Re: [PATCH v3] drm: of: Properly try all possible cases for bridge/panel detection

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

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Hi Bjorn,

On Thu 31 Mar 22, 20:16, Bjorn Andersson wrote:
> On Tue 29 Mar 06:27 PDT 2022, 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.
> > 
> > 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.
> > 
> > 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.
> > 
> 
> Thanks for your patch Paul, it fixed a regression on a device where I
> have a eDP bridge with an of_graph and a aux-bus defined.
> 
> But unfortunately it does not resolve the regression I have for the
> USB based DisplayPort setup described below.
> 
> 
> In the Qualcomm DisplayPort driver We're calling:
> 
> 	devm_drm_of_get_bridge(dev, dev->of_node, 1, 0);
> 
> and with the following DT snippet the behavior changed:
> 
> displayport-controller@ae90000 {
> 	compatible = "qcom,sc8180x-dp";
> 	...
> 
> 	operating-points-v2 = <&dp0_opp_table>;
> 
> 	ports {
> 		#address-cells = <1>;
> 		#size-cells = <0>;
> 
> 		port@0 {
> 			reg = <0>;
> 			dp0_in: endpoint {
> 				remote-endpoint = <&display_driver>;
> 			};
> 		};
> 	};
> 
> 	dp0_opp_table: opp-table {
> 		...;
> 	};
> };
> 
> Prior to the introduction of 80253168dbfd ("drm: of: Lookup if child
> node has panel or bridge") this would return -ENODEV, so we could
> differentiate the case when we have a statically defined eDP panel from
> that of a dynamically attached (over USB) DP panel.
> 
> Prior to your change, above case without the opp-table node would have
> still returned -ENODEV.
> 
> But now this will just return -EPROBE_DEFER in both cases.

Oh that's right, the -ENODEV case was just completely removed by my change.
Initially this would happen if !of_graph_is_present or if the remote node
doesn't exist.

Now that we are also checking for child nodes, we can't just return -ENODEV
when the graph or remote node is missing: we must also check that there is no
child node that is a panel/bridge.

For the graph remote case, we can reliabily return -EPROBE_DEFER when
of_graph_is_present and the remote exists and of_device_is_available.
Otherwise we can go for -ENODEV. I think getting -EPROBE_DEFER at this point
should stop the drm_of_find_panel_or_bridge process.

On the other hand for the child panel/bridge node case, I don't see how we
can reliably distinguish between -EPROBE_DEFER and -ENODEV, because
of_drm_find_panel and of_drm_find_bridge will behave the same if the child
node is a not-yet-probed panel/bridge or a totally unrelated node.
So I think we should always return -EPROBE_DEFER in that case.

As a result you can't get -ENODEV if using the of graph while having any
(unrelated) child node there, so your issue remains.

Do you see any way we could make this work?

> I thought the appropriate method of referencing the dsi panel was to
> actually reference that using the of_graph, even though it's a child of
> the dsi controller - that's at least how we've done it in e.g. [1].
> I find this to be much nicer than to just blindly define that all
> children of any sort of display controller must be a bridge or a panel.

Yes I totally agree. Given that using the child node directly apparently
can't allow us to distinguish between -EPROBE_DEFER/-ENODEV I would be in
favor of dropping this mechanism and going with explicit of graph in any case
(even if it's a child node). I don't see any downside to this approach.

What do yout think?

Paul

> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/qcom/sdm845-mtp.dts#n436
> 
> Regards,
> Bjorn
> 
> > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > Fixes: 80253168dbfd ("drm: of: Lookup if child node has panel or bridge")
> > ---
> > 
> > Changes since v2:
> > - Removed unnecessary else statement and added a comment about
> >   clearing the panel pointer on error.
> > 
> > Changes since v1:
> > - Renamed remote to node;
> > - Renamed helper to find_panel_or_bridge;
> > - Cleared bridge pointer early;
> > - Returned early to make the code more concise;
> > 
> > ---
> >  drivers/gpu/drm/drm_of.c | 99 ++++++++++++++++++++--------------------
> >  1 file changed, 50 insertions(+), 49 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> > index 9d90cd75c457..8716da6369a6 100644
> > --- a/drivers/gpu/drm/drm_of.c
> > +++ b/drivers/gpu/drm/drm_of.c
> > @@ -219,6 +219,29 @@ int drm_of_encoder_active_endpoint(struct device_node *node,
> >  }
> >  EXPORT_SYMBOL_GPL(drm_of_encoder_active_endpoint);
> >  
> > +static int find_panel_or_bridge(struct device_node *node,
> > +				struct drm_panel **panel,
> > +				struct drm_bridge **bridge)
> > +{
> > +	if (panel) {
> > +		*panel = of_drm_find_panel(node);
> > +		if (!IS_ERR(*panel))
> > +			return 0;
> > +
> > +		/* Clear the panel pointer in case of error. */
> > +		*panel = NULL;
> > +	}
> > +
> > +	/* No panel found yet, check for a bridge next. */
> > +	if (bridge) {
> > +		*bridge = 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
> > @@ -241,66 +264,44 @@ int drm_of_find_panel_or_bridge(const struct device_node *np,
> >  				struct drm_panel **panel,
> >  				struct drm_bridge **bridge)
> >  {
> > -	int ret = -EPROBE_DEFER;
> > -	struct device_node *remote;
> > +	struct device_node *node;
> > +	int ret;
> >  
> >  	if (!panel && !bridge)
> >  		return -EINVAL;
> > +
> >  	if (panel)
> >  		*panel = NULL;
> > -
> > -	/**
> > -	 * 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;
> > +	if (bridge)
> > +		*bridge = NULL;
> > +
> > +	/* Check for a graph on the device node first. */
> > +	if (of_graph_is_present(np)) {
> > +		node = of_graph_get_remote_node(np, port, endpoint);
> > +		if (node) {
> > +			ret = find_panel_or_bridge(node, panel, bridge);
> > +			of_node_put(node);
> > +
> > +			if (!ret)
> > +				return 0;
> > +		}
> >  	}
> >  
> > -	/*
> > -	 * 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 = 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. */
> > +	for_each_available_child_of_node(np, node) {
> > +		if (of_node_name_eq(node, "port") ||
> > +		    of_node_name_eq(node, "ports"))
> > +			continue;
> >  
> > -	if (panel) {
> > -		*panel = of_drm_find_panel(remote);
> > -		if (!IS_ERR(*panel))
> > -			ret = 0;
> > -		else
> > -			*panel = NULL;
> > -	}
> > -
> > -	/* No panel found yet, check for a bridge next. */
> > -	if (bridge) {
> > -		if (ret) {
> > -			*bridge = of_drm_find_bridge(remote);
> > -			if (*bridge)
> > -				ret = 0;
> > -		} else {
> > -			*bridge = NULL;
> > -		}
> > +		ret = find_panel_or_bridge(node, panel, bridge);
> > +		of_node_put(node);
> >  
> > +		/* Stop at the first found occurrence. */
> > +		if (!ret)
> > +			return 0;
> >  	}
> >  
> > -	of_node_put(remote);
> > -	return ret;
> > +	return -EPROBE_DEFER;
> >  }
> >  EXPORT_SYMBOL_GPL(drm_of_find_panel_or_bridge);
> >  
> > -- 
> > 2.35.1
> > 

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

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