GL.iNet-Router-Screen-Saver-BE3600

Tearing: what is possible on the BE3600’s display

Short answer: hardware double buffering is not possible on this display with this firmware. The project does what can be done in software.

What was measured

native/fbprobe.c asks the framebuffer driver what it supports. On a GL-BE3600 (driver fb_st7789p3, firmware 4.8.3, kernel 5.4.213):

Question	Answer	Meaning
Framebuffer memory (`smem_len`)	43,168 bytes	exactly one frame; no spare memory for a second page
Virtual height vs visible height	284 = 284	no extra rows to draw into off-screen
Panning steps (`ypanstep`)	0	the driver does not support panning
`FBIOPAN_DISPLAY`	rejected (`EINVAL`)	so pages cannot be flipped
Asking for a taller virtual screen	accepted but ignored	the driver will not grow the buffer
`FBIO_WAITFORVSYNC`	not supported	there is no vertical-blank signal to wait for
`mmap` of the framebuffer	works	not a way around the above

So the usual tear-free techniques (draw the next frame off-screen and flip, or wait for vsync before writing) are all unavailable. Doing them would need a different display driver, which is a kernel change, not something a screen saver can install.

To reproduce (with the screen owners stopped so nothing is drawing):

be3600-anim off
/etc/init.d/gl_screen stop
./fbprobe --try-flip /dev/fb0      # restores the original mode afterwards
/etc/init.d/gl_screen start
be3600-anim on

fbprobe without --try-flip only asks questions and is safe to run any time.

What the project does instead

One burst per frame. A full frame is written with a single write(), and a BEA2 delta writes only the ranges that changed, all back to back. Small framebuffer drivers like this one typically push the framebuffer to the panel a little after a write rather than instantly, so writes that arrive together should be sent together (this behaviour was not itself measured).
Less data changes. With BEA2, an animation that moves a small object only rewrites that object’s bytes (the bundled animation averages a few kilobytes a frame instead of 43 KB), which shortens the window in which the panel could catch a half-updated picture.
Exact timing. The native player uses absolute deadlines, so frames arrive at a steady rhythm instead of drifting against the panel’s refresh.

What is not known

Whether tearing is visible has not been measured; that needs a high-speed camera or a photodiode, not software. Slow or moderate motion (like the bundled eyes) shows none to the eye. Fast full-screen motion is the case most likely to show it; if you see it, lowering fps or changing less of the screen per frame is the practical remedy.

This site is open source. Improve this page.