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

[Tomi Valkeinen]

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.

  Tomi