On LV_DISPLAY_RENDER_MODE_DIRECT render mode & double buffer, flush the same buffer

qingsong · July 9, 2025, 3:32am

LVGL VERSION

v9.2.2

Platform

bes2800, Arm cortex-m55

what happened?

I set lvgl with LV_DISPLAY_RENDER_MODE_DIRECT & double buffer. When devices run, i find some area is not display or some area is not disappear correctly.

what I test for this problems?

1

I add some log and find that buffer is not switch correctly when device flush to screen. For example, buffer1 is addr A, buffer2 is addr B. The log show: Addr A, Addr A, Addr B, Addr B, Addr A, Addr A…

2

I switch to LV_DISPLAY_RENDER_MODE_FULL mode & double buffer. This problem is gone.
And buffer Addr log is: Addr A, Addr B, Addr A…

I see the same issue on github/lvgl/issue.

github.com/lvgl/lvgl

LV_DISPLAY_RENDER_MODE_DIRECT

opened 10:42AM - 02 Jul 25 UTC

hansd56

### LVGL version 9.3.0 ### Platform STM32H743, 32MB SDRAM 16 Bit access, 8MB …QSPI flash. ### What happened? Hi, In the LV_DISPLAY_RENDER_MODE_DIRECT I am having a hard time to understand what is written in the documentation and what is happening in disp_flush and what I am seeing on the screen. I have two full display buffers in external SRAM. Using LV_DISPLAY_RENDER_MODE_FULL the two display buffers are swapped for each call to disp_flush. Only the buffer swap is required and no issues. For LV_DISPLAY_RENDER_MODE_DIRECT it is stated that virtually the same is needed by the user and the rest is done by lvgl. That doesn't seem to be the case. As long as it is a full page like changing background colours it works the same way. As soon as a small area only is changed lvgl might be calling disp_flush a few times with the same buffer address. My question is in LV_DISPLAY_RENDER_MODE_DIRECT does the user need to update the buffers in any way? ### How to reproduce? Just set it up on a board with some more ram and change the buffers with each call or just swap the buffer that px_map delivers.