Amdgpu module is references even after unbinding the vtcon

[Alex Deucher]

On Tue, Jan 24, 2023 at 11:15 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>
> Hi
>
> 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?
>
> No real clue, sorry.
>
> But does it change if you use a shadow buffer on top of the amdgpu BO?
> Set prefer_shadow_fbdev = 1. [1]   I once tried to run generic fbdev
> without prefer_shadow_fbdev and it never worked. I suspected that some
> reference counting got wrong, but could never pin it down.  Maybe your
> issue is similar.

Is that equivalent to setting mode_config.prefer_shadow = 1?  If so,
we already do that.

Alex

>
> That said, generic fbdev is not so super-optimal for TTM-based drivers.
> I'm working on improving that, but it's not there yet.
>
> Best regards
> Thomas
>
>
> [1]
> https://elixir.bootlin.com/linux/v6.0/source/include/drm/drm_mode_config.h#L890
>
> >
> > Thanks,
> > Christian.
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
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