linux-kernel - Re: [PATCH 00/32] drm/mipi-dsi: avoid DSI host drivers to have pointers to DSI devices

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Message-ID: <20250725173248.4cdca9e4@booty>
Date: Fri, 25 Jul 2025 17:32:48 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maxime Ripard <mripard@...nel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann
 <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter
 <simona@...ll.ch>, Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong
 <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, Laurent
 Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman
 <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, Inki Dae
 <inki.dae@...sung.com>, Jagan Teki <jagan@...rulasolutions.com>, Marek
 Szyprowski <m.szyprowski@...sung.com>, Jani Nikula
 <jani.nikula@...ux.intel.com>, Dmitry Baryshkov <lumag@...nel.org>, Hui Pu
 <Hui.Pu@...ealthcare.com>, Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-sunxi@...ts.linux.dev, Kevin Hilman <khilman@...libre.com>, Jerome
 Brunet <jbrunet@...libre.com>, Martin Blumenstingl
 <martin.blumenstingl@...glemail.com>, linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH 00/32] drm/mipi-dsi: avoid DSI host drivers to have
 pointers to DSI devices

Hi Maxime,

On Fri, 25 Jul 2025 17:22:47 +0200
Maxime Ripard <mripard@...nel.org> wrote:

...

> > The question is: should a DSI host bridge driver:
> > 
> >  A) wait for a DSI device to .attach before drm_bridge_add()ing itself,
> >     or
> >  B) drm_bridge_add() itself unconditionally, and let the DSI device
> >     .attach whenever it happens?
> > 
> > A) is what many drivers (IIRC the majority) does. It implies the card
> > will not be populated until .attach, which in the hotplug case could
> > happen very late
> > 
> > B) is done by a few drivers and allows the card to appear in the
> > hotplug case without the device, which is needed for hotplug.
> > 
> > I had tried simply moving drm_bridge_add() from .attach to probe in
> > the samsung-dsim driver in the pase but that would not work. Now I did
> > yet another check at the code and I suspect it can be done with a small
> > additional change, but cannot access the hardware to test it currently.
> > 
> > Answering this last question might change and simplify the requirements
> > discussed in the (very lengthy, sorry about that) discussion above.  
> 
> You're not going to like the answer too much, but I think that it is
> that "nobody knows".
> 
> The bad news is that I can't give you an answer, but the good one is
> that it gives us some leeway.
> 
> As I said in my previous mail, we should probably figure it out,
> document it, and then make it work for your drivers. Once we have a
> documentation we can point to, the rest will fall in line.

Thanks for taking time to reply this!

I just sent a patch to do the mentioned change in samsung-dsim [0] to
start discussion on real code. I'd appreciate if you could prioritize
that patch over all other patches I have sent.

[0] https://lore.kernel.org/all/20250725-drm-bridge-samsung-dsim-add-in-probe-v1-1-b23d29c23fbd@bootlin.com/

Best regards,
Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com