Multiple HW plane enable BKM in open source Weston

Pekka Paalanen ppaalanen at
Tue Dec 31 12:40:58 UTC 2019

On Mon, 30 Dec 2019 11:29:02 +0200
Marius Vlad <marius.vlad at> wrote:

> Hi,
> Yes, overlays are supported in weston but do note that this mailing list
> is also about other compositors and the protocol itself.
> Clients do not specify where to "place" the surfaces, the
> compositor back-end is the only one who can make those decisions
> based on what HW is capable of, what is currently in use, and
> what other surfaces are currently being displayed. The back-end,
> in weston, got a new face lift, and exposes a little bit better
> the conditions in which a view can make it to a HW plane: is a
> (HW) plane available, is the pixel format suitable, etc. If you
> feel  adventurous that should be place to learn more.
> Together with this face lift weston can now place an AR24 type of
> pixel buffer on the overlay plane, so if HW capable of using that
> pixel format should be able to place the view into an overlay
> plane. I'm mentioning this because `weston-simple-egl` will
> default to that when started, and a fairly recent i915 driver
> should have an overlay supporting that.
> We have in weston debug scopes capable of displaying large
> amounts of debug information. Start weston with with `--debug`
> and use the client `weston-debug` with `-a` to list all current
> debug scopes. `drm-backend' is the one useful in this case. It
> will display which views made to it a HW plane, or why was the
> reasons for not doing so. Should be able to correlate a bit this
> information with decision tree currently done in weston.
> Hope this helps.
> On 12/30/19 4:05 AM, Tang, Jun wrote:
> > Hi Wayland Experts,
> > 
> > I checked the open source version of Weston code, seems there
> > is atomic mode setting API to add new plane properties. I
> > believe overlay HW plane should be supported by Weston. But as
> > a newbie for Wayland. Still not sure how application can put
> > their surface on specified HW plane. Could you please share me
> > some BKM on Wayland client side? Any quick ideas would be
> > appreciated!
> > 


Marius is right. I'd just clarify, that there are no client
examples because there is nothing a client needs to do to make use
of hardware overlays.

Well, almost. There is no API for overlays for clients at all, but
clients do need to use buffers that are possible to be put on
hardware overlays. In practise that means they need to use dmabuf
buffers with a compatible format.

Creating compatible buffers is a game of luck for now, but there is
an effort to make it more predictable:

Once that improvement is done, it will likely be used automatically
inside the Mesa EGL implementation, meaning that applications that
use the EGL Wayland platform, e.g. weston-simple-egl, will get the
benefit without any code changes.

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