linux-kernel - Re: [PATCH v2] drm/bridge: ti-sn65dsi86: Add HPD for DisplayPort connector type

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Message-ID: <mwh35anw57d6nvre3sguetzq3miu4kd43rokegvul7fk266lys@5h2euthpk7vq>
Date: Mon, 26 May 2025 10:41:49 +0200
From: Ernest Van Hoecke <ernestvanhoecke@...il.com>
To: Jayesh Choudhary <j-choudhary@...com>
Cc: dianders@...omium.org, andrzej.hajda@...el.com, 
	neil.armstrong@...aro.org, rfoss@...nel.org, Laurent.pinchart@...asonboard.com, 
	dri-devel@...ts.freedesktop.org, tomi.valkeinen@...asonboard.com, max.krummenacher@...adex.com, 
	jonas@...boo.se, jernej.skrabec@...il.com, maarten.lankhorst@...ux.intel.com, 
	mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch, 
	kieran.bingham+renesas@...asonboard.com, linux-kernel@...r.kernel.org, max.oss.09@...il.com, 
	devarsht@...com, dmitry.baryshkov@....qualcomm.com, ernest.vanhoecke@...adex.com
Subject: Re: [PATCH v2] drm/bridge: ti-sn65dsi86: Add HPD for DisplayPort
 connector type

Hi Jayesh,

First of all, thanks for your patch. I applied it to our 6.6-based
downstream kernel supporting a board I have here, and noticed some
strange behaviour with eDP now.

On Thu, May 08, 2025 at 05:24:33PM +0530, Jayesh Choudhary wrote:
> +	if (pdata->bridge.type == DRM_MODE_CONNECTOR_eDP)
> +		regmap_update_bits(pdata->regmap, SN_HPD_DISABLE_REG, HPD_DISABLE,
> +				   HPD_DISABLE);
>  

On my setup it seems that `pdata->bridge.type` is not yet set here,
because it executes before `ti_sn_bridge_probe`. For the DP use case,
this is not a problem because the type field is 0 (i.e., not
DRM_MODE_CONNECTOR_eDP) in that case. But for eDP, it means that we are
unexpectedly not disabling HDP.

With working HDP, everything is fine in the end for both DP and eDP. But
when the HDP line is not connected, eDP no longer works. So I wonder if
this breaks some functionality for weird eDP panels or board
implementations.

I could certainly be missing something; from my understanding it looks
like without a good HPD signal, the `ti_sn_bridge_probe` and quoted code
are stuck in a loop. `ti_sn65dsi86_enable_comms` runs but does not
disable HDP, after which the probe runs but fails and does not set the
type field, so the next `enable_comms` run fails to disable HDP again,
etc.

Kind regards,
Ernest