universal planes in drm backend

zou lan nancy.lan.zou at gmail.com
Tue Sep 17 08:20:37 UTC 2019

Hi scott

yes, one CRTC bind to one primary plane. But if not all the crtcs are
connected and universal planes is enable, the spare primary plane could be
an overlay plane.

for example, plane 0 ~ 5 for 6 CRTCs, they are all primary planes. But
weston only enable 3 output, then plane 3 ~ 5 could treat as overlay plane.

Thank you!

Best regards

Scott Anderson <scott.anderson at collabora.com> 于2019年9月17日周二 下午3:50写道:

> On 17/09/19 7:38 pm, zou lan wrote:
> > Hi Daniel & all
> >
> > I find the function drm_output_prepare_overlay_view() only use the plane
> > type of WDRM_PLANE_TYPE_OVERLAY. it could be a waste for some planes of
> > type WDRM_PLANE_TYPE_PRIMARY if the universal planes is enable.
> >
> > For example, the kernel define 6 crtcs, and each crtc will have one
> > primary type plane, but not all of the crtcs are used by weston_output.
> > Some crtcs may never used, if we reserve  all the primary type planes as
> > scanout plane, that could waste some of them.
> >
> > Could the open source drm backend modify the logic of judge the overlay
> > plane? let the primary plane equal to overlay plane or judge in
> > drm_output_prepare_overlay_view(), if the plane is not used by outputs,
> > it could be used by overlay?
> >
> > Thank you!
> >
> > Best regards
> > Nancy
> Hi,
> As far as I'm aware, the kernel never advertises more than one primary
> plane per CRTC and they're never possible to be used with multiple
> CRTCs:
> https://www.kernel.org/doc/html/latest/gpu/drm-kms.html#plane-abstraction
>  >All drivers should provide one primary plane per CRTC to avoid
> surprising userspace too much
> Perhaps that restriction is not as strict as I interpret it to be, but
> I'm not aware of anything which does not have a one-to-one relationship
> between primary planes and CRTCs.
> Scott
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