[PATCH v6 6/6] drm/doc: Define KMS atomic state set

[Michel Dänzer]

On 11/13/23 10:18, Simon Ser wrote:
> On Monday, October 23rd, 2023 at 10:25, Simon Ser <contact at emersion.fr> wrote:
> 
>>>>>>>>>>> +An atomic commit with the flag DRM_MODE_PAGE_FLIP_ASYNC is allowed to
>>>>>>>>>>> +effectively change only the FB_ID property on any planes. No-operation changes
>>>>>>>>>>> +are ignored as always. [...]
>>>>>>>>>>> During the hackfest in Brno, it was mentioned that a commit which re-sets the same FB_ID could actually have an effect with VRR: It could trigger scanout of the next frame before vertical blank has reached its maximum duration. Some kind of mechanism is required for this in order to allow user space to perform low frame rate compensation.
>>>>>>>>>
>>>>>>>>> Xaver tested this hypothesis in a flipping the same fb in a VRR monitor
>>>>>>>>> and it worked as expected, so this shouldn't be a concern.
>>>>>>>>> Right, so it must have some effect. It cannot be simply ignored like in
>>>>>>>>> the proposed doc wording. Do we special-case re-setting the same FB_ID
>>>>>>>>> as "not a no-op" or "not ignored" or some other way?
>>>>>>>>> There's an effect in the refresh rate, the image won't change but it
>>>>>>>>> will report that a flip had happened asynchronously so the reported
>>>>>>>>> framerate will be increased. Maybe an additional wording could be like:
>>>>>>>
>>>>>>> Flipping to the same FB_ID will result in a immediate flip as if it was
>>>>>>> changing to a different one, with no effect on the image but effecting
>>>>>>> the reported frame rate.
>>>>>>
>>>>>> Re-setting FB_ID to its current value is a special case regardless of
>>>>>> PAGE_FLIP_ASYNC, is it not?
>>>>>
>>>>> No. The rule has so far been that all side effects are observed
>>>>> even if you flip to the same fb. And that is one of my annoyances
>>>>> with this proposal. The rules will now be different for async flips
>>>>> vs. everything else.
>>>>
>>>> Well with the patches the async page-flip case is exactly the same as
>>>> the non-async page-flip case. In both cases, if a FB_ID is included in
>>>> an atomic commit then the side effects are triggered even if the property
>>>> value didn't change. The rules are the same for everything.
>>>
>>> I see it only checking if FB_ID changes or not. If it doesn't
>>> change then the implication is that the side effects will in
>>> fact be skipped as not all planes may even support async flips.
>>
>> Hm right. So the problem is that setting any prop = same value as
>> previous one will result in a new page-flip for asynchronous page-flips,
>> but will not result in any side-effect for asynchronous page-flips.
>>
>> Does it actually matter though? For async page-flips, I don't think this
>> would result in any actual difference in behavior?
> 
> To sum this up, here is a matrix of behavior as seen by user-space:
> 
> - Sync atomic page-flip
>   - Set FB_ID to different value: programs hw for page-flip, sends uevent
>   - Set FB_ID to same value: same (important for VRR)
>   - Set another plane prop to same value: same

A page flip is programmed even if FB_ID isn't touched?


>   - Set another plane prop to different value: maybe rejected if modeset required
> - Async atomic page-flip
>   - Set FB_ID to different value: updates hw with new FB address, sends
>     immediate uevent
>   - Set FB_ID to same value: same (no-op for the hw)

No-op implies it doesn't trigger scanning out a frame with VRR, if scanout is currently in vertical blank. Is that the case? If so, async flips can't reliably trigger scanning out a frame with VRR.


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer