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

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Date:   Thu, 28 Apr 2022 13:55:18 +0530
From:   Jagan Teki <jagan@...rulasolutions.com>
To:     Marek Szyprowski <m.szyprowski@...sung.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>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Robert Foss <robert.foss@...aro.org>
Subject: Re: [PATCH 2/2] Revert "drm: of: Lookup if child node has panel or bridge"

Hi Marek,

On Thu, Apr 28, 2022 at 11:47 AM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> Hi Maxime,
>
> On 27.04.2022 16:34, 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
> >> <paul.kocialkowski@...tlin.com> 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
> >>>>>> <bjorn.andersson@...aro.org> 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?
> >
> >> 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?
>
> It broke getting the panel if it is a direct child of the DSI device
> node. It worked before because it used following code:
>
> dsi->panel = of_drm_find_panel(device->dev.of_node);
>
> which got replaced by devm_drm_of_get_bridge().

Yes, we need to revert that change back to find the individual panel
and bridge. I'm preparing a patch for it.

Jagan.