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Date:   Thu, 10 Mar 2022 15:54:23 +0100
From:   Maxime Ripard <maxime@cerno.tech>
To:     Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc:     dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Linus Walleij <linus.walleij@linaro.org>,
        Jagan Teki <jagan@amarulasolutions.com>
Subject: Re: [PATCH] drm: of: Properly try all possible cases for
 bridge/panel detection
Message-ID: <20220310145423.but7r7ul4j7h3wxw@houat>
References: <20220309143200.111292-1-paul.kocialkowski@bootlin.com>
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Hi Paul,

On Wed, Mar 09, 2022 at 03:32:00PM +0100, Paul Kocialkowski wrote:
> While bridge/panel detection was initially relying on the usual
> port/ports-based of graph detection, it was recently changed to
> perform the lookup on any child node that is not port/ports
> instead when such a node is available, with no fallback on the
> usual way.
>=20
> This results in breaking detection when a child node is present
> but does not contain any panel or bridge node, even when the
> usual port/ports-based of graph is there.
>=20
> In order to support both situations properly, this commit reworks
> the logic to try both options and not just one of the two: it will
> only return -EPROBE_DEFER when both have failed.
>=20
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> Fixes: 80253168dbfd ("drm: of: Lookup if child node has panel or bridge")

Thanks, it's in pretty good shape now, but I have a few bike sheds to paint=
 :)

> ---
>  drivers/gpu/drm/drm_of.c | 93 +++++++++++++++++++++-------------------
>  1 file changed, 49 insertions(+), 44 deletions(-)
>=20
> diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> index 9d90cd75c457..67f1b7dfc892 100644
> --- a/drivers/gpu/drm/drm_of.c
> +++ b/drivers/gpu/drm/drm_of.c
> @@ -219,6 +219,35 @@ int drm_of_encoder_active_endpoint(struct device_nod=
e *node,
>  }
>  EXPORT_SYMBOL_GPL(drm_of_encoder_active_endpoint);
> =20
> +static int drm_of_find_remote_panel_or_bridge(struct device_node *remote,
> +					      struct drm_panel **panel,
> +					      struct drm_bridge **bridge)

This function performs its look up directly on the struct device_node
passed as argument, so I don't think the "remote" in the name is great.
Since it's static, we can just call it find_panel_or_bridge, what do you
think?

> +{
> +	int ret =3D -EPROBE_DEFER;
> +
> +	if (panel) {
> +		*panel =3D of_drm_find_panel(remote);
> +		if (!IS_ERR(*panel))
> +			ret =3D 0;

return 0?

> +		else
> +			*panel =3D NULL;
> +
> +	}
> +
> +	/* No panel found yet, check for a bridge next. */
> +	if (bridge) {
> +		if (ret) {

And the return above allows to remove that test

> +			*bridge =3D of_drm_find_bridge(remote);
> +			if (*bridge)
> +				ret =3D 0;

return 0?

> +		} else {
> +			*bridge =3D NULL;
> +		}
> +
> +	}
> +
> +	return ret;

And here we can just return -EPROBE_DEFER

> +}
> +

>  /**
>   * drm_of_find_panel_or_bridge - return connected panel or bridge device
>   * @np: device tree node containing encoder output ports
> @@ -249,57 +278,33 @@ int drm_of_find_panel_or_bridge(const struct device=
_node *np,
>  	if (panel)
>  		*panel =3D NULL;
> =20
> -	/**
> -	 * 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;
> -
> -		goto of_find_panel_or_bridge;
> +	/* Check for a graph on the device node first. */
> +	if (of_graph_is_present(np)) {
> +		remote =3D of_graph_get_remote_node(np, port, endpoint);
> +		if (remote) {
> +			ret =3D drm_of_find_remote_panel_or_bridge(remote, panel,
> +								 bridge);
> +			of_node_put(remote);
> +		}
>  	}
> =20
> -	/*
> -	 * 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;
> +	/* Otherwise check for any child node other than port/ports. */
> +	if (ret) {
> +		for_each_available_child_of_node(np, remote) {
> +			if (of_node_name_eq(remote, "port") ||
> +			    of_node_name_eq(remote, "ports"))
> +				continue;
> =20
> -	if (panel) {
> -		*panel =3D of_drm_find_panel(remote);
> -		if (!IS_ERR(*panel))
> -			ret =3D 0;
> -		else
> -			*panel =3D NULL;
> -	}
> +			ret =3D drm_of_find_remote_panel_or_bridge(remote, panel,
> +								 bridge);
> +			of_node_put(remote);
> =20
> -	/* 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;
> +			/* Stop at the first found occurrence. */
> +			if (!ret)
> +				break;
>  		}
> -
>  	}
> =20
> -	of_node_put(remote);
>  	return ret;
>  }

So the diff is fairly hard to read, but it ends up as:

>        int ret =3D -EPROBE_DEFER;
>        struct device_node *remote;
>
>        if (!panel && !bridge)
>                return -EINVAL;
>        if (panel)
>                *panel =3D NULL;
>
>        /* Check for a graph on the device node first. */
>       if (of_graph_is_present(np)) {
>                remote =3D of_graph_get_remote_node(np, port, endpoint);
>                if (remote) {
>                        ret =3D drm_of_find_remote_panel_or_bridge(remote,=
 panel,
>                                                                 bridge);
>                        of_node_put(remote);

I think we can simplify this by doing

                        if (!ret)
			        return ret;

>                }
>        }
>
>        /* Otherwise check for any child node other than port/ports. */
>        if (ret) {

And thus we won't have to check for ret here

>                for_each_available_child_of_node(np, remote) {

I'm a bit reluctant with variables that we reuse from one loop to
another, especially since it's a bit misleading here. What about using a
(loop local) remote variable in the of_graph path, and a loop-local
variable node or child here?

>                        if (of_node_name_eq(remote, "port") ||
>                            of_node_name_eq(remote, "ports"))
>                                continue;
>
>                        ret =3D drm_of_find_remote_panel_or_bridge(remote,=
 panel,
>                                                                 bridge);
>                        of_node_put(remote);
>
>                        /* Stop at the first found occurrence. */
>                        if (!ret)
>                                break;

Ditto, let's just return here

>                }
>       }
>
>        return ret;

And then we can just return EPROBE_DEFER here (and get rid of ret entirely)

Maxime

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