linux-kernel - Re: [PATCH] drm/mipi-dsi: Fix devm unregister & detach

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Date: Wed, 26 Jun 2024 17:07:20 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	Tomi Valkeinen <tomi.valkeinen+renesas@...asonboard.com>
Subject: Re: [PATCH] drm/mipi-dsi: Fix devm unregister & detach

On Wed, Jun 26, 2024 at 12:55:39PM GMT, Tomi Valkeinen wrote:
> On 26/06/2024 11:49, Maxime Ripard wrote:
> > Hi,
> > 
> > On Wed, Jun 19, 2024 at 12:07:48PM GMT, Tomi Valkeinen wrote:
> > > From: Tomi Valkeinen <tomi.valkeinen+renesas@...asonboard.com>
> > > 
> > > When a bridge driver uses devm_mipi_dsi_device_register_full() or
> > > devm_mipi_dsi_attach(), the resource management is moved to devres,
> > > which releases the resource automatically when the bridge driver is
> > > unbound.
> > > 
> > > However, if the DSI host goes away first, the host unregistration code
> > > will automatically detach and unregister any DSI peripherals, without
> > > notifying the devres about it. So when the bridge driver later is
> > > unbound, the resources are released a second time, leading to crash.
> > 
> > That's super surprising. mipi_dsi_device_unregister calls
> > device_unregister, which calls device_del, which in turn calls
> > devres_release_all.
> 
> Hmm, right.
> 
> > If that doesn't work like that, then it's what needs to be fixed, and
> > not worked around in the MIPI-DSI bus.
> 
> Well, something causes a crash for both the device register/unregister case
> and the attach/detach case, and the call stacks and debug prints showed a
> double unregister/detach...
> 
> I need to dig up the board and check again why the devres_release_all() in
> device_del() doesn't solve this. But I can probably only get back to this in
> August, so it's perhaps best to ignore this patch for now.
> 
> However, the attach/detach case is still valid? I see no devres calls in the
> detach paths.

I'm not sure what you mean by the attach/detach case. Do you expect
device resources allocated in attach to be freed when detach run?

Maxime

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