backend-drm and scanning really large resolutions

[Matt Hoosier]

Hi all,

I'm confronting a situation where the hardware with which I work is capable
of driving connectors at 4K or 8K, but doing so requires bonding the
scanning of multiple planes together.

The scenario is that you'd have a big primary framebuffer whose size is too
large for an individual hardware scanning pipeline on the display
controller to traverse within its maximum allowed clock rate.

The hardware supplier's approach is to assign multiple planes, which in the
KMS driver map to hardware scanning pipelines, to each be responsible for
scanning a smaller section of the framebuffer. The planes are all assigned
to the same CRTC, and in concert with each other they cover the whole area
of the framebuffer and CRTC.

This sounds a little bit wild to me. I hadn't been aware it's even legal to
have more than one plane treated a the source of scanout for a single
framebuffer. Maybe that distinction isn't really relevant nowadays with
universal plane support.

I'm wondering if anybody here knows whether this a legit approach for a
compositor's DRM backend to take?

-Matt
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