Message-ID: <11143499.CDJkKcVGEf@phil>
Date: Thu, 17 Jul 2025 20:40:59 +0200
From: Heiko Stuebner <heiko@...ech.de>
To: Quentin Schulz <quentin.schulz@...rry.de>
Cc: robh@...nel.org, krzk+dt@...nel.org, devicetree@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org,
 linux-kernel@...r.kernel.org, damon.ding@...k-chips.com,
 Heiko Stuebner <heiko.stuebner@...rry.de>
Subject:
 Re: [PATCH 2/2] arm64: dts: rockchip: add RK3588 DP carrier from Theobroma
 Systems

Hi Quentin,

Am Donnerstag, 17. Juli 2025, 14:21:22 Mitteleuropäische Sommerzeit schrieb Quentin Schulz:
> On 7/16/25 8:50 PM, Heiko Stuebner wrote:
> > From: Heiko Stuebner <heiko.stuebner@...rry.de>
> > 
> > The DisplayPort carrier is a very simple baseboard only providing serial,
> > ethernet and a displayport output.
> > 
> > But its main functionality is that it routes the Analogix eDP controller
> > to this DisplayPort output, which allows to test that controller simply
> > by hooking it up to a suitable monitor.
> > 
> > The Analogix-DP controller supports eDP 1.3 and DP 1.2, so can drive
> > both eDP displays as well as full DP monitors. It does not support DP+
> > so passive DP-to-HDMI adapters won't work.
> > 
> > Signed-off-by: Heiko Stuebner <heiko.stuebner@...rry.de>
> > ---

> > +&edp0 {
> > +	/*
> > +	 * Note that this board doesn't respect Q7 standard as Q7 has
> > +	 * lane 2 of HDMI on the same pin as lane 0 of DP and lane 0 of HDMI
> > +	 * on lane 2 of DP but this board uses HDMI lane numbering instead of
> > +	 * DP lane numbering for its DP lanes.
> > +	 *
> > +	 * It should be possible to match Q7 standard if required by requesting
> > +	 * the eDPTX controller to swap lanes with:
> > +	 *
> > +	 * data-lanes = <2 1 0 3>;
> 
> I don't think that's true currently?
> 
> The driver only seems to be supporting swapping the lanes, c.f. 
> analogix_dp_lane_swap() which routes it to either 0 1 2 3 or 3 2 1 0 but 
> no mix like the one we would need. Also, this function is never called 
> with anything but "do not swap" argument. So we would need to fix the 
> driver to support that.
> 
> Also, this is the kind of information we should have in Tiger DTSI and 
> not some "random" baseboard DTS which will actually not make use of it. 
> We should have data-lanes = <2 1 0 3>; in Tiger DTSI actually but 
> considering 1) the driver doesn't support that, 2) we don't have HW to 
> validate it works; I guess we'll just leave it at that.
> 
> > +	 */
> > +
> 
> Aren't we missing the data-lanes property here? I don't see a default 
> and I would assume we should be providing one?

The analogix-dp controller binding does not specify data-lanes at all
and the current driver also does not read it, or use anything
regarding it.


> Documentation/devicetree/bindings/media/video-interfaces.yaml specifies 
> it either starts at 0 or 1, depending if there's a clock lane or not. As 
> far as I could tell, DP has 4 data lanes and the AUX lane as well, but 
> nothing clock-specific so I assume starting at 0 would be correct?

Looks like it. I looked at the vendor-kernel which does allow data-lane
settings and specifies the default as <0 1 2 3> .

So for now, I'll just drop the comment.


> On a side note, is the audio output on eDP TX controller already 
> supported like on HDMI? If so, we should add the edp0-sound node like in 
> our downstream fork. But looking at the driver, I don't think we 
> implement any dp_audio_* callbacks from drm_bridge_funcs which I assume 
> would be the way to implement that.

Heiko

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