Re: [PATCH] Revert "drm/core: Preserve the framebuffer after removing it."

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 19-05-16 17:33, Daniel Vetter wrote:
> On Thu, May 19, 2016 at 5:15 PM, Hans de Goede <hdegoede@redhat.com> wrote:
>> This reverts commit 13803132818c ("drm/core: Preserve the framebuffer
>> after removing it.").
>>
>> This commit assumes that going through drm_framebuffer_remove() is not
>> necessary because "the fbdev code or any system compositor should restore
>> the planes anyway so there's no need to do it twice". But this is not true
>> for secondary GPUs / slave outputs.
>>
>> This revert fixes the dgpu no longer suspending on laptops with
>> switchable graphics after an external output which is connected
>> to the dgpu has been used.
>>
>> And it fixes the WARN_ON to detect drm_framebuffer leaks in
>> drm_mode_config_cleanup() triggering when unplugging an USB displaylink
>> device; or when rmmod-ing the secondary GPU kms driver on laptops with
>> switchable-graphics.
>>
>> Also this part of the reverted commit's commit-msg: "The old fb_id is
>> zero'd, so there's no danger of being able to restore the fb from fb_id."
>> is no longer true, the zero-ing does not happen until drm_framebuffer_free
>> gets called, which does not happen until the last ref is dropped, so
>> if a crtc's primary->fb is still pointing to this fb, the id will not
>> get zero'd and userspace could potentially gain access to the removed
>> fb again.
>>
>> Cc: stable@vger.kenrel.org
>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>
> We have a proper fix in drm-next:
>
> commit f2d580b9a8149735cbc4b59c4a8df60173658140
> Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Date:   Wed May 4 14:38:26 2016 +0200
>
>     drm/core: Do not preserve framebuffer on rmfb, v4.
>
> That thing took forever to get merged since no one seemed to have
> cared and bothered with a tested-by. But it's on its way to stable
> kernels now.

Ah, excellent, the important thing is that this gets fixed :)

Regards,

Hans
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