[PATCH xf86-video-ati 5/6] Make all active CRTCs scan out an all-black framebuffer in LeaveVT

[Michel Dänzer]

On 29/08/17 07:56 PM, Emil Velikov wrote:
> On 28 August 2017 at 10:23, Michel Dänzer <michel at daenzer.net> wrote:
>> From: Michel Dänzer <michel.daenzer at amd.com>
>>
>> And destroy all other FBs. This is so that other DRM masters can only
>> get access to this all-black FB, not to any other FB we created, while
>> we're switched away and not DRM master.
>>
> Isn't the issue applicable overall - be that in X, wayland compositors, other?

Potentially.


> IIRC the vmwgfx's kernel driver, which has extra locking [1] in order
> to address that.
> Would a similar approach like that be applicable for radeon/amdgpu?

I don't see how that's related.

The issue is that if the caller of DRM_IOCTL_MODE_GETFB is DRM master
(or root, or using a DRM control device node), the ioctl returns a valid
GEM handle for the framebuffer, so the caller can use the underlying
buffer arbitrarily. This is handled by core DRM code, there's nothing
kernel drivers can do about it.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer