Amdgpu module is references even after unbinding the vtcon

[Slivka, Danijel]

[AMD Official Use Only - General]

Hi Christian,

I have tested the proposed patch, the issue is not reproducible and everything else seems to work fine.

BR,
Danijel

>-----Original Message-----
>From: Christian König <ckoenig.leichtzumerken at gmail.com>
>Sent: Thursday, January 26, 2023 1:20 PM
>To: Slivka, Danijel <Danijel.Slivka at amd.com>; Thomas Zimmermann
><tzimmermann at suse.de>
>Cc: Deucher, Alexander <Alexander.Deucher at amd.com>; dri-devel <dri-
>devel at lists.freedesktop.org>; Sharma, Shashank <Shashank.Sharma at amd.com>
>Subject: Re: Amdgpu module is references even after unbinding the vtcon
>
>Am 26.01.23 um 10:49 schrieb Slivka, Danijel:
>> [AMD Official Use Only - General]
>>
>> Hi Thomas,
>>
>> I have checked what you mentioned.
>> When loading amdgpu we call  drm_client_init() during fbdev setup [1], the
>refcnt for drm_kms_helper increases from 3 -> 4.
>> When we unbind vtcon, refcnt for drm_kms_helper drops 4 -> 3, but the
>drm_client_release() [2] is not called.
>> The drm_client_release() is called only when unloading the amdgpu driver.
>>
>> Is this expected?
>
>Yes, the client can't be released because it is possible that the vtcon is bound to
>this fbdev again.
>
>Please test the handle work around I've send around internally. At least for me
>that approach seems to work.
>
>Regards,
>Christian.
>
>>
>> There is a comment for drm_client_release with regards to fbdev :
>> * This function should only be called from the unregister callback. An exception
>>   * is fbdev which cannot free the buffer if userspace has open file descriptors.
>>
>> Could this be relevant for our use case, although as Application/X/GDM are
>stopped at that point and no fd should be open.
>>
>> Thank you,
>> BR,
>> Danijel
>>
>>> -----Original Message-----
>>> From: Thomas Zimmermann <tzimmermann at suse.de>
>>> Sent: Wednesday, January 25, 2023 8:48 PM
>>> To: Christian König <ckoenig.leichtzumerken at gmail.com>
>>> Cc: Deucher, Alexander <Alexander.Deucher at amd.com>; Slivka, Danijel
>>> <Danijel.Slivka at amd.com>; dri-devel
>>> <dri-devel at lists.freedesktop.org>; Sharma, Shashank
>>> <Shashank.Sharma at amd.com>
>>> Subject: Re: Amdgpu module is references even after unbinding the
>>> vtcon
>>>
>>> Hi Christian
>>>
>>> Am 24.01.23 um 15:12 schrieb Christian König:
>>>> Hi Thomas,
>>>>
>>>> we ran into a problem with the general fbcon/fbdev implementation
>>>> and though that you might have some idea.
>>>>
>>>> What happens is the following:
>>>> 1. We load amdgpu and get our normal fbcon.
>>>> 2. fbcon allocates a dump BO as backing store for the console.
>>>> 3. GDM/X/Applications start, new framebuffers are created BOs
>>>> imported, exported etc...
>>>> 4. Somehow X or GDM iterated over all the framebuffer objects the
>>>> kernels knows about and export them as DMA-buf.
>>>> 5. Application/X/GDM are stopped, handles closed, framebuffers
>>>> released etc...
>>>> 6. We unbind vtcon.
>>>>
>>>> At this point the amdgpu module usually has a reference count of 0
>>>> and can be unloaded, but since GDM/X/Whoever iterated over all the
>>>> known framebuffers and exported them as DMA-buf (for whatever reason
>>>> idk) we now still have an exported DMA-buf and with it a reference to the
>module.
>>>>
>>>> Any idea how we could prevent that?
>>> Here's another stab in the dark.
>>>
>>> The big difference between old-style fbdev and the new one is that
>>> the old fbdev setup (e.g., radeon) allocates a GEM object and puts
>>> together the fbdev data structures from the BO in a fairly hackish
>>> way. The new style uses an in-kernel client with a file to allocate
>>> the BO via dumb buffers; and holds a reference to the DRM module.
>>>
>>> Maybe the reference comes from the in-kernel DRM client itself. [1]
>>> Check if the client resources get released [2] when you unbind vtcon.
>>>
>>> Best regards
>>> Thomas
>>>
>>> [1]
>>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_cl
>>> ient.c#L87
>>> [2]
>>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_cl
>>> ient.c#L16
>>> 0
>>>
>>>> Thanks,
>>>> Christian.
>>> --
>>> Thomas Zimmermann
>>> Graphics Driver Developer
>>> SUSE Software Solutions Germany GmbH
>>> Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg)
>>> Geschäftsführer: Ivo Totev