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

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Message-ID: <20210729140024.6a635be4@eldfell>
Date:   Thu, 29 Jul 2021 14:00:24 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
Cc:     Christian König <christian.koenig@....com>,
        Michel Dänzer <michel@...nzer.net>,
        Rob Clark <robdclark@...omium.org>,
        Matthew Brost <matthew.brost@...el.com>,
        Jack Zhang <Jack.Zhang1@....com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Luben Tuikov <luben.tuikov@....com>, Roy Sun <Roy.Sun@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Tian Tao <tiantao6@...ilicon.com>,
        Lee Jones <lee.jones@...aro.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [RFC 0/4] dma-fence: Deadline awareness

On Thu, 29 Jul 2021 12:14:18 +0200
Christian König <ckoenig.leichtzumerken@...il.com> 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.

Thanks,
pq

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