drm_hwcomposer: Question about buffer importer

[Andrii Chepurnyi]

Hello,

I'm working on Android in a virtualized environment under Xen hypervisor.
I suppose that drm_hwcomposer will be the convenient choice as a generic solution in that case.

I've implemented some platfomrXXX for our case, in which implemented Importer:
I.e. ImportBuffer and ReleaseBuffer for native buffers(i.e. ION allocated memory) in/from DRM.
I use the same ImportImage as in platformgeneric.cpp implementation(but I suppose it not used in our use-cases for now).
Please note, we use custom paravirtual DRM driver on our platform.
It has only one main plane so far.
Taking that into account, I suppose that all composition is done in SurfaceFlinger in that case.
I.e.: hwc report that it won't take any layers and SurfaceFlinger will do composition and propagate one buffer as framebuffer target(via SetClientTarget)
to hwc. That buffer will be placed on the main plane.

Using drm_hwcomposer I see buffer registration/deregistration(DrmHwcBuffer::ImportBuffer / DrmHwcBuffer::Clear) for each composition.
That cause some performance issues, which may be specific to our platform/DRM realization/our use-case:
In our virtualized environment Framebuffer registration/deregistration(drmModeAddFB2/drmModeRmFB) is  performance costly. And de facto useless to do it for every frame, because SurfaceFlinger will not reallocate that framebuffer-target's(at least until display resolution won't change).

For example: If I allocate several buffers for final composition and register them once in DRM. Further, I will copy composition from framebuffer surface (provided by SurfaceFlinger) to one of that preallocated buffers and put it on a plane, it will be more optimal than do drmModeAddFB2/drmModeRmFB for every buffer per composition.

Could you please suggest me how to overcome that situation with buffers registration/deregistration?

Kind regards,
Andrii.

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