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Date:   Wed, 27 Apr 2022 16:34:10 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Jagan Teki <jagan@amarulasolutions.com>
Cc:     Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Thierry Reding <thierry.reding@gmail.com>,
        Rob Clark <robdclark@gmail.com>, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org,
        Linus Walleij <linus.walleij@linaro.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Robert Foss <robert.foss@linaro.org>
Subject: Re: [PATCH 2/2] Revert "drm: of: Lookup if child node has panel or
 bridge"
Message-ID: <20220427143410.bectqumq62qmlv5x@houat>
References: <20220420231230.58499-1-bjorn.andersson@linaro.org>
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On Tue, Apr 26, 2022 at 01:40:31PM +0530, Jagan Teki wrote:
> On Tue, Apr 26, 2022 at 1:24 PM Paul Kocialkowski
> <paul.kocialkowski@bootlin.com> wrote:
> >
> > Hi,
> >
> > On Thu 21 Apr 22, 10:59, Paul Kocialkowski wrote:
> > > Hi Maxime,
> > >
> > > On Thu 21 Apr 22, 10:23, Maxime Ripard 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 ch=
ild 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 eve=
r finding
> > > > > > a reference to the panel.
> > > > > >
> > > > > > This reverts commit '80253168dbfd ("drm: of: Lookup if child no=
de has
> > > > > > panel or bridge")', in favor of using an explicit graph referen=
ce 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.
> > >
> > > Oh wow I feel stupid for not thinking about that.
> > >
> > > Yeah I agree that it seems like the best option.
> >
> > Should I prepare a patch with such a new helper?
> >
> > The idea would be to keep drm_of_find_panel_or_bridge only for the of g=
raph
> > case and add one for the child node case, maybe:
> > drm_of_find_child_panel_or_bridge.
> >
> > I really don't have a clear idea of which driver would need to be switc=
hed
> > over though. Could someone (Jagan?) let me know where it would be neede=
d?
>=20
> sun6i_mipi_dsi

It doesn't look like sun6i_mipi_dsi is using devm_drm_of_get_bridge at all?

> exynos_drm_dsi

If you reference 711c7adc4687, I don't see why we would need to switch
it back to the old behaviour. It wasn't iterating over its child node
before, so what does the switch to drm_of_get_bridge broke exactly?

> mcde_dsi (as of now)

Yeah, we do need to revert 3730bc6147b0 and 3d7039e1e649

Maxime