[PATCH] present: Execute right away if target_msc equals current_msc

[Axel Davy]

Yes,

I assume divisor = 0.

Initially the code was like that:
. if crtc_msc > target_msc, set target_msc to crtc_msc, and if 
PresentOptionAsync not requested, then increase target_msc.
. if ddx doing sync flips only, and we plan to do a flip, then decrease 
target_msc
. if target_msc was already reached, do a copy immediately (or async flip)

Lets give as example a few scenarios:
the screen is currently at msc crtc_msc.

Let's assume ddx is not able to do async flips:

. Client presents for target_msc <= crtc_msc
-> target_msc set to crtc_msc
-> if Async not requested, target_msc increased by one
=> We present immediately (copy) if Async requested
else if Async not requested:
-> if we plan to do a flip, we decrement target_msc (because sync flips 
delay effect by one msc)
-> if we don't plan to flip, we wait next vblank and copy then.


. Client presents for target_msc = crtc_msc + 1
-> PresentOptionAsync doesn't change anything
-> if doing flip then target_msc is decremented (same reason than before).

In these two cases, when at the end of present_pixmap, we have 
target_msc = crtc_msc, then we are
in the case where we must copy immediately and won't be doing flip. That 
explains the " target_msc > crtc_msc" check.

I don't know if that system worked perfectly when ddx was async flips able.
I haven't checked yet if the new code is ok for sync flips.

Yours,

Axel
On 02/11/2015 09:56, Zhou, Jammy wrote :
> Thanks Axel. It is consistent with my understanding.
>
> For the 'pixmap' check in your previous change, it is used to differentiate present_pixmap and present_notify_msc, right? It looks like "target_msc >= crtc_msc" was used at the first place for present_pixmap, do you know the background about this?
>
> Regards,
> Jammy
>
> -----Original Message-----
> From: Axel Davy [mailto:axel.davy at ens.fr]
> Sent: Monday, November 02, 2015 4:45 PM
> To: Zhou, Jammy; Michel Dänzer
> Cc: xorg-devel at lists.x.org; Mario Kleiner
> Subject: Re: [PATCH] present: Execute right away if target_msc equals current_msc
>
> Hi,
>
> As you can see here:
> http://cgit.freedesktop.org/xorg/proto/presentproto/tree/presentproto.txt#n212
>
> It means to present immediately instead of waiting for the next vblank (if needed).
>
> In practice when needed to present immediately, if the ddx is not able to do async flips, then we do a copy to the screen buffer, else we use that ability.
>
> Yours,
>
> Axel
>
> On 02/11/2015 09:26, Zhou, Jammy wrote :
>> By the way, does PresentOptionAsync mean the same thing with the async flip capability in kernel side? Or is it just a flag to indicate X/DDX to do present immediately instead of waiting for the next vblank?
>>
>> Regards,
>> Jammy
>>
>> -----Original Message-----
>> From: Axel Davy [mailto:axel.davy at ens.fr]
>> Sent: Thursday, October 29, 2015 3:42 PM
>> To: Michel Dänzer; Zhou, Jammy
>> Cc: xorg-devel at lists.x.org; Mario Kleiner
>> Subject: Re: [PATCH] present: Execute right away if target_msc equals
>> current_msc
>>
>> Hi,
>>
>> What is the current conscensus for how should PresentOptionAsync work ?
>>
>> If I remember correctly, the semantic used to be:
>>
>> . if we present at the current or past msc with the flag, two options:
>> -> if the ddx doesn't support async swap, we do copy to the screen
>> pixmap right away
>> -> if the ddx does support it, we do an async swap right away
>>
>> . if we present at another msc, things behave as without PresentOptionAsync, that is we schedule a swap, and ask at msc-1 to the ddx to swap for msc.
>>
>> With "Fix use of vsynced pageflips", I get the impression it shifted to:
>>
>> . Async requested, but driver not having the async option -> do screen copy (no flips, whatever the msc) . if Async not requested and flip planned, present at msc-1, else at msc. (so for flips and Async flag, we do always plan to flip at msc ?
>> That means always tear, right ?)
>>
>> That doesn't make a lot of sense to me, can someone clarify ?
>>
>> I'm afraid this patch could be a workaround to currently broken behaviour, and not the correct fix.
>>
>> I CCed Mario Kleiner.
>>
>> Yours,
>>
>> Axel Davy
>>
>> On 29/10/2015 03:53, Michel Dänzer wrote:
>>> On 28.10.2015 19:39, Jammy Zhou wrote:
>>>> It is according to the protocol:
>>>>
>>>> "If 'options' contains PresentOptionAsync, and the 'target-msc'
>>>> is less than or equal to the current msc for 'window', then the
>>>> operation will be performed as soon as possible, not necessarily
>>>> waiting for the next vertical blank interval."
>>>>
>>>> Signed-off-by: Jammy Zhou <Jammy.Zhou at amd.com>
>>>> ---
>>>>     present/present.c | 2 +-
>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/present/present.c b/present/present.c index
>>>> beb4ff0..5900c22 100644
>>>> --- a/present/present.c
>>>> +++ b/present/present.c
>>>> @@ -871,7 +871,7 @@ present_pixmap(WindowPtr window,
>>>>     
>>>>         xorg_list_add(&vblank->event_queue, &present_exec_queue);
>>>>         vblank->queued = TRUE;
>>>> -    if ((pixmap && target_msc >= crtc_msc) || (!pixmap && target_msc > crtc_msc)) {
>>>> +    if (target_msc > crtc_msc) {
>>>>             ret = present_queue_vblank(screen, target_crtc, vblank->event_id, target_msc);
>>>>             if (ret == Success)
>>>>                 return Success;
>>>>
>>> Looks good to me, but Cc'ing Axel Davy, who made the last change to
>>> this code.
>>>
>>> Reviewed-by: Michel Dänzer <michel.daenzer at amd.com>
>>>
>>>

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