered on a DVI connector instead of a HDMI
>> one. I'd consider that hardware.
> Do you need to set this if you have DVI monitor attached to the HDMI
> connector via the passive cable?

In our testing, the setting didn't appear to have any effect on the video 
signals. Activating the I2C interface blocks the pinstrapping setting of this 
bit, that's the only reason it's there.

I've been digging through the datasheet. It appears that the DVI mode disables 
an I2C slave on the DDC bus, when in HDMI_SEL mode the datasheet says:  
"Device configured for HDMI (Adaptor ID block is readable through I2C or 
I2C-over-AUX."

Apparently a better name for the pin would be "disable-adaptor-id-block".

Another approach would be to just always disable it in the driver. It appears 
to make the retimer "visible" in the DDC path and even allows to change some 
settings that way (most noticeably, turn it on/off), which will interfere with 
the driver's functions. If we're directly controlling the device over I2C, one 
shouldn't be using this secondary interface.


> As for the connector type, you can check it in the .atomic_enable by
> checking drm_connector::connector_type.

Assuming this is the last bridge, yes. But see above, I suggest removing this 
from the bindings and just always set it in the driver.

M.