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Subject: Re: Warnings in DRM code when removing/unbinding a driver
To:     Daniel Vetter <daniel@ffwll.ch>
CC:     Ezequiel Garcia <ezequiel@collabora.com>,
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        Thomas Zimmermann <tzimmermann@suse.de>,
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        Gerd Hoffmann <kraxel@redhat.com>, <dbueso@suse.de>
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From:   John Garry <john.garry@huawei.com>
Message-ID: <fcca5732-c7dc-6e1d-dcbe-bfd914a4295b@huawei.com>
Date:   Thu, 19 Dec 2019 10:03:33 +0000
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On 19/12/2019 09:54, Daniel Vetter wrote:
> On Wed, Dec 18, 2019 at 7:08 PM John Garry <john.garry@huawei.com> wrote:
>>
>> +
>>
>> So the v5.4 kernel does not have this issue.
>>
>> I have bisected the initial occurrence to:
>>
>> commit 37a48adfba6cf6e87df9ba8b75ab85d514ed86d8
>> Author: Thomas Zimmermann <tzimmermann@suse.de>
>> Date:   Fri Sep 6 14:20:53 2019 +0200
>>
>>       drm/vram: Add kmap ref-counting to GEM VRAM objects
>>
>>       The kmap and kunmap operations of GEM VRAM buffers can now be called
>>       in interleaving pairs. The first call to drm_gem_vram_kmap() maps the
>>       buffer's memory to kernel address space and the final call to
>>       drm_gem_vram_kunmap() unmaps the memory. Intermediate calls to these
>>       functions increment or decrement a reference counter.
>>
>> So this either exposes or creates the issue.
> 
> Yeah that's just shooting the messenger.

OK, so it exposes it.

  Like I said, for most drivers
> you can pretty much assume that their unload sequence has been broken
> since forever. It's not often tested, and especially the hotunbind
> from a device (as opposed to driver unload) stuff wasn't even possible
> to get right until just recently.

Do you think it's worth trying to fix this for 5.5 and earlier, or just 
switch to the device-managed interface for 5.6 and forget about 5.5 and 
earlier?

Thanks,
John