On Tue, Sep 03, 2024 at 11:27:23AM +0300, Tomi Valkeinen wrote: > On 03/09/2024 10:40, Simona Vetter wrote: > > On Mon, Sep 02, 2024 at 03:31:28PM +0300, Tomi Valkeinen wrote: > > > Hi, > > > > > > On 02/09/2024 13:50, Daniel Vetter wrote: > > > > On Mon, Sep 02, 2024 at 11:26:11AM +0200, Maxime Ripard wrote: > > > > > Hi, > > > > > > > > > > On Wed, Aug 07, 2024 at 03:19:23PM GMT, Tomi Valkeinen wrote: > > > > > > On 25/07/2024 14:28, Maxime Ripard wrote: > > > > > > > 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@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. > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > Does that mean that you're ok with the patch, or that something should be > > > > > > improved? > > > > > > > > > > No, I meant that at the very least the commit log needs to be updated to > > > > > reflect what is actually going on, because at least my understanding of > > > > > it doesn't match what actually happens. > > > > > > > > > > We want a solution to the problem you're facing, but it's not clear to > > > > > me what the problem is exactly at this point, so it's hard to review a > > > > > solution. > > > > > > > > So I haven't looked at the full thing, but I think the proper fix is to > > > > make both detach and unregister cope with being called multiple times. I > > > > think devm_ here is a red herring, the underlying issues is that we can > > > > unregister/detach from two sides: > > > > > > > > - when the host dsi goes away > > > > - when individual dsi devices on a given host go away > > > > > > > > So there needs to be book-keeping and locking to make sure no matter which > > > > order things disappear, we don't try to unregister/detach a dsi device > > > > twice. > > > > > > I think that is what my patch does (for devm_). > > > > Yep, except I think you should just do it for everyone, not just for the > > special case where one of the calls is done through devm. > > > > > Some vocabulary first: > > > > > > dsi peripheral device - The device that represents the DSI peripheral. It is > > > a bridge or a panel, and (usually) an i2c or platform device. > > > > > > dsi peripheral driver - The driver handling the dsi peripheral device. > > > > > > dsi device - Runtime created device instance that represents the DSI > > > peripheral. So in my case we have the i2c bridge device, and a dsi device is > > > created for it in the setup code. > > > > > > dsi controller device - A device that has a DSI bus (usually a platform or > > > i2c device, I would guess). > > > > > > dsi controller driver - A driver for the dsi controller device. Creates the > > > dsi host. > > > > > > dsi host - represents the DSI host side, owned by the dsi controller driver. > > > > > > When a dsi peripheral driver uses devm_mipi_dsi_device_register_full() or > > > devm_mipi_dsi_attach(), the dsi device is created and attached to the dsi > > > host. When the dsi peripheral device-driver is unbound, devres will call > > > unregister and detach are called automatically. This works fine. > > > > > > But when the device-driver for the dsi controller is unbound, the dsi > > > controller driver will unregister the dsi host, and the unregistration will > > > also unregister and detach the dsi device. But the devres is not told about > > > that. So when the dsi peripheral is later unbound, its devres will again > > > unregister and detach. > > > > > > To fix that this patch uses devm_remove_action() to remove the devres action > > > when the host side goes away first. > > > > > > Now, after writing the above, I realized that all this won't help with the > > > non-devm versions: the host side has unregistered and detached the dsi > > > device, but if the dsi peripheral driver calls mipi_dsi_detach() or > > > mipi_dsi_device_unregister(), it will again crash. > > > > > > Handling the attach/detach should be quite easy, and in fact the code > > > already handles it, but it uses WARN_ON() there so that has to go. But > > > attach/detach will crash anyway if the dsi device has already been freed, > > > which happens when the dsi controller driver calls > > > mipi_dsi_device_unregister(). > > > > Hm I thought we have a full struct device, so refcounted, and also with > > struct device unregister should be separate from the final kfree when the > > last reference drops away. Hence I thought this should just work. > > > > We might need to grab a reference in attach/detach to sort this out? > > I think there's a bit more to it. A non-dsi-device bridge driver would do: > > (devm_)mipi_dsi_device_register_full() > ... > (devm_)mipi_dsi_attach() > > The DSI host side could unregister and free the dsi_device right after the > call to mipi_dsi_device_register_full(), and mipi_dsi_attach() would > probably just crash. > > If I understand this correctly, the main issue is that the bridge driver > doesn't own an exclusive reference to the dsi_device, even if it looks like > it does, but rather both the dsi host and the bridge driver share the same > reference. So, we could get an extra reference, so that each side has its > own. > > But I think there's more. The bridge driver will call > mipi_dsi_device_unregister() (manually or via devres), and that does > device_unregister(). However, the dsi host will also call > mipi_dsi_device_unregister() when tearing down, which would result in > another device_unregister(). > > The above is not a problem if the bridge does away first, as then the dsi > bus won't contain the dsi_device anymore, and the dsi host will not > unregister it. But if it's the other way around, the dsi host will do > device_unregister, and later the bridge will do it too as it thinks it owns > the dsi_device. > > I presume that can be solved by tracking whether we have unregistered the > dsi_device or not. However, what would happen if the bridge driver calls any > of the other mipi_dsi_* functions with the dsi_device that has already been > unregistered by the dsi host? Nothing good, probably. So all those functions > should start to fail graciously when the dsi_device has been unregistered. > Or the bridge driver should somehow get a notification about the > unregistration so that it knows not to call those functions. Ah yeah I got confused, I thought attach/detach was for the dsi_device, not the dsi_host. But it's all ok, because mipi_dsi_driver is a full-blown driver, so if we unregister the device all the drivers will be unbound, which means they should first detach and stop using the mipi_dsi_device. If they continue to do that, they're busted. Where things fall apart is for the non-dsi drivers which call mipi_dsi_attach directly, after having called mipi_dsi_device_register_full, bypassing the driver model. Those just blow up, and I don't think you can fix that without using the driver model properly. With that you should have all the pieces: - No matter who calls unregister, the driver gets unbound and can clean up the mess. After the unregister call there should be nothing attached anymore, so we can make that a WARN_ON. - mipi_dsi_device_register_full needs to be changed to not transfer the reference, meaning using device_register instead of device_add. This way the non-dsi drivers retain a reference of their own. - the non-dsi driver in it's non-dsi remove hook needs to call mipi_dsi_unregister and then mipi_dsi_put (which is just a device_put). - we need some locking around mipi_dsi_unregister to make sure it's not called twice and cannot race between a non-dsi driver and a bridge driver. > I have to say I feel a bit uncertain about the whole ownership model here. Yeah I misunderstood a few things too, but I think now it's clearer for me. Essentially unless you have a guarantee that both the host and dsi-device will be removed in the same order always (by both being instantiated from one probe hook like with pci drivers) you have to use a full dsi_driver even for non-dsi cases. Otherwise you won't get the ->remove hook and things will blow up. Also the manual call to mipi_dsi_detach in mipi_dsi_remove_device_fn is a bug and should be replaced by a WARN_ON(dsi->attached) after we've called mipi_dsi_device_unregister: - For the case of a native driver we've called that driver's ->remove hook as part of unregister, which should have called mipi_dsi_detach. If it didn't, it's a bug in that drivers ->remove hook Aside: we should have a devm_mipi_dsi_attach to make this easier, which also checks that you only call this on a mipi_dsi_device and nothing else (because anything else is a bug). - for the case of a non-dsi driver that also registers the host (the only case which does not result in lifetime bugs on removal) that non-dsi driver's remove function should make sure it's nuking everything in the right order. So first explicit dsi_device unregister, then explicit dsi_host_unregister. Otherwise it will at least look buggy. It's a bit of work, but I think this should be solid and we should be able to get there in fairly small steps. Cheers, Sima -- Simona Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
