Amdgpu module is references even after unbinding the vtcon

[Thomas Zimmermann]

Hi

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?
> 
> 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.

This looks like the bug to me.

I'm not sure why the client code takes the module reference in the first 
place. Drivers invoke client interface directly. Shouldn't that imply 
that they have a module reference already?

Best regards
Thomas

> 
> 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_client.c#L87
>> [2]
>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_client.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

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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