On Mon, Dec 13, 2021 at 02:28:39PM +0200, Laurent Pinchart wrote:
> Hi Jagan,
> 
> Thank you for the patch.
> 
> On Mon, Dec 13, 2021 at 05:46:13PM +0530, Jagan Teki wrote:
> > Some OF graphs don't require 'ports' to represent the
> > downstream panel or bridge; instead it simply adds a child
> > node on a given parent node.
> > 
> > drm_of_find_panel_or_bridge can lookup panel or bridge for
> > a given node based on the OF graph port and endpoint and it
> > fails to use if the given node has a child panel or bridge.
> > 
> > This patch add support to lookup that given node has child
> > panel or bridge however that child node cannot be a 'port'
> > alone or it cannot be a 'port' node too.
> > 
> > Example OF graph representation of DSI host, which doesn't
> > have 'ports' and has child panel.
> > 
> > dsi {
> > 	compatible = "allwinner,sun6i-a31-mipi-dsi";
> > 	#address-cells = <1>;
> > 	#size-cells = <0>;
> > 
> > 	port {
> > 		dsi_in_tcon0: endpoint {
> > 			remote-endpoint = <tcon0_out_dsi>;
> > 	};
> > 
> > 	panel@0 {
> > 		reg = <0>;
> > 	};
> > };
> > 
> > Example OF graph representation of DSI host, which doesn't
> > have 'ports' and has child bridge.
> > 
> > dsi {
> > 	compatible = "allwinner,sun6i-a31-mipi-dsi";
> > 	#address-cells = <1>;
> > 	#size-cells = <0>;
> > 
> > 	port {
> > 		dsi_in_tcon0: endpoint {
> > 			remote-endpoint = <tcon0_out_dsi>;
> > 	};
> > 
> > 	bridge@0 {
> > 		reg = <0>;
> > 
> > 		ports {
> > 			#address-cells = <1>;
> > 			#size-cells = <0>;
> > 
> > 			bridge_out: port@1 {
> > 				reg = <1>;
> > 
> > 				bridge_out_panel: endpoint {
> > 					remote-endpoint = <&panel_out_bridge>;
> > 				};
> > 			};
> > 		};
> > 	};
> > };
> > 
> > Example OF graph representation of DSI host, which doesn't
> > have 'ports' or 'port' and has child panel.
> > 
> > dsi0 {
> > 	compatible = "ste,mcde-dsi";
> > 	#address-cells = <1>;
> > 	#size-cells = <0>;
> > 
> > 	panel@0 {
> > 		reg = <0>;
> > 	};
> > };
> > 
> > Example OF graph representation of LTDC host, which doesn't
> > have 'ports' or child panel/bridge and has 'port'.
> > 
> > ltdc {
> > 	compatible = "st,stm32-ltdc";
> > 	#address-cells = <1>;
> > 	#size-cells = <0>;
> > 
> > 	port {
> > 	};
> > };
> > 
> > Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
> > ---
> > Changes for v2:
> > - drop of helper
> > https://patchwork.kernel.org/project/dri-devel/cover/20211207054747.461029-1-jagan@amarulasolutions.com/
> > - support 'port' alone OF graph
> > - updated comments
> > - added simple code
> > 
> >  drivers/gpu/drm/drm_of.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> > index 59d368ea006b..7d018ff8bc83 100644
> > --- a/drivers/gpu/drm/drm_of.c
> > +++ b/drivers/gpu/drm/drm_of.c
> > @@ -249,6 +249,27 @@ int drm_of_find_panel_or_bridge(const struct device_node *np,
> >  	if (panel)
> >  		*panel = NULL;
> >  
> > +	/**
> > +	 * Some OF graphs don't require 'ports' to represent the downstream
> > +	 * panel or bridge; instead it simply adds a child node on a given
> > +	 * parent node.
> > +	 *
> > +	 * Lookup that child node for a given parent however that child
> > +	 * cannot be a 'port' alone or it cannot be a 'port' node too.
> > +	 */
> > +	if (!of_get_child_by_name(np, "ports")) {
> > +		if (of_get_child_by_name(np, "port") && (of_get_child_count(np) == 1))
> 
> This messes up reference counting of device_node.
> 
> > +			goto of_graph_get_remote;
> > +
> > +		for_each_available_child_of_node(np, remote) {
> > +			if (of_node_name_eq(remote, "port"))
> > +				continue;
> > +
> > +			goto of_find_panel_or_bridge;
> > +		}
> > +	}
> 
> This really looks like a hack to me, I'm worried it may cause issues. It
> would be better, I think, to split the drm_of_find_panel_or_bridge()
> function in two, with the of_graph_get_remote_node() call moved to a
> wrapper function, calling an inner function that takes the remote
> device_node pointer. For the DSI use case, you could either look up the
> panel DT node in the display driver and call the inner function
> directly, or implement a DSI-specific wrapper.

I disagree. The whole point of drm_of_find_panel_or_bridge was that it
was a helper for the encoder / upstream bridge to retrieve whatever is
there next. It's useful and removes boilerplate.

We definitely want to have something just as convenient for DSI.

Maxime