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

[Tomi Valkeinen]

On 26/06/2024 18:07, Maxime Ripard wrote:
> 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@xxxxxxxxxxxxxxxx>
> > > >
> > > > 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?

Ah, never mind, the devres_release_all() would of course deal with that too.

However, I just realized/remembered why it crashes.

devm_mipi_dsi_device_register_full() and devm_mipi_dsi_attach() are 
given a device which is used for the devres. This device is probably 
always the bridge device. So when the bridge device goes away, so do 
those resources.

The mipi_dsi_device_unregister() call deals with a DSI device, which was 
created in devm_mipi_dsi_device_register_full(). Unregistering that DSI 
device, which does happen when the DSI host is removed, does not affect 
the devres of the bridge.

So, unloading the DSI host driver causes mipi_dsi_device_unregister() 
and mipi_dsi_detach() to be called (as part of 
mipi_dsi_host_unregister()), and unloading the bridge driver causes them 
to be called again via devres.

 Tomi