Re: [RFC 0/4] dma-fence: Deadline awareness

[Christian König]

Am 29.07.21 um 13:00 schrieb Pekka Paalanen:
> On Thu, 29 Jul 2021 12:14:18 +0200
> Christian König <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
>
> > Am 29.07.21 um 11:15 schrieb Pekka Paalanen:
> > > If the app happens to be frozen (e.g. some weird bug in fence handling
> > > to make it never ready, or maybe it's just bugged itself and never
> > > drawing again), then the app is frozen, and all the rest of the desktop
> > > continues running normally without a glitch.
> > But that is in contradict to what you told me before.
> >
> > See when the window should move but fails to draw it's new content what
> > happens?
> >
> > Are the other windows which would be affected by the move not drawn as well?
> No, all the other windows will continue behaving normally just like
> they always did. It's just that one frozen window there that won't
> update; it won't resize, so there is no reason to move that other
> window either.
>
> Everything continues as if the frozen window never even sent anything
> to the compositor after its last good update.
>
> We have a principle in Wayland: the compositor cannot afford to wait
> for clients, the desktop as a whole must remain responsive. So there is
> always a backup plan even for cases where the compositor expects the
> client to change something. For resizes, in a floating-window manager
> it's easy: just let things continue as they are; in a tiling window
> manager they may have a timeout after which... whatever is appropriate.
>
> Another example: If a compositor decides to make a window maximized, it
> tells the client the new size and state it must have. Until the client
> acks that specific state change, the compositor will continue managing
> that window as if nothing changed. Given the asynchronous nature of
> Wayland, the client might even continue submitting updates
> non-maximized for a while, and that will go through as if the
> compositor didn't ask for maximized. But at some point the client acks
> the window state change, and from that point on if it doesn't behave
> like maximized state requires, it will get a protocol error and be
> disconnected.

Yeah and all of this totally makes sense.

The problem is that not forwarding the state changes to the hardware 
adds a CPU round trip which is rather bad for the driver design, 
especially power management.

E.g. when you submit the work only after everybody becomes available the 
GPU becomes idle in between and might think it is a good idea to reduce 
clocks etc...

How about doing this instead:

1. As soon as at least one window has new committed state you submit the 
rendering.
        As far as I understand it that is already the case anyway.

2. Before starting rendering the hardware driver waits with a timeout 
for all the window content to become ready.
        The timeout is picked in a way so that we at least reach a 
reasonable fps. Making that depending on the maximum refresh rate of the 
display device sounds reasonable to me.

3a. If all windows become ready on time we draw the frame as expected.
3b. If a timeout occurs the compositor is noted of this and goes on a 
fallback path rendering only the content known to be ready.

4. Repeat.

This way we should be able to handle all use cases gracefully, e.g. the 
hanging client won't cause the server to block and when everything 
becomes ready on time we just render as expected.

Regards,
Christian.

> Thanks,
> pq