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

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

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Hi Maxime,

On Fri 18 Mar 22, 16:14, Maxime Ripard wrote:
> On Wed, Mar 16, 2022 at 04:40:49PM +0100, Paul Kocialkowski wrote:
> > Hi Maxime,
> > 
> > Thanks for the review!
> > 
> > On Thu 10 Mar 22, 15:54, Maxime Ripard wrote:
> > > 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.
> > > > 
> > > > 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.
> > > > 
> > > > 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(-)
> > > > 
> > > > 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_node *node,
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(drm_of_encoder_active_endpoint);
> > > >  
> > > > +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?
> > 
> > From a quick look at other DRM code I got the impression that static functions
> > also usually carry the drm prefix but I might be wrong.
> 
> Not necessarily, see handle_conflicting_encoders, commit_tail, commit_work,
> convert_clip_rect_to_rect, edid_load, etc.
> 
> Most functions do, but it's not a rule or a convention.

Okay then, I'm fine with find_panel_or_bridge.

> > > > +{
> > > > +	int ret = -EPROBE_DEFER;
> > > > +
> > > > +	if (panel) {
> > > > +		*panel = of_drm_find_panel(remote);
> > > > +		if (!IS_ERR(*panel))
> > > > +			ret = 0;
> > > 
> > > return 0?
> > 
> > The idea was to still go through the "*bridge = NULL;" path if a bridge
> > pointer is provided, to preserve the original behavior of the function.
> > There may or may not not be any hard expectation on that, in any case
> > I feel like it would be good to avoid out-of-scope functional changes here.
> 
> Then we could just clear it just like we clear the panel pointer in
> drm_of_find_panel_or_bridge. It would be more consistent.

Oh absolutely, I agree that would be best.

> > > > +		else
> > > > +			*panel = NULL;
> > > > +
> > > > +	}
> > > > +
> > > > +	/* No panel found yet, check for a bridge next. */
> > > > +	if (bridge) {
> > > > +		if (ret) {
> > > 
> > > And the return above allows to remove that test
> > > 
> > > > +			*bridge = of_drm_find_bridge(remote);
> > > > +			if (*bridge)
> > > > +				ret = 0;
> > > 
> > > return 0?
> > > 
> > > > +		} else {
> > > > +			*bridge = 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 = 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;
> > > > +	/* Check for a graph on the device node first. */
> > > > +	if (of_graph_is_present(np)) {
> > > > +		remote = of_graph_get_remote_node(np, port, endpoint);
> > > > +		if (remote) {
> > > > +			ret = drm_of_find_remote_panel_or_bridge(remote, panel,
> > > > +								 bridge);
> > > > +			of_node_put(remote);
> > > > +		}
> > > >  	}
> > > >  
> > > > -	/*
> > > > -	 * 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. */
> > > > +	if (ret) {
> > > > +		for_each_available_child_of_node(np, remote) {
> > > > +			if (of_node_name_eq(remote, "port") ||
> > > > +			    of_node_name_eq(remote, "ports"))
> > > > +				continue;
> > > >  
> > > > -	if (panel) {
> > > > -		*panel = of_drm_find_panel(remote);
> > > > -		if (!IS_ERR(*panel))
> > > > -			ret = 0;
> > > > -		else
> > > > -			*panel = NULL;
> > > > -	}
> > > > +			ret = drm_of_find_remote_panel_or_bridge(remote, panel,
> > > > +								 bridge);
> > > > +			of_node_put(remote);
> > > >  
> > > > -	/* 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;
> > > > +			/* Stop at the first found occurrence. */
> > > > +			if (!ret)
> > > > +				break;
> > > >  		}
> > > > -
> > > >  	}
> > > >  
> > > > -	of_node_put(remote);
> > > >  	return ret;
> > > >  }
> > > 
> > > So the diff is fairly hard to read, but it ends up as:
> > 
> > Yeah I agree, not sure what I can do about that.
> 
> Nothing, really. I don't expect any change there, it just happens sometimes :)

All right then :)

I'll send another iteration soon.

Cheers,

Paul

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

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