Possible Cache Decoherence Issue

[johanderson]

Problem Summary

When trying to either:

• add new tests to the demo in lv_demo_benchmark to evaluate other items
• remove the demo call to begin creating my own UI here:

  • lv_port_stm32u5g9j-dk2/Core/Src/main.c

    Line 143 in ab35c3e

    lv_demo_benchmark();

I have noticed that sometimes the text is rendered as garbled text such as:

Reproduction steps:

Comment out the line

lv_port_stm32u5g9j-dk2/Core/Src/main.c

Line 143 in ab35c3e

lv_demo_benchmark();

Replace with example Bar code: https://docs.lvgl.io/master/details/widgets/bar.html#styling-a-bar

contains surrounding context:

  /* USER CODE BEGIN 2 */

  lvgl_port_init();

  static lv_style_t style_bg;
  static lv_style_t style_indic;

  lv_style_init(&style_bg);
  lv_style_set_border_color(&style_bg, lv_palette_main(LV_PALETTE_BLUE));
  lv_style_set_border_width(&style_bg, 2);
  lv_style_set_pad_all(&style_bg, 6); /*To make the indicator smaller*/
  lv_style_set_radius(&style_bg, 6);
  lv_style_set_anim_duration(&style_bg, 1000);

  lv_style_init(&style_indic);
  lv_style_set_bg_opa(&style_indic, LV_OPA_COVER);
  lv_style_set_bg_color(&style_indic, lv_palette_main(LV_PALETTE_BLUE));
  lv_style_set_radius(&style_indic, 3);

  lv_obj_t * bar = lv_bar_create(lv_screen_active());
  lv_obj_remove_style_all(bar);  /*To have a clean start*/
  lv_obj_add_style(bar, &style_bg, 0);
  lv_obj_add_style(bar, &style_indic, LV_PART_INDICATOR);

  lv_obj_set_size(bar, 200, 20);
  lv_obj_center(bar);
  lv_bar_set_value(bar, 100, LV_ANIM_ON);

  //lv_demo_benchmark();

Solution discussion

I have isolated the problem down to when

lv_port_stm32u5g9j-dk2/Core/Src/main.c

Line 131 in ab35c3e

MX_DCACHE2_Init();

this line is commented out, disabling the use of DCACHE2, the text garbling goes away. Note that DACHE1 is coupled with the CPU while DCACHE2 is coupled with the DMA2D.

This makes me think that there may be a cache decoherence problem that is occurring between DCACHE1, SRAM and DCACHE2. I am wondering if you have run into this problem before. What is interesting is that the demo as is seems to work just fine, but I only run into issues when I begin modifying this project.

In order to make use of the DCACHE2's performance increase, I am wondering if we have to add a strategy to managing the caches such as:

However I am not sure where would be the most appropriate or best lvgl practiced place to put this.

[johanderson]

One possible fix that I am investigating is adding the following:

HAL_DCACHE_Invalidate(&hdcache2);

to:
https://github.com/lvgl/lvgl/blob/cd35a9a25c877d6ab447cbb55fc98d1867305142/src/drivers/display/st_ltdc/lv_st_ltdc.c#L142

along with

extern DCACHE_HandleTypeDef hdcache2;

earlier in the file.

This forces DCache2 to constantly reload itself with new data. This seems to get rid of the rendering bug, but I am further investigating if this is the most appropriate solution.