[PATCH 4/5] drm/amdgpu: Wait for end of last waited-for vblank before programming flip

[Michel Dänzer]

On 10.06.2016 23:43, Daniel Vetter wrote:
> On Fri, Jun 10, 2016 at 05:57:12PM +0900, Michel Dänzer wrote:
>> From: Michel Dänzer <michel.daenzer at amd.com>
>>
>> If userspace wants a page flip to take effect during vblank sequence n,
>> it has to wait for vblank seqno n-1 before calling the
>> DRM_IOCTL_MODE_PAGE_FLIP ioctl.
>>
>> This change makes sure that we do not program the flip to the hardware
>> before the end of vblank seqno n-1 in this case, to prevent the flip
>> from taking effect too early.
>>
>> On the other hand, if the DRM_IOCTL_MODE_PAGE_FLIP ioctl is called
>> during vblank, but userspace didn't wait for the current vblank seqno
>> before, this change would still allow the flip to be programmed during
>> the current vblank seqno.
> 
> This just sounds like you're sending vblank events out a bit too early.
> And watching vblank waits that userspace does works, but it's fragile,
> add-hoc and I don't really jump in joy about adding that to the vblank
> core. Is there no way you can adjust sending out the vblank events
> similarly, to make sure userspace can never sneak in a pageflip too early?

What you call "too early" is actually "during the vertical blank period
waited for". IMHO only notifying userspace of a vertical blank period
when it's already over would defeat the purpose.


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