Re: [PATCH 2/2] Revert "drm: of: Lookup if child node has panel or bridge"

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

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Hi Jagan,

On Wed 27 Apr 22, 17:22, Jagan Teki wrote:
> On Wed, Apr 27, 2022 at 12:29 PM Jagan Teki <jagan@amarulasolutions.com> wrote:
> >
> > On Thu, Apr 21, 2022 at 1:54 PM Maxime Ripard <maxime@cerno.tech> wrote:
> > >
> > > On Thu, Apr 21, 2022 at 01:15:54PM +0530, Jagan Teki wrote:
> > > > + Linus
> > > > + Marek
> > > > + Laurent
> > > > + Robert
> > > >
> > > > On Thu, Apr 21, 2022 at 4:40 AM Bjorn Andersson
> > > > <bjorn.andersson@linaro.org> wrote:
> > > > >
> > > > > Commit '80253168dbfd ("drm: of: Lookup if child node has panel or
> > > > > bridge")' attempted to simplify the case of expressing a simple panel
> > > > > under a DSI controller, by assuming that the first non-graph child node
> > > > > was a panel or bridge.
> > > > >
> > > > > Unfortunately for non-trivial cases the first child node might not be a
> > > > > panel or bridge.  Examples of this can be a aux-bus in the case of
> > > > > DisplayPort, or an opp-table represented before the panel node.
> > > > >
> > > > > In these cases the reverted commit prevents the caller from ever finding
> > > > > a reference to the panel.
> > > > >
> > > > > This reverts commit '80253168dbfd ("drm: of: Lookup if child node has
> > > > > panel or bridge")', in favor of using an explicit graph reference to the
> > > > > panel in the trivial case as well.
> > > >
> > > > This eventually breaks many child-based devm_drm_of_get_bridge
> > > > switched drivers.  Do you have any suggestions on how to proceed to
> > > > succeed in those use cases as well?
> > >
> > > I guess we could create a new helper for those, like
> > > devm_drm_of_get_bridge_with_panel, or something.
> >
> > I think using the same existing helper and updating child support is
> > make sense, as there is a possibility to use the same host for child
> > and OF-graph bindings.
> >
> > I can see two possible solutions (as of now)
> >
> > 1. adding "dcs-child-type" bindings for child-based panel or bridge
> > 2. iterate child and skip those nodes other than panel or bridge. or
> > iterate sub-child to find it has a panel or bridge-like aux-bus (which
> > is indeed hard as this configuration seems not 'standard' i think )
> >
> > Any inputs?
> 
> Checking aux-bus with the sub-node panel can be a possible check to
> look at it, any comments?

That looks very fragile and oddly specific. Also why base changes on the
original patch that you made?

With the follow-up fixes, we are checking the of graph first and only
considering child nodes if the of graph and remote are missing, so there isn't
really a need to be more specific in the child noise discrimination.

Actually I should also make a new version of "drm: of: Improve error handling in
bridge/panel detection" to also return -ENODEV if of_graph_get_remote_node
fails, so that it doesn't try to use the child node when the graph is defined
but not remote is defined.

Paul

> --- a/drivers/gpu/drm/drm_of.c
> +++ b/drivers/gpu/drm/drm_of.c
> @@ -244,6 +244,25 @@ 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;
> +
> +               if (!(of_node_name_eq(remote, "aux-bus") &&
> +                     of_get_child_by_name(remote, "panel")))
> +                       continue;
> +
> +               goto of_find_panel_or_bridge;
> +       }
> +
>         /*
>          * 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,
> @@ -254,6 +273,8 @@ int drm_of_find_panel_or_bridge(const struct
> device_node *np,
>                 return -ENODEV;
> 
>         remote = of_graph_get_remote_node(np, port, endpoint);
> +
> +of_find_panel_or_bridge:
>         if (!remote)
>                 return -ENODEV;
> 
> Jagan.

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

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