D714 - A drawn colour target is written back at the flip

Status: decided Date: 2026-09-24

A frame stays on the device across a frame's submissions and is written into guest memory at the flip, when a submission draws into a different target, or before command-processor work touches the pending target. Unchanged is detected by comparing the target's bytes with those last written or read, and ORBISTOUN_TARGET_WRITEBACK=submit writes back after every submission.

Why: a GL title submits dozens of times a frame into one target, and an 8 MB readback and retile per submission capped it near one frame a second. Draws still run before their fence retires, so only the copy moves. A CPU read of the target between submissions sees the last written-back frame; the operator chose this knowing it, and submit serves such a guest.

Rejected:

• Write-back after every submission: exact, and the dominant cost of a frame.
• Assuming the target unchanged between submissions: misses writes by anything else.