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

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Message-ID: <4fa96259-5dfd-485f-ab41-c039f4a1f23c@topic.nl>
Date: Wed, 3 Sep 2025 13:12:33 +0200
From: Mike Looijmans <mike.looijmans@...ic.nl>
To: Maxime Ripard <mripard@...nel.org>
CC: dri-devel@...ts.freedesktop.org, Andrzej Hajda <andrzej.hajda@...el.com>,
 Conor Dooley <conor+dt@...nel.org>, David Airlie <airlied@...il.com>,
 Jernej Skrabec <jernej.skrabec@...il.com>, Jonas Karlman <jonas@...boo.se>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Neil Armstrong <neil.armstrong@...aro.org>, Rob Herring <robh@...nel.org>,
 Robert Foss <rfoss@...nel.org>, Simona Vetter <simona@...ll.ch>,
 Thomas Zimmermann <tzimmermann@...e.de>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] dt-bindings: drm/bridge: ti-tmds181: Add TI
 TMDS181 and SN65DP159 bindings

On 03-09-2025 08:17, 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.

Looking back at the datasheet - once you enable the I2C interface of the pin, 
many of the pinstrapping options are no longer available (such as slew-rate, 
dvi/hdmi mode, equalizer and termination). The devicetree properties allow 
these settings to be applied again.

> ...
>
M.