Date:   Tue, 26 Apr 2022 15:04:17 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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 26 Apr 22, 14:55, Maxime Ripard wrote:
> On Tue, Apr 26, 2022 at 02:54:01PM +0200, Maxime Ripard wrote:
> > On Tue, Apr 26, 2022 at 02:41:44PM +0200, Paul Kocialkowski wrote:
> > > 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?
> > 
> > I don't think Laurent's point was to move the child node away from its
> > DSI controller, that part doesn't make much sense. The panel or bridge
> > is still accessed through the DSI bus, so it very much belongs there.
> > 
> > What he meant I think was that we mandate the OF graph for all panels,
> > so for panels/bridges controlled through DCS, you would still list the
> > output through the graph.
> 
> Also, we're already in a bit of a mess right now. I don't think rushing
> that kind of patches in a (late) rc is making much sense, but as I said,
> if you want to start working on this, then I'll take a revert for the
> next rc, and then we can work calmly on this.

As I understand it we either have some broken stuff because of the revert of:
- drm: of: Lookup if child node has panel or bridge
- drm: of: Properly try all possible cases for bridge/panel detection

because the child node is already used in places, or we can have broken stuff
because with the patches because with these two patches -ENODEV is no longer
returned.

Now with the extra patch that I sent:
- drm: of: Improve error handling in bridge/panel detection

we get -ENODEV back, except for the connector case but this one should be
handled in drivers directly and drm_of_find_panel_or_bridge should not be
called in that situation.

So all in all it seems that all the pieces are there, unless I'm missing
something.

What do you think?

Paul

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

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