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 14:41:44 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.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"

Hi,

On Tue 26 Apr 22, 14:33, Laurent Pinchart wrote:
> 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.

I just went over the thread on "drm: of: Improve error handling in bridge/panel
detection" again and I'm no longer sure there's actually still an issue that
stands, with the fix that allows returning -ENODEV when possible.

The remaining issue that was brought up was with a connector node, but it should
be up to the driver to detect that and avoid calling drm_of_find_panel_or_bridge
in such situations.

So with that in mind it feels like the child node approach can be viable
(and integrated in the same helper).

We might still want to favor an explicit OF graph approach, but note that
dsi-controller.yaml also specifies extra properties that are specific to
MIPI DSI and I'm not sure there are equivalent definitions for the OF graph
approach.

What do you think?

Paul

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

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