RP1 DPI: interlaced HSync/VSync alignment (6.12) #6695
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njhollinghurst
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Instead of trying to minimize the delay between seeing HSYNC edge and asserting VSYNC, try to predict the next HSYNC edge precisely. This eliminates the round-trip delay but introduces mode-dependent rounding error. HSYNC->VSYNC lag reduced from ~30ns to -5ns..+10ns (plus up to 5ns synchronization jitter as before). This may benefit e.g. SCART HATs, particularly those that generate Composite Sync using a XNOR gate. Signed-off-by: Nick Hollinghurst <[email protected]>
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popcornmix
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See: raspberrypi/linux#6689 kernel: Revert "firmware: raspberrypi: Add backward compatible get_throttled" See: raspberrypi/linux#6697 kernel: RP1 DPI: interlaced HSync/VSync alignment (6.12) See: raspberrypi/linux#6695 kernel: dtoverlays: ov9281: Add continuous clock option as an override See: raspberrypi/linux#6700 kernel: Fix clang compilation error for rpi5 See: raspberrypi/linux#6699 kernel: drm/vc4: Correct one logging message that got promoted from dbg to err See: raspberrypi/linux#6702
popcornmix
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See: raspberrypi/linux#6689 kernel: Revert "firmware: raspberrypi: Add backward compatible get_throttled" See: raspberrypi/linux#6697 kernel: RP1 DPI: interlaced HSync/VSync alignment (6.12) See: raspberrypi/linux#6695 kernel: dtoverlays: ov9281: Add continuous clock option as an override See: raspberrypi/linux#6700 kernel: Fix clang compilation error for rpi5 See: raspberrypi/linux#6699 kernel: drm/vc4: Correct one logging message that got promoted from dbg to err See: raspberrypi/linux#6702
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Instead of trying to minimize the HSync -> VSync lag, count cycles to predict the next HSync.
It reduces the HSync->VSync offset from about (+30ns .. +35ns), to about (-5ns .. +10ns). There is some mode-dependent rounding error, but for a given video mode the jitter is ~5ns as before.
This is mainly for the benefit of SCART Hats, particularly those that use an XNOR gate to generate Composite Sync, which otherwise could produce glitchier output when used in an interlaced mode with Raspberry Pi 5.
A fringe benefit is that 6 PIO instructions are now free (enough for some RGB LEDs but not for PWM...)