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

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Message-ID: <CAMty3ZAkw0rssCzR_ka7U9JeoGxJr5JPM7GWDfd1dob9goL-BQ@mail.gmail.com>
Date:   Wed, 27 Apr 2022 17:22:32 +0530
From:   Jagan Teki <jagan@...rulasolutions.com>
To:     Maxime Ripard <maxime@...no.tech>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Thierry Reding <thierry.reding@...il.com>,
        Rob Clark <robdclark@...il.com>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robert Foss <robert.foss@...aro.org>
Subject: Re: [PATCH 2/2] Revert "drm: of: Lookup if child node has panel or bridge"

On Wed, Apr 27, 2022 at 12:29 PM Jagan Teki <jagan@...rulasolutions.com> wrote:
>
> On Thu, Apr 21, 2022 at 1:54 PM Maxime Ripard <maxime@...no.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@...aro.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?

--- 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.