[PATCH v4] drm: of: Lookup if child node has panel or bridge

[Paul Kocialkowski]

On Fri 04 Mar 22, 12:00, Paul Kocialkowski wrote:
> Hi Maxime,
> 
> On Fri 04 Mar 22, 09:54, Maxime Ripard wrote:
> > Hi Paul,
> > 
> > On Thu, Mar 03, 2022 at 09:26:30PM +0100, Paul Kocialkowski wrote:
> > > On Wed 02 Feb 22, 21:34, Jagan Teki wrote:
> > > > Devices can also be child nodes when we also control that device
> > > > through the upstream device (ie, MIPI-DCS for a MIPI-DSI device).
> > > > 
> > > > drm_of_find_panel_or_bridge can lookup panel or bridge for a given
> > > > device has port and endpoint and it fails to lookup if the device
> > > > has a child nodes.
> > > 
> > > This patch breaks the logicvc drm driver that I'm currently developping.
> > > The symptom is that drm_of_find_panel_or_bridge now always returns
> > > -EPROBE_DEFER even after the panel has probed and is running well.
> > > It seems that the function can no longer find the panel.
> > > 
> > > I haven't figured out the details, but reverting your patch makes
> > > it work again. I suspect other drivers might be affected as well, so
> > > it would probably be a good idea to revert the patch until the root
> > > cause is clearly understood and the patch can be adapted accordingly.
> > > 
> > > Here is what the device-tree looks like:
> > > 
> > > / {
> > > 	panel: panel-lvds {
> > > 		compatible = "panel-lvds";
> > > 
> > > 		[...]
> > > 
> > > 		port {
> > > 			#address-cells = <1>;
> > > 			#size-cells = <0>;
> > > 
> > > 			panel_input: endpoint at 0 {
> > > 				reg = <0>;
> > > 				remote-endpoint = <&logicvc_output>;
> > > 			};
> > > 		};
> > > 	};
> > > };
> > > 
> > > &amba {
> > > 	logicvc: logicvc at 43c00000 {
> > > 		compatible = "xylon,logicvc-3.02.a", "syscon", "simple-mfd";
> > > 		reg = <0x43c00000 0x6000>;
> > > 
> > > 		#address-cells = <1>;
> > > 		#size-cells = <1>;
> > > 
> > > 		[...]
> > > 
> > > 		logicvc_display: display-engine at 0 {
> > > 			compatible = "xylon,logicvc-4.01.a-display";
> > > 
> > > 			[...]
> > 
> > I think the issue lies in what you left out here: you have another node
> > aside from the port one, called layers. I *think* the issue is that the
> > code will now pick up the layers node, and try to use it as a panel,
> > which will never probe.
> > 
> > I've had a look at all the other bindings though, it seems like this
> > driver is the only one that can be affected: the anx7625 seems to be the
> > only other driver that has a child node that isn't either a port or a
> > panel (aux-bus) but it doesn't use drm_of_find_panel_or_bridge either.
> 
> Thanks a lot for looking into this so quickly!
> 
> After some testing it clearly appears that you're right and the layers
> node is the one conflicting with the patch. Removing it brings the
> behavior back to normal. I'll try to dig-in a bit more to understand
> why this is happening since it's really not obvious when just looking
> at the patch.

Ah wait I do understand it actually. The patch will take the *first* node
that doesn't have ports/port in it and use that as remote instead of
of_graph_get_remote_node.

So maybe the fix would be to first look via of_graph_get_remote_node and
if nothing is returned then it should try to use the first node as remote.
tl;dr just inverting the order of the logic.

Do you think that would work?

Cheers,

Paul

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