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

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi,

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@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.

Does that mean that you're ok with the patch, or that something should 
be improved?

  Tomi