linux-kernel - Re: [PATCH v2] drm/bridge: Fix assignment of the of

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Message-ID: <CAA8EJprEDV2JViB9kQS2H1p=NgF+PcataEejC97DBo=aU5g5kw@mail.gmail.com>
Date: Fri, 1 Nov 2024 12:27:15 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Sui Jingfeng <sui.jingfeng@...ux.dev>, Johan Hovold <johan@...nel.org>, neil.armstrong@...aro.org, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Robert Foss <rfoss@...nel.org>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Abel Vesa <abel.vesa@...aro.org>, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] drm/bridge: Fix assignment of the of_node of the
 parent to aux bridge

On Fri, 1 Nov 2024 at 11:20, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> On Fri, Nov 01, 2024 at 11:49:07AM +0800, Sui Jingfeng wrote:
> >
> > On 2024/11/1 00:23, Johan Hovold wrote:
> > > On Thu, Oct 31, 2024 at 11:06:38PM +0800, Sui Jingfeng wrote:
> > >
> > >> But I think Johan do need more times to understand what exactly
> > >> the real problem is. We do need times to investigate new method.
> > > No, I know perfectly well what the (immediate) problem is here (I was
> > > the one adding support for the of_node_reused flag some years back).
> > >
> > > I just wanted to make sure that the commit message was correct and
> > > complete before merging (and also to figure out whether this particular
> > > patch needed to be backported).
> >
> > Well under such a design, having the child device sharing the 'OF' device
> > node with it parent device means that one parent device can *only*
> > create one AUX bridge child device.
> >
> > Since If you create two or more child AUX bridge, *all* of them will
> > call devm_drm_of_get_bridge(&auxdev->dev, auxdev->dev.of_node, 0, 0),
> > then we will *contend* the same next bridge resource.
> >
> > Because of the 'auxdev->dev.of_node' is same for all its instance.
> > While other display bridges seems don't has such limitations.
>
> Brainstorming a bit, I wonder if we could create a swnode for the
> auxiliary device, instead of reusing the parent's OF node.

This will break bridge lookup which is performed by following the OF
graph links. So the aux bridges should use corresponding of_node or
fwnode.

> This would
> require switching the DRM OF-based APIs to fwnode, but that's easy and
> mostly a mechanical change.

-- 
With best wishes
Dmitry