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Date:   Thu, 21 Apr 2022 09:11:52 +0200
From:   Paul Kocialkowski <paul.kocialkowski@bootlin.com>
To:     Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Thierry Reding <thierry.reding@gmail.com>,
        Rob Clark <robdclark@gmail.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] Revert "drm: of: Properly try all possible cases for
 bridge/panel detection"
Message-ID: <YmEDuBeAhQr8Ev1d@aptenodytes>
References: <20220420231230.58499-1-bjorn.andersson@linaro.org>
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Hi Bjorn,

On Wed 20 Apr 22, 16:12, Bjorn Andersson wrote:
> Commit '80253168dbfd ("drm: of: Lookup if child node has panel or
> bridge")' introduced the ability to describe a panel under a display
> controller without having to use a graph to connect the controller to
> its single child panel (or bridge).
>=20
> The implementation of this would find the first non-graph node and
> attempt to acquire the related panel or bridge. This prevents cases
> where any other child node, such as a aux bus for a DisplayPort
> controller, or an opp-table to find the referenced panel.
>=20
> Commit '67bae5f28c89 ("drm: of: Properly try all possible cases for
> bridge/panel detection")' attempted to solve this problem by not
> bypassing the graph reference lookup before attempting to find the panel
> or bridge.
>=20
> While this does solve the case where a proper graph reference is
> present, it does not allow the caller to distinguish between a
> yet-to-be-probed panel or bridge and the absence of a reference to a
> panel.
>=20
> One such case is a DisplayPort controller that on some boards have an
> explicitly described reference to a panel, but on others have a
> discoverable DisplayPort display attached (which doesn't need to be
> expressed in DeviceTree).
>=20
> This reverts commit '67bae5f28c89 ("drm: of: Properly try all possible
> cases for bridge/panel detection")', as a step towards reverting commit
> '80253168dbfd ("drm: of: Lookup if child node has panel or bridge")'.
>=20
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Acked-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>

Cheers,

Paul

> ---
>  drivers/gpu/drm/drm_of.c | 99 ++++++++++++++++++++--------------------
>  1 file changed, 49 insertions(+), 50 deletions(-)
>=20
> diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> index f4df344509a8..026e4e29a0f3 100644
> --- a/drivers/gpu/drm/drm_of.c
> +++ b/drivers/gpu/drm/drm_of.c
> @@ -214,29 +214,6 @@ int drm_of_encoder_active_endpoint(struct device_nod=
e *node,
>  }
>  EXPORT_SYMBOL_GPL(drm_of_encoder_active_endpoint);
> =20
> -static int find_panel_or_bridge(struct device_node *node,
> -				struct drm_panel **panel,
> -				struct drm_bridge **bridge)
> -{
> -	if (panel) {
> -		*panel =3D of_drm_find_panel(node);
> -		if (!IS_ERR(*panel))
> -			return 0;
> -
> -		/* Clear the panel pointer in case of error. */
> -		*panel =3D NULL;
> -	}
> -
> -	/* No panel found yet, check for a bridge next. */
> -	if (bridge) {
> -		*bridge =3D of_drm_find_bridge(node);
> -		if (*bridge)
> -			return 0;
> -	}
> -
> -	return -EPROBE_DEFER;
> -}
> -
>  /**
>   * drm_of_find_panel_or_bridge - return connected panel or bridge device
>   * @np: device tree node containing encoder output ports
> @@ -259,44 +236,66 @@ int drm_of_find_panel_or_bridge(const struct device=
_node *np,
>  				struct drm_panel **panel,
>  				struct drm_bridge **bridge)
>  {
> -	struct device_node *node;
> -	int ret;
> +	int ret =3D -EPROBE_DEFER;
> +	struct device_node *remote;
> =20
>  	if (!panel && !bridge)
>  		return -EINVAL;
> -
>  	if (panel)
>  		*panel =3D NULL;
> -	if (bridge)
> -		*bridge =3D NULL;
> -
> -	/* Check for a graph on the device node first. */
> -	if (of_graph_is_present(np)) {
> -		node =3D of_graph_get_remote_node(np, port, endpoint);
> -		if (node) {
> -			ret =3D find_panel_or_bridge(node, panel, bridge);
> -			of_node_put(node);
> -
> -			if (!ret)
> -				return 0;
> -		}
> -	}
> =20
> -	/* Otherwise check for any child node other than port/ports. */
> -	for_each_available_child_of_node(np, node) {
> -		if (of_node_name_eq(node, "port") ||
> -		    of_node_name_eq(node, "ports"))
> +	/**
> +	 * Devices can also be child nodes when we also control that device
> +	 * through the upstream device (ie, MIPI-DCS for a MIPI-DSI device).
> +	 *
> +	 * Lookup for a child node of the given parent that isn't either port
> +	 * or ports.
> +	 */
> +	for_each_available_child_of_node(np, remote) {
> +		if (of_node_name_eq(remote, "port") ||
> +		    of_node_name_eq(remote, "ports"))
>  			continue;
> =20
> -		ret =3D find_panel_or_bridge(node, panel, bridge);
> -		of_node_put(node);
> +		goto of_find_panel_or_bridge;
> +	}
> +
> +	/*
> +	 * of_graph_get_remote_node() produces a noisy error message if port
> +	 * node isn't found and the absence of the port is a legit case here,
> +	 * so at first we silently check whether graph presents in the
> +	 * device-tree node.
> +	 */
> +	if (!of_graph_is_present(np))
> +		return -ENODEV;
> +
> +	remote =3D of_graph_get_remote_node(np, port, endpoint);
> +
> +of_find_panel_or_bridge:
> +	if (!remote)
> +		return -ENODEV;
> +
> +	if (panel) {
> +		*panel =3D of_drm_find_panel(remote);
> +		if (!IS_ERR(*panel))
> +			ret =3D 0;
> +		else
> +			*panel =3D NULL;
> +	}
> +
> +	/* No panel found yet, check for a bridge next. */
> +	if (bridge) {
> +		if (ret) {
> +			*bridge =3D of_drm_find_bridge(remote);
> +			if (*bridge)
> +				ret =3D 0;
> +		} else {
> +			*bridge =3D NULL;
> +		}
> =20
> -		/* Stop at the first found occurrence. */
> -		if (!ret)
> -			return 0;
>  	}
> =20
> -	return -EPROBE_DEFER;
> +	of_node_put(remote);
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(drm_of_find_panel_or_bridge);
> =20
> --=20
> 2.35.1
>=20

--=20
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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