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

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Date:   Tue, 26 Apr 2022 14:33:31 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     Maxime Ripard <maxime@...no.tech>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        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 Tue, Apr 26, 2022 at 09:54:36AM +0200, Paul Kocialkowski wrote:
> On Thu 21 Apr 22, 10:59, Paul Kocialkowski wrote:
> > 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 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.
> > 
> > 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 graph
> 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 switched
> over though. Could someone (Jagan?) let me know where it would be needed?
> 
> Are there cases where we could both expect of graph and child node?
> (i.e. does the new helper also need to try via of graph?)

I still think we should use OF graph uncondtionally, even in the DSI
case. We need to ensure backward-compatibility, but I'd like new
bindings (and thus new drivers) to always use OF graph.

-- 
Regards,

Laurent Pinchart