RFC: page-flip with damage?

[Pekka Paalanen]

On Tue, 26 Sep 2017 09:07:45 -0700
Thomas Hellstrom <thellstrom at vmware.com> wrote:

> On 09/26/2017 01:18 AM, Daniel Vetter wrote:
> > On Sun, Sep 24, 2017 at 07:41:45PM +0200, Thomas Hellstrom wrote:  
> >> Hi, list!
> >>
> >> Page flips, while efficient on real hardware, aren't that efficient in other
> >> situations, like for virtual devices with local, or even worse, remote
> >> desktops.
> >> We might ending up forwarding or encoding a couple of full frames worth of
> >> data instead of a small region at a cursor blink.
> >>
> >> Now there is this extension EGL_KHR_swap_buffers_with_damage, and
> >> gnome-shell/wayland on KMS also has a damage region that it forwards all the
> >> way down to the function where page-flip is called.

> > Wrt single dirty rect vs. rectlist: I'd opt for a single rect since that
> > makes the interface easier. Currently most drivers collapes the list
> > passed to fb_funcs->dirty to a single rect, so that seems good enough, and
> > a nice simplification of the interface (both uapi and driver).  
> 
> I think multiple cliprects had its benefits for old X type rendering: 
> Frontbuffer-type diagonal
> lines benchmarked with x11perf and similar. Now that everybody's 
> compositing that use-case is mostly gone, and as you say
> most driver coalesce anyway. Even we to some extent, at least on newer 
> "hardware" versions.

Hi,

it still is awfully easy to create a pathological use case where
collapsing into a single rect adds a thousand-fold more pixels to the
damage than having more than one rectangle would allow: two tiny
animations at the opposite corners of a screen.

I'm thinking Wayland compositors here.

Simplifying damage regions while not crashing down to just one
rectangle is quite possible I believe. Let userspace do simplification
from hundreds down to a few rectangles, and then collapse into one
rectangle in the driver if absolutely necessary. That's what I'd hope.

It is totally fine to require non-overlapping rectangles. You could
even demand a specific layout of rectangles, e.g. the form Pixman
regions use. You can also put an arbitrary cap on how many rectangles
are allowed.


Thanks,
pq
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