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Date:   Fri, 29 Apr 2022 19:05:59 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Jagan Teki <jagan@amarulasolutions.com>,
        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>,
        Robert Foss <robert.foss@linaro.org>
Subject: Re: [PATCH 2/2] Revert "drm: of: Lookup if child node has panel or
 bridge"
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Hi Maxime,

On Fri, Apr 29, 2022 at 05:46:45PM +0200, Maxime Ripard wrote:
> On Fri, Apr 29, 2022 at 01:17:26AM +0300, Laurent Pinchart wrote:
> > On Thu, Apr 28, 2022 at 02:09:42PM +0530, Jagan Teki wrote:
> > > On Wed, Apr 27, 2022 at 8:04 PM Maxime Ripard wrote:
> > > > On Tue, Apr 26, 2022 at 01:40:31PM +0530, Jagan Teki wrote:
> > > > > On Tue, Apr 26, 2022 at 1:24 PM 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?
> > > > >
> > > > > sun6i_mipi_dsi
> > > >
> > > > It doesn't look like sun6i_mipi_dsi is using devm_drm_of_get_bridge at all?
> > > 
> > > Correct, patch for this on the mailing list.
> > 
> > I've lost track of how we're solving the fallout of this for v5.18. I
> > have received a report that the original commit (80253168dbfd) also
> > broke the rcar-du driver. Could you please provide a git branch (based
> > on drm-fixes or drm-misc-fixes) with any patch that you plan to get
> > merged in v5.18, to let me test them locally ?
> 
> Was that report about 5.18 or drm-misc-next? It appears that all the
> drivers conversions are in drm-misc-next.

v5.18-rc2. I've double-checked, and it has been bisected to commit
67bae5f28c89, which is a fix of the commit this patch reverts
(80253168dbfd).

-- 
Regards,

Laurent Pinchart