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

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Date:   Fri, 29 Apr 2022 01:17:26 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     Maxime Ripard <maxime@...no.tech>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.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 Jagan,

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 ?

> > > 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?
> 
> Exynos bindings have a child node (unlike OF-graph), the old code is
> checking panel and bridge individually so it broke once switch to
> devm_drm_of_get_bridge

-- 
Regards,

Laurent Pinchart