Amdgpu module is references even after unbinding the vtcon

[Thomas Zimmermann]

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#L160

> 
> Thanks,
> Christian.

-- 
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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