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

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Message-ID: <66ab4206-d1c8-4aad-99a7-c4c316e343a9@ideasonboard.com>
Date: Wed, 26 Jun 2024 12:55:39 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.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>,
 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 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.

  Tomi