universal planes in drm backend

[Scott Anderson]

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