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

Wed Jun 15 08:03:41 UTC 2016

On 14.06.2016 17:06, Daniel Vetter wrote:
> On Tue, Jun 14, 2016 at 04:25:28PM +0900, Michel Dänzer wrote:
>> On 14.06.2016 14:53, Daniel Vetter wrote:
>>> On Tue, Jun 14, 2016 at 11:09:10AM +0900, Michel Dänzer wrote:
>>>> On 06/13/16 23:06, Daniel Vetter wrote:
>>>>> On Mon, Jun 13, 2016 at 05:58:29PM +0900, Michel Dänzer wrote:
>>>>>> On 06/13/16 17:06, Daniel Vetter wrote:
>>>>>>>
>>>>>>> Afaiui the rules are:
>>>>>>> - The timestamp for vblank event needs to agree with whatever oml_sync
>>>>>>>   requries.
>>>>>>> - The event delivery itself needs to be consistent with what page_flip
>>>>>>>   takes, i.e. if userspace sees an event and immediately issues a
>>>>>>>   page_flip then it should not be able to hit the same vblank with that
>>>>>>>   pageflip.
>>>>>>> [...]
>>>>>>
>>>>>>> I assume you're goal is to not delay page_flips unecessarily, without
>>>>>>> breaking requirement 2 here. Imo a simpler fix would be to delay the
>>>>>>> vblank handling to end of vblank. Fixes everything without hacks, [...]
>>>>>>
>>>>>> Except it breaks the original purpose of the wait for vblank
>>>>>> functionality, which is to wait for the beginning of a vertical blank
>>>>>> period. [0] You're focusing too much on page flips and suggesting to
>>>>>> throw out the vblank baby with the bathwater. I really don't see the big
>>>>>> issue which would justify that.
>>>>>>
>>>>>>
>>>>>> [0] As an analogy, how useful would e.g. calendar notifications be if
>>>>>> they arrived at the end of the events they're about? "Hey, that meeting
>>>>>> you were supposed to attend? It just finished!"
>>>>>
>>>>> Ok, what exactly is the use-case for waiting for vblanks _without_
>>>>> scheduling a flip afterwards? At least in drm the rule is that ABI is what
>>>>> userspace observes and actually cares about.
>>>>
>>>> E.g.: In cases where page flipping cannot be used, Xorg / the DDX driver
>>>> waits for the target vertical blank period before emitting the drawing
>>>> commands for a buffer swap operation. If the vblank notification only
>>>> arrives when the vertical blank period is already over, this is very
>>>> likely to result in tearing.
>>>>
>>>> Some X compositors and AFAIK even applications such as media players can
>>>> use DRM_IOCTL_WAIT_VBLANK similarly. Obviously it's not intended to be
>>>> used directly like that, but nonetheless it is.
>>>
>>> Is there really anything using it like that outside of -ati?
>>
>> Yes. With DRI3/Present, it's driver independent code in
>> xserver/present/. With DRI2, it's theoretically up to the DDX driver,
>> but all drivers seem to have basically the same logic; the modesetting
>> driver certainly does.
> 
> Hm, didn't know that everyone does that. Seemed to silly an idea to waste
> all that gpu bandwidth by waiting for vblank ...

Actually it's kind of the other way around: One of the reasons for using
sync-to-vblank is to save power for rendering stuff which cannot be seen
anyway.

Also note that only the specific client using DRI2 or Present for
presentation is blocked, nothing else.

>>> I didn't know that we pass vblank waits to X clients. Either way annoying,
>>> since it means you need to keep things working like this for amd drivers
>>> forever.
>>
>> Please don't try to single out our drivers, it seems like rather your
>> driver is the odd man out in this case.
> 
> Hm, why/where is i915 special?

By only triggering the vblank interrupt at the end of a vertical blank
period, breaking the original DRM_IOCTL_WAIT_VBLANK functionality.

>>> How bad would it really be to just always delay the page_flip past vblank?
>>
>> In the variable refresh rate case it could be pretty bad, basically
>> dropping to the minimum refresh rate.
> 
> Variable refresh rate is entirely undefined right now in kms.

I don't think that really matters for the issue I'm thinking of.

My understanding is that variable refresh rate basically works by
transmitting the frame contents using the maximum refresh rate timing,
and then dynamically extending the vertical blank period until either
the next flip arrives, or the time since the last frame was transmitted
corresponds to the minimum refresh rate. So if we always wait for the
end of vertical blank before programming a flip, we'll probably end up
degrading to the minimum refresh rate whenever we're already in vertical
blank by the time we're ready to program the flip. Which will happen
with any app which cannot sustain a framerate > the maximum refresh
rate. So we'll end up running at either the minimum or maximum refresh
rate (or possibly even worse, flip-flopping between the two), defeating
the purpose of variable refresh rate.

Also, as I explained before, even without variable refresh rate, always
delaying flip programming past vblank can cause flips to be
unnecessarily delayed by one frame.

>>> If that's not good enough I'd say we should add a
>>> faster-than-vblank-but-still-synced page_flip flag. Then userspace could
>>> tell you exactly whether you should always wait (no flags), or never wait
>>> (with this new flag).
>>
>> That would be an inferior solution compared to my series, e.g.: If
>> userspace calls DRM_IOCTL_MODE_PAGE_FLIP before the target vblank seqno
>> is reached, it cannot use the new flag, otherwise the flip might take
>> effect too early. However, if we are then already in the target vblank
>> period when the fences have signalled and we are ready to program the
>> flip, we have to wait for the end of vblank first, and the flip will be
>> delayed by one frame.
>>
>> If we're going to change the userspace interface, it would be better to
>> re-purpose the reserved field of struct drm_mode_crtc_page_flip for
>> explicitly specifying the target vblank seqno (via a new cap and flag).
>> Then the kernel and userspace would no longer need to second-guess each
>> other.
>>
>> But even if we take that route, this series would be desirable for
>> getting us most of the way there for existing userspace.
> 
> Well that's what I mean. You'r patches here shoehorn what you want into
> existing api, trying to second-guess userspaces intentions inferred from
> what it does. Ime that tends to end in trouble. It would be much better to
> have a clear uabi for this. And my flag was just a suggestion.
> 
> I just had a discussion on irc with Dave about another topic were Dave
> complained about some of the inferred abi rules SNA uses (entirely
> differently, in probe code). And I thought it was a nice idea, since hey
> it works and its easier. But really it's not, since in the end kms is
> supposed to be somewhat generic. And usually your nice idea for inferring
> behaviour then tends to break down somewhere. At least I think the generic
> kms rules are:
> 
> - vblank events fire at lockstep [...].
> - pageflip immediately after a vblank wait needs to hit the next vblank.
> - pageflip in a loop needs to result in at most 1 flip per vblank, and if
>   you're too fast then the kernel should return -EBUSY. [...]

My series doesn't break any of these rules (or any others I'm aware of).
It avoids unnecessarily delaying flips in some cases within the confines
of these rules.

> Imo everything else (in this case: make the flip complete on the same
> frame as the vblank, if you hit the vblank window) needs special flags,
> with clear meaning of what they do. The specific flip target sounds like a
> good idea, except that current userspace can't be fixed, so we need to
> make it work without any flag.

Hey, that was my point above! So you agree that (something like) this
series will be needed anyway, even if new flags are added to make it
more explicit? :)

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