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

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Message-ID: <20241101144315.GK2473@pendragon.ideasonboard.com>
Date: Fri, 1 Nov 2024 16:43:15 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
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, Nov 01, 2024 at 12:27:15PM +0200, Dmitry Baryshkov wrote:
> On Fri, 1 Nov 2024 at 11:20, Laurent Pinchart 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.

We can also expand the lookup infrastructure and handle more platform
integration and driver architecture options. I'm not sure how it would
look like, but all these are in-kernel APIs, so they can be extended and
modified if needed.

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

-- 
Regards,

Laurent Pinchart