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Provide a way for userspace to choose synchronous flushing/pageflips. This helps save CPU and power. It is also useful for test scripts since userspace can know when a flush has happended and wait before doing the next visual test. Cc: Linus Walleij <linus.walleij@linaro.org> Signed-off-by: Noralf Trønnes <noralf@tronnes.org> Reviewed-by: Peter Stuge <peter@stuge.se> Link: https://patchwork.freedesktop.org/patch/msgid/20210703141321.35494-2-noralf@tronnes.org