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

[Maxime Ripard]

On Mon, Jul 15, 2024 at 11:32:34AM GMT, Tomi Valkeinen wrote:
> On 02/07/2024 14:43, Maxime Ripard wrote:
> > Hi Tomi,
> > 
> > On Wed, Jun 26, 2024 at 06:53:40PM GMT, Tomi Valkeinen wrote:
> > > 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 at ideasonboard.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?
> > > 
> > > 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.
> > 
> > Sorry, that's one of the things I don't quite get. Both functions are
> > exclusively(?) called from I2C bridges, so the device passed there
> > should be a i2c_client instance, and thus the MIPI-DSI host going away
> > will not remove those i2c devices, only the MIPI-DSI ones, right?
> 
> Yes.
> 
> > So if we remove the host, the MIPI-DSI device will be detached and
> > removed through the path you were explaing with the i2c client lingering
> > around. And if we remove the I2C device, then devm will kick in and will
> > detach and remove the MIPI-DSI device.
> 
> Right.
> 
> > Or is it the other way around? That if you remove the host, the device
> > is properly detached and removed, but there's still the devm actions
> > lingering around in the i2c device with pointers to the mipi_dsi_device
> > that was first created, but since destroyed?
> > 
> > And thus, if the i2c device ever goes away, we get a use-after-free?
> 
> Hmm, I'm not sure I understand what you mean here... Aren't you describing
> the same thing in both of these cases?
> 
> In any case, to expand the description a bit, module unloading is quite
> fragile. I do get a crash if I first unload the i2c bridge module, and only
> then go and unload the other ones in the DRM pipeline. But I think module
> unloading will very easily crash, whatever the DRM drivers being used are,
> so it's not related to this particular issue.
> 
> In my view, the unload sequence that should be supported (for development
> purposes, not for production) is to start the unload from the display
> controller module, which tears down the DRM pipeline, and going from there
> towards the panels/connectors.
> 
> Of course, it would be very nice if the module unloading worked perfectly,
> but afaics fixing all that's related to module unloading would be a
> multi-year project... So, I just want to keep the sequence I described above
> working, which allows using modules while doing driver development.

FTR, I'm all for supporting module unloading. The discussion above was
about what is broken exactly, so we can come up with a good solution.

Maxime
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