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

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Message-ID: <CAMty3ZCLEMv4cqUcUGUAkLtH8tmh1WO582cDjZWynAifZJy=_w@mail.gmail.com>
Date:   Wed, 27 Apr 2022 12:29:04 +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 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?

Thanks,
Jagan.