On 15/11/19 4:04 am, Drew DeVault wrote:
On Fri Nov 15, 2019 at 12:39 AM, Scott Anderson wrote:
A hint is merely a hint. The compositor can abide or not by that.
This flag will explicitly close the client connection if the buffer
can't be scanned out when this flag is passed.
This flag doesn't sound like a hint to me, but a hard requirement. From
the discussion I saw on the MR [1], if we don't abide, we risk being killed.
I agree: that's not a hint, it's a demand.
I think that there would be value in being able to suggest that a
particluar buffer be scanned out for performance reasons. But, as a
suggestion, and not a demand, and definitely not for secrecy reasons.
wlroots-based compositors would reserve the right to read your buffers
whenever we want. If I want to read your buffer for a frame to take a
screenshot, it's not going to be the end of the world for performance
and I don't want to end up segfaulting because of it.
>
But without the excuse for performance, that also raises another issue
for me about content-protection living in a "wp" protocol. The
governance thing hasn't officially been applied yet, and I wouldn't even
be the official spokesperson for wlroots, but I would personally NACK
that. Content-protection is a niche use case not generally useful to
Wayland implementations. I think a "ext" "wl_buffer factory for
protected dmabufs" would be a better place for this. It means you could
advertise a correct list of formats+modifiers too.
As an official spokesperson for wlroots:
If this protocol change were for the purpose of _suggesting_ scan-out,
I'd be on board with it. But this is not that - if it were, we would
dispense with any ideas around enforcing it with memory protections.
Then we could get an improvement for performance without a regression
for usability.
Rather, this just seems to be a DRM-enabling change, and the official
policy of wlroots will always be to NACK those for the wp and xdg
namespaces.
I don't think a hint for "please scan me out" has any value. A Wayland
compositor that is capable of scanning out will try to scan out
everybody as often as they can anyway, making the hint meaningless. A
Wayland compositor that is not capable of scanning out is also going to
ignore the hint, also making it meaningless.
The only situation I could possibly think of where this could make a
difference is if you have two surfaces, and the compositor's policy
would normally be to scan out the focused surface, but chooses to scan
out e.g. a video player surface with this flag instead. That only works
if the video player is not occluded.
But that's also cheapened by the fact that this flag sounds like "free
performance", and everybody would just set it.
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